Arrow Pushing in Inorganic Chemistry : a Logical Approach to the Chemistry of the Main Group Elements

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Arrow Pushing in
Inorganic Chemistry

Arrow Pushing in
Inorganic Chemistry
A Logical Approach to the Chemistry
of the Main-Group Elements
Abhik Ghosh
Steffen Berg

Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved
Published by John Wiley & Sons, Inc., Hoboken, New Jersey
Published simultaneously in Canada
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Contents

FOREWORD

xi

PREFACE

xiii

ACKNOWLEDGMENTS

xvii

1. A Collection of Basic Concepts
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
1.10
1.11
1.12
1.13
1.14
1.15
1.16
1.17

Nucleophiles and Electrophiles: The SN 2 Paradigm
What Makes for a Good Nucleophile?
Hard and Soft Acids and Bases: The HSAB Principle
pKa Values: What Makes for a Good Leaving Group?
Redox Potentials
Thermodynamic Control: Bond Dissociation Energies (BDEs)
Bimolecular 𝛽-Elimination (E2)
Proton Transfers (PTs)
Elementary Associative and Dissociative Processes (A and D)
Two-Step Ionic Mechanisms: The SN 2-Si Pathway
Two-Step Ionic Mechanisms: The SN 1 and E1 Pathways
Electrophilic Addition to Carbon–Carbon Multiple Bonds
Electrophilic Substitution on Aromatics: Addition–Elimination
Nucleophilic Addition to Carbon–Heteroatom Multiple Bonds
Carbanions and Related Synthetic Intermediates
Carbenes
Oxidative Additions and Reductive Eliminations

1
2
5
8
9
11
11
14
15
16
19
20
22
23
24
26
29
30

Sections marked with an asterisk (*) may be skipped on first reading.

v

vi

CONTENTS

1.18
1.19
1.20
1.21
1.22
1.23
1.24
1.25
1.26
1.27
1.28

Migrations
Ligand Exchange Reactions
Radical Reactions
Pericyclic Reactions
Arrow Pushing: Organic Paradigms
Inorganic Arrow Pushing: Thinking Like a Lone Pair
Definitions: Valence, Oxidation State, Formal Charge, and Coordination
Number
Elements of Bonding in Hypervalent Compounds
The 𝜆 Convention
The Inert Pair Effect
Summary
Further Reading

2. The s-Block Elements: Alkali and Alkaline Earth Metals
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8

Solubility
The s-Block Metals as Reducing Agents
Reductive Couplings
Dissolving Metal Reactions
Organolithium and Organomagnesium Compounds
Dihydrogen Activation by Frustrated Lewis Pairs (FLPs)
A MgI –MgI Bond
Summary
Further Reading

3. Group 13 Elements
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9

Group 13 Compounds as Lewis Acids
Hydroboration
Group 13-Based Reducing Agents
From Borazine to Gallium Arsenide: 13–15 Compounds
Low-Oxidation-State Compounds
The Boryl Anion
Indium-Mediated Allylations
Thallium Reagents
Summary
Further Reading

4. Group 14 Elements
4.1
4.2
4.3

Silyl Protecting Groups
A Case Study: Peterson Olefination
Silanes

32
33
35
37
38
38
40
41
45
46
47
48
50
51
52
53
56
58
61
63
64
65
66
67
70
73
76
80
87
88
89
94
94
96
98
103
104

CONTENTS

4.4
4.5
4.6
4.7
4.8∗
4.9∗
4.10
4.11
4.12

The 𝛽-Silicon Effect: Allylsilanes
Silyl Anions
Organostannanes
Polystannanes
Carbene and Alkene Analogs
Alkyne Analogs
Silyl Cations
Glycol Cleavage by Lead Tetraacetate
Summary
Further Reading

5A. Nitrogen
5A.1 Ammonia and Some Other Common Nitrogen Nucleophiles
5A.2 Some Common Nitrogen Electrophiles: Oxides, Oxoacids,
and Oxoanions
5A.3 N–N Bonded Molecules: Synthesis of Hydrazine
5A.4 Multiple Bond Formation: Synthesis of Sodium Azide
5A.5 Thermal Decomposition of NH4 NO2 and NH4 NO3
5A.6 Diazonium Salts
5A.7 Azo Compounds and Diazene
5A.8∗ Imines and Related Functional Groups: The Wolff–Kishner
Reduction and the Shapiro Reaction
5A.9 Diazo Compounds
5A.10 Nitrenes and Nitrenoids: The Curtius Rearrangement
5A.11 Nitric Oxide and Nitrogen Dioxide
5A.12 Summary
Further Reading
5B. The Heavier Pnictogens
5B.1 Oxides
5B.2 Halides and Oxohalides
5B.3 Phosphorus in Biology: Why Nature Chose Phosphate
5B.4 Arsenic-Based DNA
5B.5 Arsenic Toxicity and Biomethylation
5B.6 Alkali-Induced Disproportionation of Phosphorus
5B.7 Disproportionation of Hypophosphorous Acid
5B.8 The Arbuzov Reaction
5B.9 The Wittig and Related Reactions: Phosphorus Ylides
5B.10 Phosphazenes
5B.11∗ The Corey–Winter Olefination
5B.12 Triphenylphosphine-Mediated Halogenations

vii

106
109
112
113
115
120
122
124
127
128
129
130
131
133
135
137
138
140
144
146
149
151
155
155
156
158
160
163
166
168
171
173
175
176
180
185
187

viii

CONTENTS

5B.13∗ The Mitsunobu Reaction
5B.14∗ The Vilsmeier–Haack Reaction
5B.15 SbF5 and Superacids
5B.16 Bismuth in Organic Synthesis: Green Chemistry
5B.17 Summary
Further Reading
6. Group 16 Elements: The Chalcogens
6.1 The Divalent State: Focus on Sulfur
6.2 The Divalent State: Hydrogen Peroxide
6.3 S2 Cl2 and SCl2
6.4 Nucleophilic Breakdown of Cyclopolysulfur Rings
6.5 Cyclooctachalcogen Ring Formation
6.6 Higher-Valent States: Oxides and Oxoacids
6.7 Sulfur Oxochlorides
6.8 Ozone
6.9 Swern and Related Oxidations
6.10 Sulfur Ylides and Sulfur-Stabilized Carbanions
6.11∗ Hydrolysis of S2 F2 : A Mechanistic Puzzle
6.12 Higher-Valent Sulfur Fluorides
6.13 Martin Sulfurane
6.14 Lawesson’s Reagent
6.15 Sulfur Nitrides
6.16∗ Selenium-Mediated Oxidations
6.17 Higher-Valent Tellurium: A Mechanistic Puzzle
6.18 Summary
Further Reading
7. The Halogens
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
7.9∗
7.10
7.11
7.12

Some Notes on Elemental Halogens
Alkali-Induced Disproportionation of Molecular Halogens
Acid-Induced Comproportionation of Halate and Halide
Hypofluorous Acid, HOF
Electrophilic Fluorinating Agents: N-Fluoro Compounds
Oxoacids and Oxoanions
Heptavalent Chlorine
Interhalogen Compounds
Halogens in Organic Synthesis: Some Classical Reactions
An Introduction to Higher-Valent Organoiodine Compounds
𝜆3 -Iodanes
𝜆5 -Iodanes: IBX and Dess–Martin Periodinane

188
191
193
195
200
200
202
204
205
209
211
213
215
219
222
226
228
231
234
236
238
240
243
247
250
251
252
254
258
260
261
264
268
271
275
276
283
284
288

CONTENTS

7.13 Periodic Acid Oxidations
7.14 Bromine Trifluoride
7.15∗ Aryl-𝜆3 -Bromanes
7.16 Summary
Further Reading
8. The Noble Gases
8.1
8.2
8.3
8.4
8.5
8.6
8.7
8.8
8.9
8.10
8.11
8.12
8.13

The Xenon Fluorides: Fluoride Donors and Acceptors
O/F Ligand Exchanges
Xenon Fluorides as F+ Donors and Oxidants
Hydrolysis of XeF2 and XeF4
Xenate and Perxenate
Disproportionation of Xenate
Hydrolysis of XeF4
Other Compounds Containing Xe–O Bonds
Xe–N Bonds
Xe–C Bonds
Krypton Difluoride
Plus Ultra
Summary
Further Reading

ix

290
291
294
298
299
300
302
303
304
306
307
308
310
311
312
313
314
316
316
316

Epilogue

318

Appendix A. Inorganic Chemistry Textbooks, with a Descriptive-Inorganic
Focus
A.1 Introductory Texts
A.2 Advanced Texts

319
319
319

Appendix B. A Short List of Advanced Organic Chemistry Textbooks

320

Index

321

Foreword

Many years ago George Hammond and I taught a course at Caltech that included discussions
of main-group chemistry. We tried to use inorganic textbooks that dealt with the subject, but
we were not happy with them, as they paid no attention to reaction mechanisms. Discussions of nucleophilic and electrophilic reagents, associative and dissociative substitutions,
reaction energy landscapes, and so on, were nowhere to be found. Faced with this problem,
we decided to base our course on reaction mechanisms, but very few instructors adopted
this approach in teaching main-group chemistry.
Now, at long last, we have a book on main-group chemistry that students can learn from!
They may even read it from cover to cover without going to sleep! The authors, Abhik
Ghosh and Steffen Berg, have clearly demonstrated how a mechanistic approach makes the
reactions of main-group elements interesting and understandable: Arrow pushing is the key!
There are many parts of the book that I like very much. The treatment of the reactions
of nitrogen compounds, largely neglected in inorganic courses, is particularly good. And
one of my favorites, the very rich chemistry of high-valent halogen and xenon molecules,
is excellent. The bottom line is that arrow pushing is a method that should be used to teach
main-group chemistry. As the authors note, their book logically can be used to supplement
standard inorganic texts. I urge instructors to try the Ghosh–Berg method when faced with
teaching the dreaded “descriptive” section of the inorganic course. Arrow pushing not only
is great fun, students who try it may actually learn main-group chemistry!
Harry B. Gray
California Institute of Technology
February 2014

xi

Preface

Inorganic chemistry at core consists of a vast array of molecules and chemical reactions. To
master the subject, students need to think intelligently about this body of facts, a feat that
is seldom accomplished in an introductory course. All too often, young students perceive
the field as an amorphous body of information that has to be memorized. We have long
been intrigued by the possibility of changing this state of affairs by means of a mechanistic
approach, specifically organic-style arrow pushing. We found that such an approach works
well for all main-group elements, that is, elements from the s and p blocks of the periodic
table. In particular, we found that arrow pushing works well for hypervalent compounds,
where the central atom has more than eight electrons in its valence shell in the Lewis structure. Over time, we came to appreciate that full implementation of a mechanistic approach
had the potential to transform the teaching of a substantial part of the undergraduate inorganic curriculum. This book is a realization of that vision.
Arrow Pushing in Inorganic Chemistry is designed as a companion to a standard inorganic text. In general, we have devoted one chapter to each group of the main-group elements. Each chapter in this book is designed to supplement the corresponding chapter in a
regular inorganic text. A student using this book is expected to have taken general chemistry
and a good, introductory course in organic chemistry at the university level. Key prerequisites include elementary structure and bonding theory, a good command of Lewis structures,
VSEPR theory, elementary thermodynamics (as usually outlined in general chemistry),
simple acid–base calculations, basic organic nomenclature, and a good but elementary
understanding of organic mechanisms. Because a basic knowledge of organic chemistry has
been assumed, the general level of this book is somewhat higher than that of an undergraduate organic text. The material included in this book (along with related content from a standard inorganic text) has been regularly taught at the University of Tromsø in about 30 h of
class time, roughly half of which has been devoted to problem-solving by students. A small
number of somewhat specialized topics and review problems have been marked with an
asterisk, to indicate that they may be skipped on first reading. We usually take up a few of
these at the end of our course and in conjunction with a second or more specialized course.
xiii

xiv

PREFACE

The approach. Many students are deeply impressed by the logic of organic chemistry.
Mechanistic rationales are available for essentially every reaction in the undergraduate
(and even graduate) organic curriculum and students learn to write reaction mechanisms
right from the beginning of their courses. A survey of current texts shows that a mechanistic approach is universally adopted in introductory organic courses. The situation with
inorganic chemistry could not be more different; not one major introductory text adopts a
mechanistic approach in presenting descriptive main-group chemistry! In a telling exercise,
we went through several textbooks that do an otherwise excellent job of presenting descriptive inorganic chemistry, without finding the words “nucleophile” and “electrophile.” Not
surprisingly, these texts do not present a single instance of arrow pushing either.
Arrow pushing above all provides a logical way of thinking about reactions, including
those as complex as the following:
P4 + 3 NaOH + 3 H2 O → 3 NaH2 PO2 + PH3
24 SCl2 + 64 NH3 → 4 S4 N4 + S8 + 48 NH4 Cl
2 HXeO4 − + 2 OH− → XeO6 4− + Xe + O2 + 2 H2 O
These reactions represent important facets of the elements involved but are typically
presented as no more than facts. (Why does boiling white phosphorus in alkali lead to
hypophosphite and not phosphate?—Current texts make no attempt to address such questions.) Arrow pushing demystifies them and places them on a larger logical scaffolding.
The transformative impact of this approach cannot be overstated. Almost to a person,
students who have gone through our introductory course say that they cannot imagine how
someone today could remain satisfied with a purely descriptive, nonmechanistic exposition
of inorganic main-group chemistry.
A mechanistic approach has done wonders for the overall tenor of our classroom—now
very much a “flipped classroom,” where arrow pushing, instead of videos, have afforded
the “flip.” Well-designed traditional lectures are still important to us and our students, but
they now account for only 50% of total contact hours, with the rest devoted to various types
of active learning. Some students solve mechanism problems on their own, others do so in
groups, and still others solve them on the blackboard in front of the class. Importantly, such
a classroom affords continual feedback from the students so we always have a good idea of
their level of understanding and can assist accordingly.
Potential concerns. Given the plethora of advantages of a mechanistic approach, it’s
worth reflecting why it has never been adopted for introductory inorganic chemistry. A
plausible reason is that, in contrast to common organic functional groups, simple p-block
compounds such as hydrides, oxides, halides, and so forth, tend to be much more reactive
and their vigorous and even violent reactions have been much less thoroughly studied. As
good scientists, inorganic chemists may have felt a certain inhibition about emphasizing an
approach that has little grounding in experimental fact. This is a legitimate objection, but
hardly a dealbreaker, in our opinion, for the following reasons.
Our ideas on main-group element reactivity are not taken out of the blue but are based on
parallels with well-studied processes in organic and organoelement chemistry. Second, it
no longer necessarily takes a prohibitive amount of resources to test a mechanistic proposal,
at least in a preliminary way. Quantum chemical calculations, particularly based on density
functional theory (DFT), very often provide an efficient and economical way of evaluating
reaction mechanisms. Third, and perhaps most important, it’s vastly better to be able to

PREFACE

xv

formulate a hypothesis on how a reaction might happen than to have no inkling whatsoever
about the mechanism.
Content and organization. Chapter 1 attempts to provide a summary of all relevant introductory concepts, paving the way for a full appreciation of the rest of the book. The chapter
begins with a discussion of nucleophiles and electrophiles, continues on to present a survey
of the major organic reaction types (substitution, elimination, addition, etc.) and of some
specifically inorganic reaction types (oxidative addition, reductive elimination, metathesis,
migrations, etc.), and concludes with an elementary discussion of hypervalent compounds.
The subsequent chapters are organized according to the groups of the periodic table, from
left to right. Chapter 2 deals with the s-block elements, providing a combined treatment
of hydrogen, the alkali metals, and the alkaline earth metals. For the p block, the chapter
number is generally the same as the old group number; thus, the chalcogens are discussed
in Chapter 6, the halogens in Chapter 7, and so on. The only exception is group 15, which
we have split up into two chapters, 5a and 5b: Chapter 5a is devoted to nitrogen and Chapter
5b to the heavier pnictogens.
As far as any given chapter is concerned, the goal has been not so much to provide a
systematic account of a given group of main-group elements (although we believe that we
have done so moderately well) as to help students figure out the inner workings of relatively
complicated-looking reactions. We have done so by organizing each chapter as a series
of vignettes, focusing on reactions that in our opinion are most conducive to sharpening
students’ arrow-pushing skills. In-chapter review problems are designed to further hone
these skills as well as to provide material for in-class discussions and recitation sections.
We have refrained from including end-of-chapter problems, in part out of a desire to limit
the book to a manageable length. Students in need of additional exercises should find an
ample supply of reactions in their regular descriptive inorganic text.
As far as our choice of reactions and topics is concerned, we have attempted to offer a
stimulating mix of the traditional and the topical. For the traditional material, we have borrowed freely from introductory and advanced texts with a “descriptive inorganic” emphasis.
These books are listed in Appendix 1. The Wikipedia has also been a valuable resource for
this purpose. On occasion, we have played science historian and thrown in an anecdote
or an amusing quote. The more cutting-edge material has been sourced from the research
literature. Examples of such topics include:
•
•
•
•
•
•
•

Jones’s Mg(I)–Mg(I) reagent
indium-mediated allylations
heavy-element carbene, alkene, and alkyne analogs
the Ruppert–Prakash and Togni reagents
BrF3 and higher valent bromine compounds as synthetic reagents
the recent arsenic-DNA controversy
the possible role of borate minerals in the origin of life (possibly even on Mars!)

Because this is an introductory text, however, we have cited the original research literature
sparingly, often settling for a short list of suggested readings at the end of each chapter.
Stylistic aspects. A few comments on stylistic aspects of the book might be helpful.
Perhaps foremost among them is the use of color in our reaction mechanisms, which include
blue, black, red, and green. In general, the first nucleophile in a given mechanism is always
indicated in blue and the first electrophile in black. Later in the mechanism, if the atoms
originating in the initial nucleophile take on a different role, such as that of an electrophile,

xvi

PREFACE

they are still indicated in blue. Thus, for any given atom or group, its color is maintained
the same throughout the mechanism so that its fate can be easily followed throughout
the reaction pathway. Curly arrows have throughout been indicated in red; certain atoms
“deserving” special attention are also indicated in red. In some cases, where a third reactant is involved, it is indicated in green. In general, the color of a newly formed bond is the
same as the color of the lone pair or other electrons from which it may be thought to have
originated for bookkeeping purposes.
In this book, curly arrows typically begin from the nucleophilic electron pair and end
on the electrophilic atom being attacked. In general, to prevent clutter, we have not shown
lone pairs unless they are specifically engaged in a nucleophilic attack.
We have made sparse use of multiple bonds involving higher valent p-block elements.
Thus, we have preferred to use the left-hand structures for POCl3 and SO2 Cl2 , as opposed
to the multiply bonded structures to the right:

O
P

−

Cl
Cl

Cl
O
−
O

S

O

+

P
Cl

−

Cl
Cl

O

2+
Cl
Cl

S
O

Cl
Cl

Despite the unrealistic formal charges, we believe that the structures on the left give a clearer
sense of the bonding, whereas the multiple bonds shown to the right are harder to appreciate.
It is not easy to explain to an undergraduate audience which specific orbitals constitute the
double bonds in the right-hand structures. To instructors who would prefer to stick to the
more conventional multiply bonded structures, we say: by all means do so; for the vast
majority of reactions, arrow pushing will work equally well for both types of structures.
The end of descriptive inorganic chemistry? An interesting question to consider is the
following: Does a mechanistic approach, making extensive use of arrow pushing, signal
of the end of descriptive inorganic chemistry? The answer, in our opinion, is both yes and
no. By emphasizing arrow pushing as a universal tool for rationalizing main-group reactivity, we have placed the field at exactly the same level as organic chemistry. Just as no one
speaks of “descriptive organic chemistry,” there is no point in treating main-group chemistry as a descriptive subject. That, of course, does not diminish the importance of facts and
having an appropriate respect for them. Facts come first, whether it’s organic or inorganic
chemistry, and mechanisms are primarily useful for understanding and rationalizing them.
In that sense, mechanisms can never supplant a descriptive exposition of chemical facts.
Abhik Ghosh and Steffen Berg
The Arctic University of Norway, Tromsø, Norway

Acknowledgments

We are indebted to many friends and colleagues who generously assisted us in the
preparation of this book. Prof. Carl Wamser of Portland State University and Dr. David
Ware of The University of Auckland read and critiqued the entire manuscript. Our debt
to these two loyal friends is immense. Others who read individual chapters and shorter
sections include Paul Deck of Virginia Tech (halogens), Penny Brothers of The University
of Auckland (Group 13 elements), Barry Rosen of Florida International University
(Group 15 elements), Ged Parkin of Columbia University (higher-valent and hypervalent
compounds), and Kyle Lancaster of Cornell University (the noble gases). We thank Steven
Benner (FFAME, Gainesville, FL; arsenic-DNA), Tristram Chivers (University of Calgary;
sulfur nitrides), Harry Gray (Caltech; higher-valent bromine reagents), Roald Hoffmann
(Cornell; aspects of halogens), Pekka Pyykkö (University of Helsinki; inert pair effect),
and Shlomo Rozen (Tel Aviv University; BrF3 ) for helpful advice and correspondence
on the topics indicated within parentheses. Our long-time friend and collaborator Prof.
Jeanet Conradie of the University of the Free State, South Africa, assisted us with the DFT
calculations we needed for a better understanding of certain reactions. Carl Wamser and
Penny Brothers also provided wonderful refuges—Portland, Oregon, and Auckland, New
Zealand—where one of us (AG) could escape to and write.
The Foreword has been written by Harry Gray, who seemed to us to be uniquely qualified for the purpose. In the 1960s, he and George Hammond tried to adopt a mechanistic
approach in teaching aspects of main-group chemistry (see, e.g., Chemical Dynamics by J.
B. Dence, H. B. Gray, and G. S. Hammond, Benjamin: 1968). Harry’s full-throated support
of our own approach means a great deal to us.
It is a pleasure to acknowledge Wiley editor Anita Lekhwani for her encouragement
and wise counsel throughout the writing process. We are similarly grateful to Sangeetha
Parthasarathy of Laserwords Pvt. Ltd. Chennai, India, for the considerable efforts involved
in the final production of the book.
Finally, we thank our families and some of our closest friends for their love and
encouragement: AG thanks Avroneel, Sheila, Ranjita, Matthew, and Daniel; and SB thanks
Kenneth, Andreas, Eirik, Tor Håvard, and above all Cathrine.
xvii

Advance praise for Arrow Pushing in
Inorganic Chemistry: A Logical Approach to
the Chemistry of the Main-Group Elements
I tell my organic students to “think like a molecule”. What are the molecules doing, and why
are they doing that? Since the essence of a chemical reaction is the reorganization of bonds
(i.e., electrons), the primary tool for understanding it is arrow pushing. It’s a real delight
to see that this fundamental approach indeed works beautifully in inorganic chemistry as
well. It makes one wonder why it hadn’t been “discovered” sooner. Congratulations to the
authors for an excellent expository textbook.— Professor Carl C. Wamser, Portland State
University
It’s great to see a key organic skill, arrow pushing, applied to inorganic chemistry, where
there’s plenty extra to think about—redox chemistry along with wide variations in atomic
size and electronegativity. The strength of the approach is that all this can be taken into
account. A powerful new way of thinking for inorganic chemists!—Dr. David Ware and
Professor Penny Brothers, University of Auckland, New Zealand
In my Metals in Biology course, I tell my students the simplest lesson of chemistry: electrons
flow from where they are to where they aren’t. This is the essence of the ‘arrow pushing’ formalism, which had its origins in physical organic chemistry. My early training in that field
led me to use the arrow pushing language in my own research in bioinorganic chemistry. I
am delighted to see this language applied much more generally to inorganic chemistry in
this very illuminating and instructive book. Students will learn where electrons want to go
and their appreciation of how reactions occur will be greatly enhanced. — Professor John
T. Groves, Princeton University
Nice up-to-date stuff, including frustrated Lewis pairs, Jones’s Mg(I) reagent, high-valent
bromine and lots more! It would have been easy for the authors to ignore the last twenty
years (or fifty) but they didn’t do that! — Professor Paul A. Deck, Virginia Tech
I was struck by the sheer amount of innovation, thought, and attention to detail that has gone
into the making of this book. In cases where arrow pushing does not immediately indicate
a unique mechanism, the authors have even resorted to DFT calculations to resolve the
ambiguity. — Professor Jeanet Conradie, University of the Free State, Republic of South
Africa
… Valence is an important concept in inorganic chemistry and it’s nice to see the authors
do full justice to the topic. They carefully distinguish valence and oxidation state, which
are often confused, and draw structures with appropriate formal charges that shed light on
the bonding. Furthermore, their treatment of the fascinating chemistry of the higher-valent
states of p-block elements is superb. — Professor Gerard Parkin, Columbia University
… The marriage between descriptive inorganic chemistry and the language of organic reaction mechanisms is convincingly consummated in this new and most useful
contribution. — Professor Peter R. Taylor, University of Melbourne

1
A Collection of Basic
Concepts
In solving a problem of this sort, the grand
thing is to be able to reason backward. That is
a very useful accomplishment, and a very easy
one, but people do not practise it much. In the
everyday affairs of life it is more useful to reason
forward, and so the other comes to be neglected.
Sherlock Holmes in A Study in Scarlet, By Sir Arthur Conan Doyle
We assume you’ve had an introductory course in organic chemistry and hope you found
it logical and enjoyable. The logic of organic chemistry is of course key to its charm, and
mechanisms are a big part of that logic. In this book, we will present a similar approach
for inorganic chemistry, focusing on the main-group elements, that is, the s and p blocks
of the periodic table (Figure 1.1). As in organic chemistry, our main tool will be the curly
arrows that indicate the movement of electrons, typically electron pairs, but on occasion
also unpaired electrons. As we shall see, this approach—arrow pushing—works well in
inorganic chemistry, especially for the main-group elements.
We want to get you started with arrow pushing in an inorganic context as quickly as
possible, but we’d also like to make sure that you are equipped with the necessary conceptual tools. In this chapter, we’ll try to provide you with that background as efficiently as
possible. Unavoidably, the concepts form a somewhat disparate bunch but they do follow a
certain logic. Sections 1.1–1.6 introduce the idea of nucleophiles and electrophiles, in the
context of the SN 2 displacement, and discuss physical concepts such as electronegativity,
polarizability, pKa , redox potentials, and bond energies in relation to chemical reactivity.
Armed with these concepts, we’ll devote the next several Sections 1.7–1.21 to survey key
mechanistic paradigms, focusing on major organic reaction types but also on a few special
inorganic ones. Sections 1.22 and 1.23 then present practical tips on arrow pushing, that
Arrow Pushing in Inorganic Chemistry: A Logical Approach to the Chemistry of the Main-Group Elements,
First Edition. Abhik Ghosh and Steffen Berg.
© 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc.

1

2

A COLLECTION OF BASIC CONCEPTS
d-Block
p-Block
s-Block

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18
He

H
Li

Be

B

C

N

O

F

Ne

Na

Mg

Al

Si

P

S

Cl

Ar

K

Ca

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

Ga

Ge

As

Se

Br

Kr

Rb

Sr

Y

Zr

Nb

Mo

Tc

Ru

Rh

Pd

Ag

Cd

In

Sn

Sb

Te

I

Xe

Cs

Ba

Lu

Hf

Ta

W

Re

Os

Ir

Pt

Au

Hg

Tl

Pb

Bi

Po

At

Rn

Fr

Ra

Lr

Rf

Db

Sg

Bh

Hs

Mt

Ds

Rg

Cn

Figure 1.1 The periodic table: group numbers and the s, p, and d blocks.

is, how you might approach a given mechanistic problem. In the course of our mechanistic survey, we’ll encounter a number of so-called hypervalent p-block compounds, which
you may not have encountered until now. These call for a brief discussion of the bonding
involved, which we will present in Sections 1.24–1.27. That said, we will not cover some
of the more elementary aspects of structure and bonding theory, including the very useful
VSEPR (valence shell electron pair repulsion) model; feel free to go back to your general
or organic chemistry text for a quick refresher.

1.1

NUCLEOPHILES AND ELECTROPHILES: THE SN 2 PARADIGM

In this book, we will be overwhelmingly concerned with polar or ionic mechanisms. These
involve the movement of electron pairs, unlike radical reactions which involve unpaired
electrons. The components of a polar mechanism can generally be classified as nucleophiles or electrophiles. A nucleophile (“nucleus-lover”) is typically an anion or a neutral
molecule that uses an electron pair to attack another atom, ion, or molecule. The species
being attacked is called an electrophile (“electron-lover”). The terms “nucleophile” and
“electrophile” often refer to the classic SN 2 reaction of organic chemistry. In the example
below (which happens to be a Williamson ether synthesis), the methoxide anion is the
nucleophile, methyl iodide is the electrophile, and iodide is the leaving group.
H
C
H

H

−
O

H
C
H

H

I

H
H

H

H

C

C
O

H
H

+ I

−

(1.1)

A key feature of the SN 2 reaction is that the nucleophile attacks from the “back side”
relative to the leaving group, leading to an umbrella-like inversion of the carbon undergoing

1.1 NUCLEOPHILES AND ELECTROPHILES: THE SN 2 PARADIGM

−
H,

−
BH4 ,

RLi ,

RMgBr , RC

R3N ,

−
AIH4

−
CN ,
−
HO ,
−
RS ,
−
Br ,

R3P,

−
ROH, RO ,
RSH , RSR′,
−
−
F ,
Cl ,
Figure 1.2

− −
C , HC
−
N

+
N

3

CO2Et
CO2Et
NR

−

HOO
−
S
C
−
I

N

S

C

−
N

Some common nucleophiles, with the nucleophilic atoms indicated in blue.

substitution. If this carbon atom is stereogenic,1 such an inversion of configuration may be
discerned experimentally, as in the example below; otherwise the inversion is not detectable,
even though it occurs.

− Br
−
RS

Br

−

(1.2)
RS

H
Me

Me

H

Several common nucleophiles are depicted in Figure 1.2, where R and R′ denote alkyl
groups. Many of them are nitrogen-based, such as ammonia, amines (RNH2 ), and azide
(N3 − ), or oxygen-based, such as water, alcohols (ROH), and alkoxide (RO− ) and carboxylate (RCO2 − ) anions. Sulfur-based nucleophiles such as thiols (RSH), thiolates (RS− ), and
thioethers (RSR′ ) are also widely used in chemical synthesis. Triphenylphosphine, a phosphorus nucleophile, is an important reagent in organic synthesis, as well as an important
transition-metal ligand. Halide ions are widely employed as both nucleophiles and leaving
groups. Hydride is used both as a base (typically as NaH or KH) and as a nucleophile (often
in complexed forms such as BH4 − or AlH4 − ).
Carbon nucleophiles play a central role in organic chemistry, as they form the basis of
carbon–carbon bond formation. A few are shown in Figure 1.2, including such carbanionic
species as organolithiums (RLi), Grignard reagents (typically written as RMgBr), and the
cyanide (CN− ) and acetylide (R–C≡C− ) anions. Other examples such as enolates, enols,
and enamines will be briefly discussed in Section 1.15.
Some common electrophiles are shown in Figure 1.3. These include protons and positively charged metal ions, electron-deficient species such as trivalent group 13 compounds
(e.g., BF3 , AlCl3 ), the cationic carbon in carbocations, the halogen-bearing carbon in alkyl
1A

stereogenic center is an atom in a molecule for which interchanging any two of its substituents leads to a
different stereoisomer. The term was introduced by Mislow and Siegel in an important foundational paper on
modern stereochemical concepts and terminology: Mislow, K.; Siegel, J. J. Am. Chem. Soc. 1984, 106, 3319–3328.

4

A COLLECTION OF BASIC CONCEPTS

Figure 1.3
green.

2+
Mg ,

+
H,

+
H3O ,

+
Li ,

BF3 ,

AlCl3 ,

TlCl3 ,

RX,

+
R ,

+
RCO,

R3SiX,

PCl5 ,

Ph3BiCl2,

SF4 ,

SO3 ,

SeCl2 ,

SeO2 ,

X2 ,

BrF3,

XeF2

R3SnX,

Pb(OAc)4,

Some common electrophiles; X is a halogen. The electrophilic atoms are indicated in

halides, the Si atom in silyl halides, molecular halogens, and even the fluorine atoms in
xenon difluoride (XeF2 ).
The ease with which a given SN 2 displacement occurs depends on multiple factors, such
as the nucleophilicity of the incoming nucleophile (which depends on both its electronic
and steric character), steric hindrance at the electrophilic carbon center, the effectiveness of
the leaving group, and the solvent and other environmental effects. By defining a standard
substrate and standard reaction conditions, the reactivity of different nucleophiles may be
quantified. One such measure of nucleophilicity is the Swain–Scott nucleophilicity constant n, for which methyl iodide is chosen as the standard substrate and reaction rates are
measured in methanol at 25 ∘ C:
nCH3 I = log

kNu
kCH3 OH

where kNu is the rate constant for the nucleophile of interest (Nu) and kCH3 OH is the rate
constant for methanol itself as the nucleophile. Table 1.1 lists nCH3 I values for a number of
representative nucleophiles, along with the pKa values of their conjugate acids (i.e., a measure of the basicity of the nucleophiles). Observe that there is only a very rough correlation
between nCH3 I and the conjugate acid pKa ; we’ll return to this point in the next section.
Table 1.2 presents a more qualitative characterization of some common nucleophiles,
classifying them from strong to very weak.
Table 1.1 shows that, for a given electrophile (CH3 I) and standard conditions, the rate
constants for common nucleophiles vary by a factor of well over a billion (109 ). This
tremendous variation of reactivity of the different nucleophiles might pose a conundrum
in relation to their synthetic utility. Note (from either Table 1.1 or 1.2) that alkoxide (RO− )
anions are some 103 –104 times more nucleophilic than neutral alcohols, and the rates for
carboxylate anions (RCO2 − ), relative to the un-ionized carboxylic acids, differ by even
more: 105 –106 . With such low rates, are alcohols and carboxylic acids, in their un-ionized
forms, at all useful as nucleophiles? The answer is a clear yes. In acidic media, many
of the anionic nucleophiles simply don’t exist; they are entirely protonated. Under such
conditions, weak nucleophiles such as alcohols and carboxylic acids react effectively with
cationic electrophiles such as carbocations. Second, although weaker nucleophiles may not
react at a useful rate with alkyl halides, many of them do react at perfectly acceptable rates

1.2 WHAT MAKES FOR A GOOD NUCLEOPHILE?

5

TABLE 1.1 Swain–Scott Nucleophilicity Constants
and Conjugate Acid pKa Values of Some Common
Nucleophiles
Nucleophile

nCH

Conjugate
Acid pKa

CH3 OH
NO3 –
F–
CH3 CO2 –
Cl –
R2 S
NH3
N3 −
C6 H5 O –
Br –
CH3 O –
HO –
NH2 OH
NH2 NH2
(CH3 CH2 )3 N
CN–
I–
HO2 –
(CH3 CH2 )3 P
C6 H5 S –
C6 H5 Se−

0.0
1.5
2.7
4.3
4.4
5.3
5.5
5.8
5.8
5.8
6.3
6.5
6.6
6.6
6.7
6.7
7.4
7.8
8.7
9.9
10.7

−1.7
−1.3
3.45
4.8
−5.7
−6 to −7
9.25
4.74
9.89
−7.7
15.7
15.7
5.8
7.9
10.7
9.3
−10.7
11.75
8.7
6.5
5.9

3I

TABLE 1.2 Qualitative Classification of Nucleophiles, Based
on the Swain–Scott Nucleophilicity Constants nCH I
3

Nucleophiles
RS− ,

HS− , I−

N3 − , CN− , RO− , OH− , Br−
NH3 , RCO2 − , F− , Cl−
ROH, H2 O
RCO2 H

Relative Rate

Characterization

>105

Strong
Good
Moderate
Weak
Very weak

104
103
1
10−2

with stronger electrophiles such as BF3 or neutral organosilicon compounds in general. The
usefulness of a given nucleophile thus depends enormously on the reaction conditions.

1.2

WHAT MAKES FOR A GOOD NUCLEOPHILE?

Nucleophilicity and electrophilicity are closely related to Lewis basicity and acidity, respectively. Nucleophiles are Lewis bases (electron-pair donors) and electrophiles are Lewis
acids (electron-pair acceptors). Now, as discussed previously, nucleophilicity is measured
in terms of the rate of a nucleophilic attack, so it’s a kinetic concept. Basicity, on the other
hand, is measured in terms of the equilibrium constant for protonation (or for association with some Lewis acid), so it is a thermodynamic concept. Another difference is that,

6

A COLLECTION OF BASIC CONCEPTS

whereas Brønsted basicity refers to the thermodynamic affinity for protons, nucleophilicity
in organic chemistry typically refers to the rate of attack on a carbon center. Moreover,
in this book, we will talk about nucleophilic attacks on pretty much any p-block element!
Understandably, therefore, you should not expect more than a rough correlation between
the nucleophilicity of a nucleophile and its basicity.
To better appreciate this point, let us go back to Table 1.1, which lists a number of
nucleophiles in increasing order of nCH3 I , an arbitrarily defined measure of nucleophilicity. Observe that the basicities of the nucleophiles, as indicated by the pKa values of their
conjugated acids, increase in a general but highly erratic way with the value of nCH3 I . To
illustrate, whereas tertiary phosphines are more nucleophilic than tertiary amines by about
two orders of magnitude, the former are less basic than the latter by roughly the same factor.
What factors then make for a good nucleophile? The following trends have been observed:
• Anions are better nucleophiles than the related neutral molecules. Thus:
RO – > ROH;

RS – > R2 S;

NH2 – > NH3

where R = H, alkyl, or aryl.
• For analogous species in a given period, nucleophilicity decreases as one progresses
to the right of the periodic table:
NH3 > H2 O;

R3 P > R2 S

The more electronegative elements hold on to their electrons more tightly.
• Nucleophilicity increases as one goes down a given group of the periodic table. Thus,
for example
PR3 > NR3 ; PhSe – > PhS – > PhO –
The larger atoms are less electronegative and the anions derived from them are more
polarizable, which translates to increasing nucleophilicity as one goes down a group.
Given that electronegativity and size (atomic radius) are the two key determinants of
nucleophilicity, it’s useful to remind ourselves how the two atomic properties vary across
the periodic table. Figure 1.4 presents Pauling electronegativities and Figure 1.5 the atomic
radii of the s- and p-block elements. Note that electronegativity increases from left to right
along a given period, and decreases down a group. Atomic radii shrink from left to right in
a given period and increase down a group.
Against this backdrop, the relative nucleophilicities of the halide anions make for somewhat of a puzzle. The Swain–Scott nucleophilicities (Tables 1.1 and 1.2), based on methanol
as solvent, are in the order:
I – > Br – > Cl – > F –
The same order is found in other protic solvents. This is also the order expected on the basis
of polarizability: the larger and more polarizable anions should be the most nucleophilic.
In polar aprotic solvents (e.g., DMSO, DMF, THF, etc.), however, the relative rates are
completely reversed:
F – ≫ Cl – > Br – > I –

1.2 WHAT MAKES FOR A GOOD NUCLEOPHILE?

H
2.20

7

He

Li
0.98

Be
1.57

B
2.04

C
2.55

N
3.04

O
3.44

F
3.98

Ne

Na
0.93

Mg
1.31

Al
1.61

Si
1.90

P
2.19

S
2.58

Cl
3.16

Ar

K
0.82

Ca
1.00

Sc

Ti

Rb
0.82

Sr
0.95

Y

Zr

Nb Mo

Tc

Cs
0.79

Ba
0.89

–

Hf

Ta

Re Os

Fr
0.7

Ra
0.9

V

Cr

W

Co

Ni

Cu

Zn

Ga
1.81

Ge
2.01

As
2.18

Se
2.55

Br
2.96

Kr
3.00

Ru Rh

Pd

Ag

Cd

In
1.78

Sn
1.96

Sb
2.05

Te
2.1

I
2.66

Xe
2.60

Pt

Au

Hg

Tl
1.62

Pb
2.33

Bi
2.02

Po
2.0

At
2.2

Rn
2.2

Mn Fe

Ir

Figure 1.4 Pauling electronegativities of the main-group elements. A relatively self-explanatory color
code has been employed to give a semiquantitative visual indication of the electronegativities.

Ne

H

31

53
Li

Be

B

C

N

O

F

Ne

167

112

87

67

56

48

42

38

Na

Mg

Al

Si

P

S

Cl

Ar

190

145

118

111

98

88

79

71

K

ca

Ga

Ge

As

Se

Br

Kr

Sc

Ti

V

Cr Mn Fe Co Ni Cu Zn

243

194

136

125

114

103

94

88

Rb

Sr

In

Sn

Sb

Te

I

Xe

265

219

156

145

133

123

115

108

Cs

Ba

Tl

Pb

Bi

Po

At

Rn

156

154

143

135

Y

–
298

253

Zr Nb Mo Tc Ru Rh Pd Ag Cd

Hf Ta W Re Os

Ir

Pt Au Hg
120

Figure 1.5 Atomic radii (pm) of s- and p-block elements. (Clementi, E.; Raimond, D. L.; Reinhardt,
W. P. J. Chem. Phys. 1967, 47, 1300–1307.)

This remarkable reversal is due to hydrogen bonding, or the lack thereof in aprotic
solvents.
As a powerful hydrogen-bond acceptor, fluoride is understandably a poor nucleophile
in protic solvents. Iodide, as the worst hydrogen-bond acceptor, is thus a much more active
nucleophile in protic solvents. In the absence of hydrogen-bonding interactions with the
solvent, which is the case in dry polar aprotic solvents, fluoride is the strongest nucleophile.

8

A COLLECTION OF BASIC CONCEPTS

To a significant extent, the high nucleophilicity of “naked” fluoride ions may be attributed
to the strength of the C–F bond (more on which in Section 1.6). Because the SN 2 transition
state involves bond formation between the incoming nucleophile and carbon, the strength
of that bond is a key determinant of nucleophilicity.
Last but not least, steric effects are yet another key determinant of nucleophilicity. We
will discuss steric effects to some extent in Section 1.7.

1.3

HARD AND SOFT ACIDS AND BASES: THE HSAB PRINCIPLE

Several of the factors affecting nucleophilicity may be nicely rolled together into the
concept of hard and soft Lewis acids and bases—HSAB, for short. The HSAB concept
was introduced by Ralph Pearson over 50 years ago and was subsequently put on a firmer
theoretical foundation by Pearson and Parr, among others. Hard acids and bases are
relatively unpolarizable and have relatively high surface charge density, positive or
negative; soft acids and bases are relatively polarizable and have low surface charge
density, positive or negative. Of course, there are many borderline cases. High surface
charge density (hardness) typically results from a high formal charge (FC), positive or
negative, and small atomic/ionic size, and the opposite is true for low surface charge density
(softness). Examples of hard, borderline, and soft acids and bases are shown in Table 1.3.
The utility of the hardness/softness concept derives from the HSAB principle, which
states that soft bases react faster and form stronger bonds with soft acids, and hard bases
react faster and form stronger bonds with hard acids. A vast amount of chemistry can be
rationalized with this principle.
The HSAB concept greatly facilitates our appreciation of nucleophilicity: softer bases
often make better nucleophiles. Phosphines, for example, are typically better nucleophiles
than the analogous, harder amines, and sulfur compounds are better nucleophiles than their
oxygen analogs.
In this book, some of the best illustrations of the HSAB principle will be provided by
the so-called ligand exchange or metathesis reactions, which are discussed in more detail in
Section 1.19. The principle helps us in deciding whether a metathesis reaction will proceed
in a given direction or not:
AB + CD → AC + BD
(1.3)

TABLE 1.3

Qualitative Listing of Hard, Intermediate, and Soft Acids and Basesa
Acids

Bases

Hard

H+ , H–X, Li+ , Na+ , R3 SiX
Mg2+ , Ca2+ , AlX3 , SnCl4 , TiCl4

NH3 , RNH2
H2 O, HO− , ROH, RO− , RCO2 −
Cl− , F− , NO3 −

Intermediate

CuX2 , ZnX2 , SnX2 , GaX3 ,
R3 C+ , R3 B

Br− , NNN− (azide), ArNH2
pyridine

Soft

RCH2 X, RSX, RSeX, I2 , Br2 , BrF3 ,
CuX, Ag+ , Pd(X/R)2 , Pt(X/R)2 ,
Hg(X/R)2 , zero-valent metals

RSH, RS− , R2 S, RSe− , I− , R3 P,
NC− , CO, RCH=CHR,
benzene

a Where

warranted, the atom of interest is indicated in bold.

1.4 pK a VALUES: WHAT MAKES FOR A GOOD LEAVING GROUP?

9

Let us take a couple of concrete examples.
SeCl2 + 2 (CH3 )3 SiBr → SeBr2 + 2 (CH3 )3 SiCl

(1.4)

PCl3 + AsF3 → PF3 + AsCl3

(1.5)

In reaction 1.4, Se is a softer Lewis acid than Si, and bromide is a softer Lewis base than
chloride. It makes sense therefore that Se and Br should link up, as should Si and Cl. In
the second example (reaction 1.5), As is a softer Lewis acid center than P, and chloride is a
softer Lewis base than fluoride. These ligand exchanges are thus consistent with the HSAB
principle.

1.4

pK a VALUES: WHAT MAKES FOR A GOOD LEAVING GROUP?

Compared with the multitude of factors affecting nucleophilicity, the efficacy of a leaving
group is much more easily predictable. In short, a weaker Brønsted base makes a better
leaving group. We can simply look up the pKa of the conjugate acid of a leaving group
to arrive at a good idea of its leaving ability. Table 1.4, a short pKa table, will serve our
purposes very well.
Observe that the best leaving groups are conjugate bases of the strongest acids. Thus,
iodide and bromide are excellent and popular leaving groups in organic chemistry. The
worst leaving groups are very strong bases, such as amide, hydride, and alkyl anions.
Hydroxide and alkoxide (RO− ) are also poor leaving groups in organic chemistry. The
Williamson ether synthesis mentioned above (reaction 1.1) illustrates this last point well.
Like all elementary reactions, the reaction is in principle reversible, but the reverse reaction,
I− displacing a CH3 O− anion, does not occur for all intents and purposes.
A couple of additional observations are worth making, again with specific reference to
organic chemistry.

TABLE 1.4 Common Leaving Groups and the pKa Values of Their
Conjugate Acids

Good

Bad

Leaving Group

Conjugated Acid

pKa

I–
Br –
Cl –
HSO4 −
p-CH3 –C6 H4 –SO3 –
H2 O
F–
CH3 COO –
NH3
HO –
CH3 O –
NH2 –
H–
CH3 –CH2 –CH2 –CH2 –

HI
HBr
HCl
H2 SO4
p-CH3 –C6 H4 –SO3 H
H3 O+
HF
CH3 COOH
NH4 +
H2 O
CH3 OH
NH3
H2
CH3 –CH2 –CH2 –CH3

−10
−9
−8
−3
−3
−1.7
3.2
4.74
9.25
15.74
15.2
38
42
50

10

A COLLECTION OF BASIC CONCEPTS

Fluoride and cyanide are very much worse leaving groups than the pKa values of HF
and HCN would imply. This presumably reflects the great strength of the C–F and C–CN
bonds.
Sulfonates are better leaving groups than the pKa values of sulfonic acids suggest. Arenesulfonates (ArSO3 − ), especially p-toluenesulfonate (also known as tosylate, TsO− ), are
popular leaving groups in organic chemistry because alkyl tosylates may be readily prepared from the corresponding alcohols. The trifluoromethanesulfonate anion (also known
as triflate, TfO− ) leaves with even greater alacrity, and even better leaving sulfonate-based
leaving groups have been developed:
F

−
SO3

F

−
SO3

F

Tosylate

Triflate

A fact that many students struggle with on their first introduction to organic chemistry
is the following: the iodide ion is both an excellent nucleophile and an excellent leaving
group; by contrast, alkoxide ions (RO− ) are good nucleophiles but lousy leaving groups.
What accounts for the difference?
The solution to this conundrum is that, although both nucleophiles and leaving groups
are Lewis bases, very different factors control their efficacy. Iodide’s nucleophilicity is
attributed primarily to its polarizability or softness. The nucleophilicity of alkoxide ions
owes more to the hard–hard interaction between O− and C𝛿+ and the resulting strength of
the C–O bond.
On the other hand, there is a clear inverse correlation between the efficacy of leaving
groups and their Brønsted basicity. Thus, iodide is an excellent leaving group because it is
a very weak base. Alkoxide anions, being strong bases, are lousy leaving groups.
Protonation greatly enhances the efficacy of leaving groups. For example, the bromide
anion by itself (e.g., in the form of NaBr) does not react with an alcohol, OH− being a
notoriously poor leaving group in organic chemistry.
H

H

OH

C
H
Br

C
− H

C
H

H

H

H

H
+ HO

C

−

(1.6)

Br

H

Protonation of the OH group by concentrated HBr, however, enables the departure of water,
a far better leaving group, as shown below:
H
H

H
C

H
Br

+
O

H

H

H

C
− HH

C
H

H

H

H
+

C
Br

O

H

(1.7)

1.6 THERMODYNAMIC CONTROL: BOND DISSOCIATION ENERGIES (BDEs)

11

Concentrated HBr is therefore a suitable reagent for converting simple alcohols to the
corresponding alkyl bromides (assuming there are no other acid-sensitive groups in the
molecule).
We’d be remiss if we didn’t make some amends for presenting a rather “organic-centered”
view of leaving groups: a variety of other factors are at play when the electrophilic center
is not carbon. Perhaps the most important of these is the fact that single bonds between
electronegative elements are typically weak and are easily cleaved. Unlike in organic
chemistry, a hydroxide anion is a fair leaving group for a substrate of the form ROOH.
Similarly, although thiolates (RS− ) are hopelessly poor leaving groups in organic chemistry, rings of divalent sulfur atoms are readily broken down by nucleophiles, with S−
leaving groups as intermediates (as discussed in Section 6.4).
1.5

REDOX POTENTIALS

Broadly speaking, nucleophilicity correlates with reduction potential. Thus, stronger reducing agents tend to make better nucleophiles, which makes sense because both properties are
related to electron donation. The correlation, however, is best limited to a set of structurally
closely related nucleophiles. For a broader correlation, Edwards and Ritchie proposed an
“oxibase” scale, which afforded a linear correlation of the reactivity of a nucleophile with
the reduction potential of its oxidized form and the pKa of its conjugate acid. Although
space doesn’t permit a more detailed discussion of this scale, redox potentials are broadly
important for the subject matter of this book.
Table 1.5 lists reduction potentials of representative inorganic substances of potential
interest in this book. Observe that the half-reactions are all written as reductions, following
current convention, as well as to avoid ambiguity. In this convention, the more positive the
reduction potential, the more the reduction is favored thermodynamically. Strong oxidants
thus exhibit high (i.e., more positive) reduction potentials. A couple of examples should
illustrate the utility of this table.
A table such as Table 1.5 provides an indication of whether an oxidant or reductant is
suitable for a given redox role. Thus, with a high reduction potential (1.19 V in Table 1.5),
ClO2 is clearly a strong oxidant, which underlies its wide use as a disinfectant.
Observe that several of the reductions involve an oxidant (e.g., O2 , H2 O2 , NO3 − ) in acid
solution. This makes sense because protonation is expected to make an oxidant even more
powerful, that is, an even more avid acceptor of electrons.
Not much more needs to be said about redox potentials at this point. We will refer back
to Table 1.5 once in a while when we make arguments based on redox potentials.
1.6 THERMODYNAMIC CONTROL: BOND DISSOCIATION ENERGIES
(BDEs)
Many of the reactions discussed in this book occur under thermodynamic control. In other
words, the activation energies for the various pathways are low enough and the temperature
is high enough so that the products formed are thermodynamically the most stable ones
possible. In contrast, under kinetic control, reaction rates determine the products observed
and certain thermodynamically favored products may not predominate because their formation is too slow under the reaction conditions (particularly temperature). Reactions under

12

A COLLECTION OF BASIC CONCEPTS

TABLE 1.5 Standard Reduction Potentials (E∘ , V) at 25 ∘ C,
1.0 M, 1 atm.
E∘ (V)

Half-Reaction
F2 (g)+ 2e− → 2F− (aq)
(aq) + 2H+

+ 2e−

+2.87
→ Xe(g) + 2HF(aq)

+2.32

O3 (g) + 2H+ (aq) + 2e− → O2 (g) + H2 O(aq)

+2.07

XeF2

H2 O2 (aq) + 2H+ (aq) + 2e− → 2H2 O

+1.78

PbO2 (s) + 4H+ (aq) + SO4 2− (aq) + 2e− → PbSO4 (s) + 2H2 O

+1.70

HClO2 (aq) + 2H+ (aq) + 2e− → HOCl(aq) + H2 O

+1.67

2HOCl(aq) + 2H+ + 2e− → Cl2 (g) + 2H2 O

+1.63

Cl2

(g) + 2e−

−

→ 2Cl (aq)

+1.36

O2 (g) + 4H+ (aq) + 4e− → 2H2 O

+1.23

Tl3+ (aq) + 2e− → Tl+ (aq)

+1.23

ClO2

(g) + H+ (aq) + e−

→ HClO2 (aq)

+1.19

Br2 (aq) + 2e− → 2Br− (aq)

+1.09

NO3 − (aq) + 4H+ (aq) + 3e− → NO(g) + 2H2 O

+0.96

O2 (g) + 2H+ (aq) + 2e− → H2 O2 (aq)

+0.68

I2

(s) + 2e−

→

2I− (aq)

+0.54

O2 (g) + 2H2 O + 4e− → 4OH− (aq)
SO2

(g) + 4H+ (aq) + 4e−

→ S(s) + 2H2 O

SO4 2− (aq) + 4H+ + 2e− → SO2 (g) + 2H2 O
4+

(aq) + 2e−

+0.40
+0.40
+0.20

2+

+0.13

𝟐H+ (aq) + 𝟐e− → H𝟐 (g)

0.00

Sn

2+

Sn

(aq) + 2e−

→ Sn (aq)
→ Sn(s)

−0.14

PbSO4 (s) + 2e− → Pb(s) + SO4 2− (aq)

−0.31

2H2 O + 2e− → H2 (g) + 2OH− (aq)

−0.83

3+

Al

(aq) + 3e−

→ Al(s)

Be2+ (aq) + 2e− → Be(s)

−1.66
−1.85

Mg2+ (aq) + 2e− → Mg(s)

−2.37

Na+ (aq) + e− → Na(s)

−2.71

Ca2+ (aq) + 2e− → Ca(s)

−2.87

K+ (aq) + e− → K(s)

−2.93

Li

+

(aq) + e−

→ Li(s)

−3.05

thermodynamic control often lead to the formation of an overall stronger set of bonds; a
general idea of bond dissociation energies (BDEs) is therefore quite useful. Table 1.6 lists
some typical single BDEs.
A few examples of BDE considerations are as follows:
Bonds between highly electronegative elements are weak and easily broken, for example,
O–O, N–O, X–X, N–X, O–X, and so on, where X is a halogen.

1.6 THERMODYNAMIC CONTROL: BOND DISSOCIATION ENERGIES (BDEs)

13

TABLE 1.6 Typical Single Bond Dissociation Energies (kJ/mol, in black) and Bond Distances
(pm, in green) for Selected p-Block Elements

H
C
N
O
F

H

C

N

O

F

Si

P

S

Cl

Br

I

436
74

414
110

389
98

464
94

569
92

323
145

318
138

339
123

431
127

368
142

297
161

347
154

293
147

351
143

439
141

289
184

264
187

259
181

330
176

276
191

238
210

159
140

201
136

272
134

187

209
180

174

201
169

243
184

203

138
132

184
130

368
183

351
176

170

205
165

180

201
199

159
128

540
181

490
174

285
168

255
163

197
178

197

176
234

213
227

226
221

360
216

289
231

250

213
220

230
214

331
209

272
224

213
243

213
208

251
203

213
218

237

243
200

218
213

209
232

192
228

180
247

Si
P
S
Cl
Br
I

151
266

Carbon, silicon, and phosphorus form strong bonds with O and F. In addition, the C=O
(∼899 kJ/mol) and P+ –O− (∼544 kJ/mol) BDEs are very high and these bonds tend to
form easily under hydrolysis. These elements are thus said to be strongly oxophilic and
fluorophilic.
The HF bond is also extremely strong (BDE 569 kJ/mol), which in part explains why,
unlike the other hydrohalic acids, HF is a weak acid. Thus, a number of p-block element
fluorides react with proton sources to yield HF.
The bond distances listed in Table 1.6 do not warrant much comment. By all means,
browse them briefly; they are simply meant to give you a sense of the comparative dimensions of different bonds. Observe that there is no particular correlation between bond distances and BDEs. For bonds between a given pair of elements, however, a longer bond does
correspond to a lower BDE.
♦♦♦

Our discussion until now has centered around the SN 2 displacement. With some of the
key physical concepts in place, we are now in a good position to survey a number of other
fundamental mechanisms. These are arranged in somewhat arbitrary order as follows:
• The E2 mechanism (Section 1.7)
• Proton transfers (PTs) (Section 1.8)

14

A COLLECTION OF BASIC CONCEPTS

•
•
•
•
•
•
•
•
•
•
•
•
•

Associative and dissociative processes (Section 1.9)
SN 2-Si (Section 1.10)
Two-step mechanisms: the SN 1 and E1 pathways (Section 1.11)
Electrophilic addition to C–C multiple bonds (Section 1.12)
Electrophilic substitution on aromatics: Addition–elimination (Section 1.13)
Nucleophilic addition to C–heteroatom multiple bonds (Section 1.14)
Carbanions and ylides (Section 1.15)
Carbenes (Section 1.16)
Oxidative addition and reductive elimination (Section 1.17)
Migrations (Section 1.18)
Ligand exchanges (Section 1.19)
Radical reactions (Section 1.20)
Pericyclic reactions (Section 1.21)
♦♦♦

1.7

BIMOLECULAR 𝜷-ELIMINATION (E2)

A process that often competes with SN 2 displacements for organic systems is E2 (which
stands for “elimination, bimolecular”). When there is one or more hydrogens 𝛽 with respect
to the leaving group (see reaction 1.8) and the incoming nucleophile is a strong enough
base, bimolecular elimination occurs, often in competition with nucleophilic displacement,
as shown below:
Me
Me
Me

C

−
O
H

H
C
β

Me
Me

H

− Br

−

C
α

Me

Me
C

OH +

H
C

(1.8)

C

Me

Br

Me

Me

H

An important point is that, although an E2 reaction involves movement of three electron
pairs, it all happens as a concerted one-step process.
A sterically hindered carbon center, such as the tertiary carbon in a t-butyl halide, is
generally not conducive to an SN 2 displacement.
Me
X
Me
Me
t-Butyl halide

1.8 PROTON TRANSFERS (PTs)

15

An E2 elimination is often the favored pathway in such cases. Similarly, a strong,
sterically hindered base also tends to favor an E2 over an SN 2 pathway. A good example
is potassium t-butoxide, which is used in reaction 1.8. Examples of sterically hindered
nitrogen bases include N,N-diisopropylethylamine (DIPEA, also known as Hünig’s
base) and the much stronger lithium diisopropylamide (LDA). The pKa of the conjugate
acid of DIPEA is about 9.0. Amidines, which are all-nitrogen analogs of carboxylic
acids, tend to be significantly more basic than tertiary amines. Two moderately sterically hindered bicyclic amidines—1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and
1,5-diazabicyclo[4.3.0]non-5-ene (DBN)—are fairly widely used as bases promoting E2
eliminations.
N

N

N

N
DBN

DBU

The pKa values of the conjugate acids of DBU and DBN have been estimated to be about
2–3 pKa units higher than those of tertiary amines such as triethylamine or DIPEA.
We will conclude this section with a couple of interesting tidbits.
The first is a piece of information: DBU is not only a synthetic base but also a natural
product, an alkaloid isolated from the Caribbean (Cuban) marine sponge Niphates digitalis.
The second is a question for you to consider. Observe that DBU and DBN have two
nitrogens each. Which is the more basic nitrogen? Why?

1.8

PROTON TRANSFERS (PTs)

PTs between electronegative atoms are fast and reversible. They are enormously important,
not only in organic and inorganic chemistry but also in biology. In terms of arrow pushing,
PTs look very much like SN 2 displacements:
−
Nu

H

H + Nu

Nu

Nu

−

(1.9)

Sometimes the proton donor is obvious: in an aqueous solution it is typically the H3 O+ ion;
under nonaqueous conditions it may be a strong acid such as H2 SO4 or HCl. In such cases,
the proton donor may not be explicitly shown but be simply indicated as H+ :

Nu

−

+
H

Nu

H

(1.10)

Some important concrete examples of PTs are as follows: carbonyl and nitrile groups
undergo protonation:

16

A COLLECTION OF BASIC CONCEPTS

(a)

+
O

R1

H
O

R1

H

H
H

O

+

+

O
H

H

R2
R2

+
O

(b)

H
R

C

N

R

H
H

(1.11)

+
N

C

H +

O
H

H

The significance of these protonations is that the cations that result are much stronger electrophiles than the uncharged functional groups. In the same vein, alcohols and thiols may
be deprotonated, as shown below, and the resulting anions are much stronger nucleophiles
than their uncharged precursors:
(a)

(b)

−
H

O
R

H

R

H

−
OR

−
EtOH + SR

S

−
EtO

H +

H

(1.12)

Being fast and reversible, PTs are typically thermodynamically controlled. In other
words, for complete PT to occur, the proton donor must be a stronger acid than the protonated proton acceptor. Stated differently, a proton wants to be bonded to the strongest base
around. Tables of pKa values are thus well suited for predicting the direction of PT reactions.
1.9 ELEMENTARY ASSOCIATIVE AND DISSOCIATIVE PROCESSES
(A AND D)
Two other elementary polar reactions are worth mentioning right away. These are the polar
association (A) and dissociation (D) reactions. An A reaction is simply a bond formation
between a nucleophile (Nu or Nu− ) or Lewis base and an electrophile (E or E+ ) or Lewis
acid, and the product may be positively or negatively charged or neutral:
−
Nu

E

−
Nu

E

Nu

E

Nu

E

+

+

Nu

E

Nu

−
E

(1.13)

+
Nu

E

+
Nu

−
E

An A reaction is one of the most common elementary reactions (i.e., a one-step process) we will encounter in this book. Whenever you encounter a reaction of the type
“A + B → products,” there is a good chance that the first step is either an SN 2 or an A
reaction. Indeed, for elements below period 2 of the periodic table, A reactions are at least

17

1.9 ELEMENTARY ASSOCIATIVE AND DISSOCIATIVE PROCESSES (A AND D)

as common as SN 2 reactions. Several examples of A reactions involving various p-block
elements are shown below:
R1
+
O

(a)

−
C

−

B
R3

R2

Si
Cl

H
(c)

Pb
AcO

H
−
F
(d)

Cl

Cl
Cl

−

+O
H
AcO

− F
F F

F

Pb

OAc
2−
OAc

(1.14)

− F
Sb
F
F

F

F
I

H

F

F

F

(e)

+
O

OAc

F

F

Cl

Si

Cl

OAc
OAc

Sb

R2
R3

Cl

F
F

B

Cl

OAc

O
O

+
O

C

Cl

−
Cl

(b)

R1

F

F

−
I

F
F

F

F
F
F

F F

F

A D reaction is the reverse of an A reaction; in other words, a D reaction consists of
the heterolytic cleavage of a molecule or ion into a nucleophile–electrophile pair, as shown
below:
Nu

E

Nu

Nu

−
E

Nu

−

−

+ E

+

+ E

+
Nu

E

Nu

+ E

+
Nu

−
E

Nu

+ E

+

(1.15)

A classic example of a D reaction is the ionization of t-butyl tosylate in a polar solvent:
Me

Me

+
OTs

Me
Me

Me

−
Me + OTs

(1.16)

18

A COLLECTION OF BASIC CONCEPTS

A more “inorganic” example might be the ionization of Martin sulfurane, a rather fancy
tetravalent-sulfur-based dehydrating agent (discussed in Section 6.13):
RF
O
RF

Ph

S

O

Ph

−

+

+

S

Ph
Ph

(1.17)

O

O
RF

RF

A very common pathway for main-group elements is an A–D sequence. Thus, a nucleophile and an electrophile come together to form a complex which then falls apart to a
different nucleophile–electrophile pair. The following fluoride ion transfer,
ClF3 + AsF5 → [ClF2 ]+ [AsF6 ] –

(1.18)

a reaction typical of halogen fluorides, is a good example of the two-step process:

F

F

F

F

Cl

Cl

+F

F

F
F

As

F

A

F

F

F
F

F

F

− F
As
F
F

D

Cl +

+

F
F

F

− F
As
F
F

(1.19)
The D–A sequence is also fairly common. Possibly the best known example of such a
sequence is the SN 1 pathway of organic chemistry, briefly described in the next section.
A fascinating situation arises when a Lewis base and a Lewis acid are too sterically
encumbered to form a bond with each other, as in the example below:
F
F

F

F

F
F

t-Bu
P
t-Bu
t-Bu

+

F

F

B

F

F F
F

F

F
F

No reaction

(1.20)

19

1.10 TWO-STEP IONIC MECHANISMS: THE SN 2-Si PATHWAY

Very recently, such “frustrated Lewis acid–base pairs (FLPs),” as they are called,
have been found to exhibit unique reactivity, including activation of molecular hydrogen
(see Section 2.6 for additional details). The following example shows associative (A)
reactions between a P/B-based FLP and CO2 .
F

F

F

F

F

F

F
CO2

B

P

F
F

F

+ P

B −

− CO2

F

F

O
O

F
F

F

F

F

F
F

F

(1.21)
The mechanism presumably consists of two successive association processes, as shown
below:
Ar
Ar
O

P
C

B
O

Ar′
Ar′

Ar
Ar
O

Ar′
Ar′

−B

P
C

O

+

(1.22)
Ar
Ar

P +
C

−B

Ar′
Ar′

O

O

Sulfur dioxide also reacts in much the same way with a P,B-based FLP. Feel free to sketch
it out.

1.10

TWO-STEP IONIC MECHANISMS: THE SN 2-Si PATHWAY

Unlike in organic chemistry, nucleophilic attack on a main-group center with a noble gas
configuration does not necessarily lead to immediate bond breakage. This is because of
the tendency of p-block elements in period 3 and below to form hypervalent molecules
(“expanded octets”). In other words, direct SN 2 displacements (such as the Williamson
ether synthesis in reaction 1.1) are far from being the norm for heavier main-group centers.
Instead, an incoming nucleophile often adds first, and the leaving group departs only in
the next step—in essence, an A–D sequence, to use the terminology of the last section.

20

A COLLECTION OF BASIC CONCEPTS

Silylation of an alcohol, a common method for OH group protection in organic chemistry,
typically involves such a mechanism:
ROH + Me3SiCl

Pyridine

(1.23)

ROSiMe3

The first step is thus an addition:
H

H

Me
O

Si

Cl

Me

R

Me

+O
R Me

Me

Si

−

(1.24)

Cl

Me

The chloride leaves only in a second or later step, as shown below:

N
H

Me

Me

+O

Si

−

Cl

−
− Cl

+
NH + O

R Me
Me

Si

(1.25)
Me
Me

R

The overall two-step or multistep mechanism is often called SN 2-Si, as this process has been
particularly well explored for Si-containing molecules. In this book, we often assume that
the SN 2-Si mechanism is operative, especially for higher-valent p-block compounds.
1.11

TWO-STEP IONIC MECHANISMS: THE SN 1 AND E1 PATHWAYS

Certain carbon–heteroatom bonds may ionize, especially in polar protic media (sometimes
referred to as solvolytic conditions), to generate carbocations. This is shown below for
t-butyl tosylate in acetic acid:
H3C
H3C
H
H

C
C
H

OTs

H3 C

−
− OTs
H
H

+
C

CH3

(1.26)

C
H

This rate-determining D step forms the first step of both the SN 1 and E1 pathways. In SN 1
(substitution, nucleophilic, unimolecular), the carbocation combines with a nucleophile,
often derived from the solvent. In E1, a base, again often derived from the solvent, deprotonates the carbocation and generates an alkene. In this case, the nucleophile or base is the
acetate anion (AcO− ):

1.11 TWO-STEP IONIC MECHANISMS: THE SN 1 AND E1 PATHWAYS

H3C
SN1
H3C

H
H

+
C

H
H

−
OAc

−

C

H3C
H3C

CH3

OAc

C
OAc

H
H

H

CH3

C
H

H3C

C
−
OAc

H

+
C

21

E1

H

(1.27)

+
C

− HOAc

C

H

H3C

CH3

C
H

H
−

CH3

C
H

OAc

Observe that an SN 1 reaction consists of a two-step D–A sequence, whereas an E1 reaction
is a D–PT sequence. Because the carbocation carbon is planar (sp2 ), the A reaction with
the nucleophile (in this case, AcO− ) can occur via both the front and back sides. Unlike an
SN 2 reaction, therefore, an SN 1 reaction is not stereospecific.
We will encounter carbocations again several times in this book, so this is a good
place for a brief refresher on carbocation stability. Carbocations are described variously as
methyl, primary (1∘ ), secondary (2∘ ), or tertiary (3∘ ), depending on the number of alkyl
(or aryl) groups attached to the cationic center. The stability of these cations increases in
the following order (3∘ most stable):
H
H

+
C

H

H

H

H
C +

C

<

C

H
+
C

<

H

H

H

H
H

H
H
H

C

H

H

<

C
H

2°

H

+

C

H

1°

H
C
H

C
H

H
3°

This stability order is typically explained in terms of hyperconjugation, which refers to the
overlap of neighboring C–H bonds with the empty p orbital at the cationic center, as shown
below:
+
H

CH3
H
H3C

H

The phenomenon is also often depicted in terms of “no-bond” resonance structures:
+

H
C
H
H3C

+
C

H
CH3
H

C
H
H3C

C

CH3
H

22

A COLLECTION OF BASIC CONCEPTS

The same phenomenon also explains the enhanced stability of 𝛽-silyl- and 𝛽-stannylsubstituted carbocations:
+
Si

Si
C

+
C

C

C

Lone-pair-bearing heteroatoms strongly stabilize carbocations via resonance or 𝜋-overlap,
as shown below:
R1
C
H

+

R2

R2

C

O

H

+

R1

R1

O +

CH
H

R2

O

1.12 ELECTROPHILIC ADDITION TO CARBON–CARBON MULTIPLE
BONDS
From an inorganic perspective, this is not an important class of reactions for the simple
reason that multiple bonds, particularly homonuclear multiple bonds, are relatively rare in
inorganic chemistry. Nevertheless, in the interest of a relatively complete picture of polar
mechanisms, a brief summary of these reactions seems appropriate.
Consider the addition of an acid HX to an alkene such as 2-methylpropene. The first step
involves the creation of a carbocation; note that a 3∘ carbocation is preferentially formed.
The carbocation then reacts with X− (in an A reaction) to generate the final product. The
overall process is essentially the E1 reaction run in reverse:
CH3

−
X
jor

CH3

H
H

C
H3C

C

X

Ma

Mi

C

CH3

+
C

H3C

H
H

X
H3C

C
C

H
H

H
3° Carbocation

H
More substituted C–X product

H

H

no

r

H
H3C
H3C

C

+
C
H

1° Carbocation

H

H3C
−
X

H3C

C
C

H
X

H
Less substituted C–X product

(1.28)
The regioselectivity of the reaction, namely, the fact that the more substituted addition product forms preferentially, is commonly referred to as Markovnikov’s rule.

23

1.13 ELECTROPHILIC SUBSTITUTION ON AROMATICS: ADDITION–ELIMINATION

Addition of halogen molecules to double bonds is often stereospecific, and the two
halogen atoms typically add to opposite faces of the double bond (so-called anti addition).
This is explained by the intermediacy of cyclic halonium ions, as shown below:
Br
+
Br

Br
H3C
C
H

C

CH3
Anti addition

H3C
H

H

Z

C

C

Br

CH3

C

H

−
Br

H3C

CH3
H3C
H
H
+
C
C
Br

H

+
Br

Br
H
C
E

H3C

Anti addition

C

H

H

CH3

Racemic

Br
H3C

Br

Br
C

H

CH3

C

H

Br

H
C

−
Br

C
CH3

H3C

H

Br

CH3

C

H3C

Br

Br
C

H

C
H

CH3

Meso

(1.29)
Recall that a racemic substance is an equal mixture of two enantiomers of a chiral molecule.
A meso compound, on the other hand, has a symmetry element such as a mirror plane or
an inversion center that prevents it from being chiral. Analysis of product stereochemistry
thus played a crucial role in establishing these mechanisms.
In other cases, addition takes place across a given face of a carbon–carbon double bond
(so-called syn addition). Epoxidation by a peroxyacid is a good example:
O

O

+

− RCOOH

O
R

H3C

H

O

CH3
H

H

Z
Meso
O
+

O
R

O

O
− RCOOH

O

H

H3C

H

H + H
H3C
CH3

CH3
H

E
Racemic

(1.30)
Once again, product stereochemistry provides key clues to the mechanism. Another important example of syn addition is osmium tetroxide-mediated cis-dihydroxylation of alkenes.
Because of their limited relevance in this book, however, we won’t discuss the mechanistic
details of these reactions, but do consult your organic text if you are curious.

1.13 ELECTROPHILIC SUBSTITUTION ON AROMATICS:
ADDITION–ELIMINATION
Electrophiles may add to aromatics, generating transient cationic intermediates. The driving
force to regain aromaticity, however, is typically too strong for subsequent addition of a

24

A COLLECTION OF BASIC CONCEPTS

nucleophile. Instead, a proton is eliminated, resulting in a substituted aromatic as the final
product. The overall two-step process is depicted below for the nitration of benzene, with
the nitronium ion (NO2 + ) as the electrophile:
O

O

N+

+

H

O
N +

O

O

−H

−

N

+

+
O

−

(1.31)
Qualitatively similar mechanisms may be written for a host of other electrophilic substitutions such as sulfonation, halogenation, Friedel–Crafts alkylation and acylation, and
thallation. Weaker electrophiles such as molecular halogens and alkyl halides may need
activation by a Lewis acid such as AlCl3 , as shown below, a point we will discuss in
Section 3.1:
R

RCl, AlCl3

(1.32)

1.14 NUCLEOPHILIC ADDITION TO CARBON–HETEROATOM MULTIPLE
BONDS
In this section, we will focus primarily on nucleophilic additions to carbonyl groups. The
carbonyl substrate may be an aldehyde or ketone, as well as various carboxylic acid derivatives such acid halides and esters. Among the variety of nucleophiles that can participate
in these reactions are hydride, hydroxide, alkoxide, and a variety of carbon-based nucleophiles. For carbonyl substrates, attack by a nucleophile typically results in an opening up
of the C–O 𝜋-bond, leading to a tetrahedral intermediate, as shown below for the addition
of cyanide to a ketone in the presence of water.
−
C

N

N
R

1

C
C

O

C
R1

R2

−
O

(1.33)

R2

The anionic alkoxide intermediate picks up a proton from water to generate the “cyanohydrin” product:
N

N
C
C

R1

R2

−
O

O
H

H

C
C
R1

R2

−
OH + HO

(1.34)

25

1.14 NUCLEOPHILIC ADDITION TO CARBON–HETEROATOM MULTIPLE BONDS

A similar reaction may also be accomplished under nonaqueous conditions with
trimethylsilyl cyanide. Though exceedingly unstable by themselves, trimethylsilyl cations
in complexed form may be profitably viewed as “fat protons” and are excellent Lewis
acids:
N
R

1

O + Me3Si

C

C

N

SiMe3

C

OTf
Me3Si
(cat.)

C
R1

R2

(1.35)

O

R2

The interaction between the carbonyl compound and the catalyst, trimethylsilyl triflate, may
be envisioned as an A–D sequence, as shown below:
R1
C

−
Me3Si

R1

OTf

Me3Si

C

O

OTf

R1

O
+

C

R2

R2

SiMe3
O

+

+

−
OTf

R2

(1.36)
A second A–D sequence involving the triflate anion and trimethylsilyl cyanide may then
produce free cyanide:
N
−
TfO

N

C

C

Me3Si

TfO

TfO

−
SiMe3

SiMe3 +

−

N

(1.37)

C

The cyanide would then attack the activated carbonyl group to yield a silylated cyanohydrin
as the final product, as shown below:
N

−
C

N
R1
C
R2

+
O

SiMe3

SiMe3

C
C
R1

(1.38)

O

R2

It’s also conceivable that the actual nucleophile is the five-coordinate species
[Me3 Si(CN)(OTf)]− , as opposed to free cyanide.
Ester hydrolysis, an important organic reaction that is also of great biological importance, may be either acid- or base-catalyzed. The base-mediated process may be represented
as follows:
R1COOR2 + NaOH
R1COONa + R2OH
(1.39)
The various oxygens have been colored so you may track how they start from a given starting material and end up in a given product. In practice, the color corresponds to the possible

26

A COLLECTION OF BASIC CONCEPTS

use of an isotopic label, typically 18 O. Thus, use of Na18 OH/H2 18 O leads to the incorporation one atom of 18 O into the carboxylic acid product, suggesting that ester cleavage occurs
across the R1 CO–OR2 bond, as shown below:
−
O

O
R2

C
R1

R1
HO

O
−
OH

O
R2

C

R1

O

R

1

−
O

H

C

+

C

O

OH

−
O

R2

O

R2

C

O

R1

−
O

+

R2
O
H

(1.40)
18 OH−

R2

simply done an SN 2 attack on the
group, the label would have been
Had the
found in the alcohol product. The mechanism shown above is by far the most common
mode of ester cleavage, although other modes of cleavage have also been observed for
certain substrates and under certain reaction conditions.
An important variant of nucleophilic additions to carbonyl compounds is the “conjugate
addition” of a nucleophile to the 𝛽-position of an 𝛼,𝛽-unsaturated carbonyl compound. The
“enolate” so produced is then protonated, producing a 𝛽-substituted carbonyl compound as
the final product.
−
Nu
R

O
β

α

−
O

Nu

R

R

β

R

R

α
H

A

O

Nu

−
−A

β

α

R

(1.41)

H

The most important example of such a conjugate addition is the Michael reaction or Michael
addition, which you may remember from your organic course; we also briefly discuss it in
the next section.

1.15

CARBANIONS AND RELATED SYNTHETIC INTERMEDIATES

In this section, we’ll take a break from our survey of reaction mechanisms and focus instead
on a class of intermediates, namely, carbanions. We will also discuss carbanion cognates
such as enols, enolates, enamines, and ylides. As classic nucleophiles, carbanions react in
highly characteristic ways, particularly via SN 2 displacements, as well as via other pathways
(e.g., carbonyl addition and conjugate addition) we have discussed above. The material in
this section will thus help you flesh out your understanding of what we have discussed
so far.

1.15 CARBANIONS AND RELATED SYNTHETIC INTERMEDIATES

27

The best known among unstabilized carbanionic derivatives are the Grignard reagents
(organomagnesium compounds) and organolithiums:
RMgBr

RLi

Alkylsodiums and alkylpotassiums are considerably more reactive and are less commonly
used. Of particular importance are stabilized carbanions derived from deprotonation of C–H
bonds adjacent to carbonyl groups; such anions are often referred to as enolates because the
negative charge is localized more on the carbonyl oxygen than on the carbanionic carbon:

−
H2C

O

−
O

C

C
R

(1.42)
R

H2C

A strong base such as an alkali metal amide (pKa ∼35–40) is typically required to quantitatively convert a simple aldehyde or ketone to an enolate. On the other hand, an alkoxide base
(pKa ∼ 16) is sufficient for deprotonating a 𝛽-diketone or a 𝛽-ketoester, since the resulting
enolate is much more stabilized by resonance:
O

O

O

−

O

O

O

−

−

(1.43)
Stabilized carbanions may also be derived from nitriles:
H3C
− C

H 3C
C

N

C

H

C

−
N

(1.44)

H

Enamines are an important class of uncharged synthetic intermediates that exhibit
carbanion-like reactivity. They are typically prepared from ketones with one or more
𝛼-hydrogens and a secondary amine under acid catalysis:
O
N
H

+
,

− H2O

H

N

(1.45)

For reasons of space, we won’t go through the mechanism of this reaction, but do look it
up in your organic text, if you wish to.

28

A COLLECTION OF BASIC CONCEPTS

Ylides are an important class of carbanion analogs, which we will encounter several
times in this book. Generally, they are not anionic, but 1,2-dipolar compounds in which a
carbanionic (or other anionic) center is stabilized by an adjacent cationic p-block center,
where both centers have full octets of electrons. The best known ylides are phosphonium
and sulfonium ylides, the following being prototypical examples:
−
CH2
P

+

−
H2C

Ph
Ph
Ph
Triphenylphosphonium
methylide

+
S

Me
Me

Dimethylsulfonium
methylide

Carbanions and related intermediates react with a variety of carbon electrophiles, such
as alkyl halides and carbonyl compounds, forming carbon–carbon bonds. In this capacity,
these intermediates are a cornerstone of organic synthesis. A classic example of a carbanion
reaction is the aldol reaction. In this, an enolate reacts with a carbonyl compound to yield
a 𝛽-hydroxycarbonyl compound, as shown in the example below:
O

O

−

LiNR2
− NHR2

R1
O
−
O

O

R

−

R1
Enolate

O

H3O

2

R2 − H2O

R1

R1

O

+

HO

R1

α

β

R2

β-Hydroxycarbonyl

(1.46)
The conjugate addition of an enolate to an 𝛼,𝛽-unsaturated carbonyl compound is called
the Michael reaction or Michael addition. A good example is the following, where an enolate derived from diethyl malonate reacts with methyl vinyl ketone.
O
O

O
EtO

O

EtONa (cat.)

+

EtOH

EtO

(1.47)

EtO

Methyl vinyl ketone
O
Diethyl malonate

EtO

O

We won’t go through the mechanism of this reaction, since it’s essentially the same as
that depicted for a generic conjugate addition in Section 1.14.

1.16 CARBENES

1.16

29

CARBENES

Divalent carbon species, or carbenes, are another classic group of organic intermediates.
Not only are they important in organic chemistry but they are also isoelectronic with other
divalent group 14 molecules (based on Ge, Sn, and Pb) and hence of considerable relevance to our further discussions. In addition, nitrogen-stabilized, so-called N-heterocyclic
carbenes (NHCs) have gained enormous popularity as transition-metal ligands; many of the
resulting complexes mediate unique transformations that define a frontier area of contemporary chemistry.
Carbenes occur in one of two low energy states, triplet or singlet. These terms refer to the
spin multiplicity of the molecule. The divalent carbon in a triplet carbene has two unpaired
electrons, whereas in a singlet carbene the two electrons are paired. Lone-pair-bearing
𝛼-heteroatom substituents strongly stabilize the singlet state, as shown by the 𝜋-type orbital
interaction below:
Cl
H

C

C

H

Cl

CH2 (triplet)

CCl2 (singlet)
Cl

Cl
C

+

Cl

(1.48)
+

Cl

−

−

C

C

Cl

Cl

Two heteroatom substituents can result in highly stable “bottle-able” singlet carbenes, two
examples of which are shown below:

N

N

N

N
R

R

R

R

Additional ionic resonance structures, which can also be drawn for these two carbenes (feel
free to draw them out), provide a rationale for their stability.
Carbenes may be accessed via a variety of routes, of which three common ones are
shown below:
O

O

R2
Δ

(a)
R1

N

+
N−

− N2

R2
R1

30

A COLLECTION OF BASIC CONCEPTS

H
C

(b)
Cl

−
C

Base
Cl
Cl

Cl

− Cl

−
C

Cl
Cl

Cl

Cl

(1.49)
(c)

R

N

N

+

NaH
− H2

R

R

N

N

R

H

Observe that the last of these reactions (1.49c) results in the formation of an NHC.
A highly characteristic reaction of carbenes is insertion, whereby a carbene inserts itself
into a C–H or C–C bond, as shown below:
H

H

H

H

CH2

H

(1.50)
H

Carbenes also add to double bonds, forming three-membered rings. We’ll have more to say
about the reactions of carbenes, particularly carbene analogs, in Chapter 4.

1.17

OXIDATIVE ADDITIONS AND REDUCTIVE ELIMINATIONS

We finally come to a pair of reactions that may be described as typically “inorganic”—
oxidative addition and reductive elimination. The two processes are the reverse of each
other:
X

Oxidative
addition

E +

X
E

Y

Reductive
elimination

(1.51)
Y

Observe that oxidative addition results in an increase in the valence of the element E by
two units, while reductive elimination results in the reverse. Thus, oxidative addition is a
1,1-addition, that is, addition of two groups to the same atom. Typical organic additions, by
contrast, are 1,2-additions or, less commonly, 1,4-additions. It’s in this sense that oxidative
addition and reductive elimination are characteristically inorganic processes.
The two processes are common for transition metals and have been studied in considerable depth for many organometallic systems. They are also important for p-block
elements. Most p-block elements exhibit multiple valence states, with the valence differing
by two units, which makes them suitable candidates for oxidative addition and reductive
elimination.

1.17 OXIDATIVE ADDITIONS AND REDUCTIVE ELIMINATIONS

31

Quite a variety of mechanisms are possible for the two processes, including radical
pathways. We’ll focus here, however, on just a couple of polar mechanisms. A concerted,
one-step mechanism may be written as follows:
X

X
E

E

(1.52)

Y
Y

Another common pathway involves a two-step polar addition. As written below, the overall
process may be viewed as a tandem SN 2–A process:

X

E

Y

SN2
−
Y

X

+
E

A

X

E

(1.53)
Y

The following chlorination reactions are good examples of oxidative addition involving
main-group elements:
PCl3 + Cl2 → PCl5

(1.54)

ICl + Cl2 → ICl3

(1.55)

As in the case of many main-group reactions, the mechanisms have not been investigated for
these two reactions. Given that Cl2 is a good electrophile, either of the above mechanisms
seems quite reasonable.
For reductive elimination, we may envision the reverse of the above two pathways. A
concerted, one-step reductive elimination may be depicted as follows:
X
E +

E

X

(1.56)
Y

Y

Alternatively, we may envision a D–SN 2 sequence:
X
D

E
Y

+
E

−
Y
X

SN 2

E + X

Y

(1.57)

32

A COLLECTION OF BASIC CONCEPTS

Good examples of reductive eliminations involving main-group elements include the following:
Cl
Cl

As

Cl
Cl

Cl

As + Cl2

Cl
Cl

Cl

(1.58)

I
Tl

+

I

Ar

Ar

Tl

I

I

1.18

MIGRATIONS

Migrations are a somewhat quirky process in which an atom or a group tears itself from
its site of origin and jumps to a neighboring atom along with its pair of bonding electrons.
Thus, typically, the migrating group acts as a nucleophile and the migration terminus as
an electrophile. The process is typically intramolecular, that is, occurring within a single
covalently bound entity.
Some of the best studied examples of migration are provided by carbocation rearrangements. Generally, less stable cations (e.g., 2∘ ) rearrange to more stable ones (3∘ ) by undergoing hydride or alkyl shifts, as shown in the example below:
H

CH3

C
H3C
H3C

+

H3C
C

C
H

H3C

+

CH3
C
H

(1.59)
H

Indeed, observation of rearranged products is often seen as “proof ” of the involvement of
carbocations and hence of an SN 1/E1-type reaction pathway; concerted SN 2/E2 pathways
are not expected to result in a rearrangement.
A typical migration involved in a carbocation rearrangement is called a 1,2-shift because
the origin and terminus of the migration are (typically, but not invariably) adjacent to each
other. We will encounter a few examples 1,2-shifts involving main-group elements. A common reaction pattern is the following:
R
M
−

M

E

E

R +L

−

(1.60)

L

The migrating group R is often an alkyl or aryl group; the E–L bond is generally weak,
which makes L a good leaving group and E a good migration terminus. The negative charge
on the migration origin M enhances the migratory aptitude of the R group. Typically, M is a
relatively electropositive p-block element such as boron or silicon, or a higher-valent state
of an electronegative element such as a halogen. A good example of this type of a 1,2-shift
is provided by hydroperoxide-mediated oxidation of organoboranes.

1.19 LIGAND EXCHANGE REACTIONS

33

A standard method of work-up for organoboranes obtained with the hydroboration reaction involves reaction with alkaline hydrogen peroxide. Mechanistically, the first step is an
A reaction between the organoborane and a hydroperoxide anion that is present in solution:

H

R

R

−
O

O

R

B

O

B

(1.61)

R
R

O

H

R

−

The anionic boron center then acts as a launchpad for a migrating R group:
R
O
H

B

R

−
− OH

−

B

R
R

O

RO

HOO

−

HOO

OR

−

R

(1.62)

B
RO

OR

Note that the leaving group here is hydroxide, normally a lousy one in organic chemistry.
Here, however, the electrophilic site is an oxygen and an O–O bond is a weak one that
is easily cleaved. We will encounter several examples of similar 1,2-shifts as we progress
through the book.

1.19

LIGAND EXCHANGE REACTIONS

Main-group element chemistry is replete with ligand exchange reactions or metatheses. We
will encounter a fair number of such reactions in this book, a few examples being as follows:
SnCl4 + SnR4 → 2 SnR2 Cl2

(1.63)

SO2 + PCl5 → SOCl2 + POCl3

(1.64)

2 (CH3 )3 SiBr + SeCl2 → 2 (CH3 )3 SiCl + SeBr2

(1.65)

IF7 + POF3 → IOF5 + PF5

(1.66)

For a mechanistic discussion, we will choose the last reaction. A reaction pathway may
not be obvious at this point, based on the mechanistic paradigms we have discussed so far.
Clearly, a series of simple D and A reactions, whereby oxide and fluoride ligands detach
from one central atom and reattach to another one, are unreasonable, given the covalent
nature of the molecules involved. An A reaction, however, is still a promising starting point:
F
F

F

F
+
P

−
O

F
I

F

F
F

F
F
F
O

−
I

F
F
F
F

F
+P

F
F

F
F

(1.67)

34

A COLLECTION OF BASIC CONCEPTS

The oxo-bridged P+ –O–I− intermediate may now react in a number of different ways. A
fluoride could depart from the anionic iodine and reattach to the cationic phosphorus center,
as shown below:
F
F
F
O

F

−
I

F
F
F

+P
F

F

F
+
P

F

F
F

−
F

I

O
F
F

F

F

I

O

F

F
P

F

F

F

F

F
F

(1.68)

F

F

F

F

A second fluoride could then attack the neutral phosphorus center, leading to the final products PF5 and IOF5 :
F

F
O
F
−
F

F

F

I
F

P

F
F

F

O

F
P
F −

F

F

F

F

F

I

F
F

F

F

(1.69)

F
F
P

F

+

F

−
O

F
I

F

F

−
+ F

F

F

F

+

F

Other pathways are also conceivable for the initially formed oxo-bridged pathway.
For example, a 1,3-shift of a fluoride provides “quick” access to the neutral intermediate
F4 P–O–IF6 :
F
F
F
O
+ P
F

−
I

F

F

F
F
F

F

F

O

F
F

F

F

I
F

P

F

(1.70)

F
F

F

A second fluoride 1,3-shift, along with cleavage of the I–O bond, could then lead to the
final observed products.
F

F
F
O
F

F

I

P
F

F

F

F
F

P

F
F

F
F
F

F

+

F

−
O

I

+ F
F

F

(1.71)

1.20 RADICAL REACTIONS

35

At present, we do not know whether fluoride 1,3-shifts provide a low energy pathway or
not. Therefore, we cannot state categorically which of the above two pathways, or for that
matter a different one, is the one that operates in reality. Note that both pathways require
the formation of an oxo-bridged intermediate. That appears to be a general feature of ligand
exchange reactions of this type. When dealing with such reactions, simply join up the two
reactants via a lone pair on one of the migrating groups; subsequent D and A reactions, or
ligand 1,3-shifts, would then lead to the final products.
A word is in order on the thermodynamic driving forces underlying the above reaction.
Relief of steric strain at the 7-coordinate iodine is a possible factor, but the main driving
force is undoubtedly the formation of two highly stable P–F bonds, whose combined BDEs
(∼490 kJ/mol each) more than outweigh that of one P+ –O− unit (∼544 kJ/mol).

1.20

RADICAL REACTIONS

Although our focus is clearly on polar or ionic mechanisms, we should not and will not
ignore radical pathways altogether. A very brief introduction is therefore provided here.
Observe that, in the discussion below, single-headed fishhook arrows indicate “movement”
of unpaired electrons.
Radicals are typically produced by thermal or photochemical homolytic cleavage of a
weak single bond:
Homolysis:

A

(1.72)

+ B

A

B

Homolysis refers to the separation of a bonding electron pair into two unpaired electrons,
that is, radicals. Heterolytic mechanisms, by contrast, are characterized by a bond-breaking
step where the electron pair constituting the bond leaves with one of the fragments, as shown
below:
A

B

A

Heterolysis:
A

A

B

+

−

+ B
+ B

−

+

(1.73)

The term “heterolytic mechanism” is thus more or less synonymous with a polar or ionic
mechanism. Some classic radical-generating reactions are as follows:
(a)

Br

Br + Br

Br
R

(b)

O

O

2 RO

(1.74)

R
R
(c)

N
R

N

2R

+ N

N

36

A COLLECTION OF BASIC CONCEPTS

Let us work our way through a radical chain reaction. A good example is the photochemical chlorination of alkanes, shown below for methane:
Cl2, hν

CH4

CH3Cl

− HCl
Cl2

Cl2

(1.75)

Cl2

CHCl3

− HCl

CH2Cl2

− HCl

CCl4

− HCl

The reaction begins with the light-induced splitting of molecular chlorine into chlorine
atoms; this is called the initiation step:
Cl

hν

Cl

+ Cl

Cl

(1.76)

A chlorine atom can then abstract a hydrogen from methane:

+

C

H

C
H

H

Cl

H

H

H

Cl

(1.77)

H

H

The methyl radical then abstracts a chlorine atom from molecular chlorine, forming
chloromethane and another chlorine atom, thus perpetuating the presence of chlorine
radicals in the system.
H

Cl

Cl

H

C

Cl + Cl

C
H

H

H

(1.78)

H

The above two steps are collectively referred to as propagation steps. A methyl radical and
a chlorine atom may also combine to form chloromethane, as shown below, in what is called
a termination step:
Cl

H

H

C
H

(1.79)

Cl

C
H

H

H

Depending on the supply of molecular chlorine, the entire process may continue, leading
to further chlorination of chloromethane, ultimately leading to carbon tetrachloride:
Cl
Cl

H
C
H

H

C

Cl
Cl

Cl

Cl

(1.80)

1.21 PERICYCLIC REACTIONS

37

Although much less commonly used in chemical synthesis relative to polar reactions,
radical reactions nonetheless form a distinct “genre” of synthetic reactions. Creatively
orchestrated, they can lead to a variety of complex structures with a surprising degree of
efficiency.

1.21

PERICYCLIC REACTIONS

Pericyclic reactions, most notably the Diels–Alder reaction, other cycloadditions, and certain sigmatropic rearrangements in which two or more electron pairs move in a more or less
concerted manner along a cyclic pathway are a cornerstone of organic synthesis. Much of
their importance derives from the efficiency with which they create two or more bonds in
one step and also in a stereospecific manner. Some examples are as follows:

Y

Y

Z

Z

Diels–Alder:

Diene Dienophile
Y

Y
Ene:

Z

Z
H
Ene Enophile
U
1,3-Dipolar
addition:

U
Y

+ V
W

(1.81)

H

−

Z

V
W

Y
Z

Dipole Dipolarophile

Pericyclic reactions provide some of the most elegant examples of the importance of
orbital symmetry in chemical reactions. Unlike in organic chemistry, however, pericyclic
reactions are not of great importance in inorganic chemistry. That said, we will encounter
a few significant examples in this book, including the reduction of carbon–carbon
double bonds by diimide (Section 5.7a) and certain selenium dioxide oxidations
(Section 6.16).
♦♦♦

That concludes our survey of the major reaction types that we are likely to encounter
in this book. We are therefore in a position now to think in somewhat more general terms
about arrow pushing. This we do in the next two sections.
♦♦♦

38

A COLLECTION OF BASIC CONCEPTS

1.22

ARROW PUSHING: ORGANIC PARADIGMS

Organic chemists have codified a number of rules and guidelines as aids to arrow pushing.
We’ll summarize a few of them here as a starting point for exploring the more diverse world
of inorganic mechanisms.
1. Stable molecules, the final products of reactions, typically have noble gas configurations on all atoms, that is, octets on all atoms B through F.
2. Nucleophilic attack on a center with an inert gas configuration (an octet) must lead to
bond breakage, that is, departure of a leaving group.
3. Stable organic molecules strive toward charge neutrality for all atoms, although
charged species such as carbocations may arise as intermediates. Like charges on
two adjacent atoms are a taboo in organic chemistry!
4. Good leaving groups are relatively nonnucleophilic and nonbasic. Thus, N2 is a superb
leaving group, and water (H2 O) and alcohols (ROH) are very good leaving groups as
well. By contrast, OH− and RO− are lousy leaving groups and H− (hydride) and R−
(alkyl anions) are far worse.
For us, the key question is: how well do these guidelines carry over to inorganic chemistry? Not too well, it turns out, as we discuss below!

1.23

INORGANIC ARROW PUSHING: THINKING LIKE A LONE PAIR

Generally speaking, the mechanisms of p-block element reactions are not particularly consistent with the rules outlined above. The reason for this boils down to the so-called first-row
anomaly, where both first- and second-period elements (H–Ne) are all somewhat unreasonably lumped together as first row. The expression means that the chemical properties
of first-row elements are anomalous relative to those of their heavier congeners. Let us go
through the above four rules one by one and see how well they hold up in a main-group
inorganic context.
1. The octet rule breaks down routinely as soon as one goes down to period 3.
Main-group centers with more than eight electrons in their valence shells abound
for period 3 and below. Molecules containing such centers are called hypervalent.
Well-known examples include SiF6 2− , PF5 , PF6 − , SF4 , SF6 , BrF3 , IF5 , IF7 , XeF2 ,
XeF4 , XeF6 , and XeF8 2− . Even this short list of paradigmatic examples should show
with complete clarity that hypervalent molecules are anything but unusual; they are
ubiquitous for p-block elements in period 3 and below. Not surprisingly, therefore,
the octet rule is essentially irrelevant for these elements. (Note: The bonding in the
hypervalent molecules listed above might seem puzzling at first sight, but we will
address that issue over the next few sections.)
2. As mentioned in Section 1.9, nucleophilic attack on a heavier main-group center
with a noble gas configuration does not necessarily lead to immediate bond breakage. Instead, the first step may be an A reaction leading to a hypervalent intermediate
with an expanded octet. The leaving group would them depart in a subsequent D process. As mentioned, this two-step process is known as SN 2-Si. In this book, we have

1.23 INORGANIC ARROW PUSHING: THINKING LIKE A LONE PAIR

39

often tacitly assumed that the SN 2-Si mechanism is operative. Although there is no
question that it is pervasive, whether it applies near-universally to all heavier p-block
scenarios has yet to be settled.
3. Compared with organic chemistry, charged centers are much more common in
main-group element chemistry. Thus, monatomic anions such as O2− , S2− , and
N3− are all stable species when stabilized by cations in a solid lattice. Ammonium,
phosphonium, and sulfonium ions are an important part of the chemistry of the
elements in question. Oxoanions are an important part of the chemistry of the great
majority of p-block elements. Thus, rule no. 3 (concerning charge-neutral reactants
and products) applies rather more weakly in inorganic chemistry than in organic
chemistry; there are many exceptions.
4. We mentioned that strong bases such as OH− and RO− generally make poor leaving
groups. These, however, are “poor leaving groups” only from an organic perspective,
that is, where the reaction center is carbon. Non-carbon p-block centers vary hugely
in their electronegativity from near-metallic or metalloidal B, Al, Sn, and so on, to
highly electronegative elements such as N, O, and F. Indeed, for N, O, or halogen
reaction centers, OH− and RO− are rather good leaving groups. The reason for this is
that bonds between two highly electronegative atoms are weak and are readily broken,
both via SN 2 displacements as well as homolytically. Rule no. 4 too thus has limited
application in inorganic chemistry.
Organic paradigms accordingly are not very helpful in providing rules of thumb for
arrow pushing in main-group inorganic chemistry, especially for elements below period 2.
Yet we are far from helpless. Simple pattern recognition skills, along with some basic ideas
about nucleophiles, electrophiles, and leaving groups, go a long way in helping us arrive at
reasonable mechanisms for reactions involving main-group elements. Our approach may
be summarized as follows:
1. Look at the product structure(s) carefully and determine what bonds have been broken
in the course of the reaction and what new bonds have been formed.
2. Identify the nucleophile and the electrophilic site of attack.
3. Apply steps (1) and (2) iteratively until you arrive at the product structures (assuming
they are known).
Step (1) consists of pattern recognition, somewhat similar to the logic involved in putting
together a puzzle. Note that in the quote at the beginning of the chapter, Sherlock Holmes
describes this ability to “reason backward” as easy! Based on many years of experience,
we would echo the same assessment. Step (2) is where your chemistry knowledge comes
in handy: Apply your knowledge of nucleophilicity, electrophilicity, leaving groups, bond
strengths (e.g., bonds between two electronegative atoms are easily cleaved), and so on.
Beyond that, we do not advocate an overly algorithmic approach. Several of the reactions discussed in this book are too complex for that. To us, a semi-intuitive approach is
what makes inorganic arrow pushing both challenging and fun. Follow your nose! Or, to
use our favorite metaphor: think like a lone pair! Where would you attack if you were a
lone pair?
♦♦♦

40

A COLLECTION OF BASIC CONCEPTS

It’s time now to think about hypervalent compounds. You have encountered a few of them
already, as products of A reactions and as intermediates in SN 2-Si mechanisms. But what is
special about such compounds? Is the term “hypervalent” synonymous with higher-valent?
(No.) To better understand these issues, we’ll take a step back in Section 1.24 and remind
ourselves what the term “valence” exactly means and how it differs from related concepts
such as coordination number (CN), FC, and oxidation state