Electrochemical Reactions and Mechanisms in Organic Chemistry - James Grimshaw.pdf

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Electrochemical Reactions and
Mechanisms in Organic Chemistry
Elsevier, 2000
Author : James Grimshaw
ISBN: 978-0-444-72007-8
Preface , Pages vii-viii
Chapter 1 - Electrochemical Oxidation and Reduction of Organic Compounds,
Pages 1-26
Chapter 2 - Oxidation of Alkanes, Haloalkanes and Alkenes, Pages 27-53
Chapter 3 - Reduction of Alkenes and Conjugated Alkenes, Pages 54-88
Chapter 4 - Reductive Bond Cleavage Processes-I, Pages 89-157
Chapter 5 - Reductive Bond Cleavage Processes-II, Pages 158-186
Chapter 6 - Oxidation of Aromatic Rings, Pages 187-238
Chapter 7 - Reduction of Aromatic Rings, Pages 239-260
Chapter 8 - Oxidation of Alcohols, Amines and Amides, Pages 261-299
Chapter 9 - Oxidation of Ketones, Aldehydes, and Carboxylic Acids, Pages 300-329
Chapter 10 - Reduction of Carbonyi Compounds, Carboxylic Acids and Their
Derivatives, Pages 330-370
Chapter 11 - Reduction of Nitro, Nitroso, Azo and Azoxy Groups, Pages 371-396
Index, Pages 397-401
by kmno4
This book is concerned with reactions carried out at an elects'ode on a prepara-
tive scale. The impact of organic electrochemistry on synthetic organic chemistry
has a long history beginning with the Kolbe reaction, which is still in the repertoire
in first year teaching. In the early 1900's electrochemical methods for the oxidative
or reductive transformation of functional groups were actively pursued.. They offer
the advantage of having no spent oxidant or reductant for disposal. However elec-
trochernicat processes fell out of favour in the face of conventional chemical reac-
tions because the outcome from electrochemistry was often far from predictable.
Now that tlre mechanisms of these processes are generally well understood, many
of the former pitfalls can be avoided.
Electrochemical processes use the electron as a reagent and so avoid a chemical
oxidant or reductant, "It~e environmental impact of electrochemistry needs to be
assessed by looking at the global cell reaction. In the electrochemical cell, every
oxidation step at the anode nmst be accompanied by a reduction at the cathode.
During an oxidation, whatever is evolved at the cathode is m effect a spent reagent.
The cathode reaction can be controlled to give a desirable product, even hydrogen
for use as a fuek During a reduction process this spent reagent is produced at the
anode. It can be oxygen, which is venmd to the atmosphere. Control of the reaction
at the counter electrode gives to electrochemical processes the advantage of being
non-polluting, relative to corresponding steps using a chemical reagent~
The discovery of the Baizer hydrodimerization process for preparation of adipo-
nitrile from acrylonitrile led to a resurgence of interest in organic electrochemistry.
This process synthesises adiponitrfle at the cathode mid the spent reagent is oxygen
evolved at the anode. Its mmrense technical success prompted extensive investiga-
tions into reaction mechanisms in o~ganic electrochemistry with a view to im-
proving the old fimctio~mI group interchange reactions. At the same time new re-
actions of potential use in organic synthesis have been discovered. In parallel with
these investigations, significant improvements have been made m the design of
electrochemical cells both for laboratory and for industrial scale use
Electrons are transferred at an electrode singly, not in pairs, The primary reac-
five species to be generated is either a delocalised radical-ion or a radical formed
by cleavage of a <s-bond, together with an ion. The first formed radicals can be
further converted to ions by electron transfer. Fhus organic electrochemistry in-
volves a study of the reactions of both radical and ionic intermediates. Electron
transfer at the electrode is a surfhce reaction while intermediates undergo chemical
reactions in the bulk solution. An appreciation of the existence of these two types
of often competing processes is required to understand the outcome of organic
electrochemical reactions.
Recent work has developed reactions for carbon-carbon bond formation or
cleavage and has introduced new routes for the introduction of functional groups,
all of which are attractive to those planning synthesis on both laboratory and in-
dustrial scales. The mechanisms of these processes are now generally well under-
~is book aims to be more than just an introduction to such current areas of re-
search, It is intended also to show how the subject of Organic Electrochemistry is
integrated across the spectrum of oxidation and reduction by a general set of
mechanisms. The discussion centres around reactions on a preparative scale and on
the mechanisms governing the outcome of such processes. The book will be of
interest to inquisitive final year undergraduates, research students and research
directors both in academia and in the fine chemicals industry. An understanding of
general organic che~s~j is assumed. Physical chemistry has to be int~roduced into
a discussion on electrode kinetics and this area is kept to a minimum. Discussions
on the preparation and properties of radical-ions are also necessary since these are
the first reactive species produced at an electrode,
The redox properties of an electrode are determined by its potential measured
relative to some reference electrode. Many different reference electrodes are used
in the literature. In order to make cross comparisons easily, most of the electrode
potential quoted for reactions have been converted to the scale based on the satu-
rated calomel electrode as reference. Electrode materials and electrolyte solutions
used by the original workers are quoted. In many cases, the electrodes could be
fabricated from more modem materials without affecting the outcome of the reac-
tions. In the not too distant past perchlorate salts were frequently used as electro-
lytes. This practise must be discouraged for preparative scale reactions because of
the danger of an explosion when perchlorates and organic compounds are mixe&
Alternative electrolytes are now readily available.
I acknowledge many discussions over the years with research students and with
the international research community on problems in organic electrochemistry. The
assistance given to me by Sheila Landy and her staff of the Science Library in
Queen's University is gratefully acknowledged. Finally, I thav& my wife fbr her
help and her patience m dealing with all the disruptions to normal life which writ-
ing this book has caused.
James Grimshaw,
Belfast, July 2000
General Technique
During an electrochemical reaction, electrons are transferred between a mole-
cule of the substrate and the electrode. Electrons are always transt?rred singly and
the substrate first is converted to an intermediate with an unpaired electron. Trans-
fomnation of this reactive ime~ediate to the final product involves a sequence of
bond forming or bond cleaving reactions and frequently further single electron
transfer steps. The complete electrochemical reaction vessel requires both an anode
and a cathode. Only one of these electrodes, the working electrode, is involved
with the chemical reaction of interest, oxidation at the anode or reduction at the
cathode. The second electrode is the counter electrode and usually some simple
inorganic reaction occurs here, such as hydrogen evolution if this is a cathode or
oxygen evolution if this is an anode. The space between the anode and cathode is
filled with an ionised salt solution and charge passes through the solution between
the electrodes by migration of ions.
The simplest design of electrochemical cell has two electrodes dipping mm the
solution containing the substrate and the supporting electrolyte. A cell of this type
is suitable for the Kolbe oxidation of carboxylate ions (see p. 316) where the anode
reaction is given by Equation I.I and the cathode reaction is the evolution of hy-
drogen (Equation 1.2), Both the substrate and the hydrocarbon product are inert
Eq. I. 1
2 ~3CO
2 e
C6HCT C.~H~3
2 CO2
2H + + 2e -----* H~
towards reduction at the cathode.
For many processes, however, it is necessary to employ a divided cell in which
the anode and cathode compartments are separated by a barrier, allowing the diffu-
sion of ions but hindering transfer of reactants and woducts between compart-
ments. This prevents undesirable side reactions. Good examples of the need for a
divided cell are seen in the reduction of nitrobenzenes to phenylhyckoxylamines (p.
379) or to anilines (p. 376), in these cases the reduction products are susceptible to
oxidation and must be prevented from approaching the anode. The cell compart-
ments can be divided with a porous separator constructed from sintered glass, po-
rous porcelain or a sintered inert polymer such as polypropene or polytetra-
fluoroethene. Another type of separator uses woven polytetrafluoroethene cloth
which has been exposed to a soluble silicate and dilute sulphuric acid so that silicic
acid precipitates into the pores [1]. On a laboratory scale porous porcelain and sin-
tered glass are the most commonly used materials.
On an industrial scale, ion-exchange membranes are most frequently used for
the separator material [2]. Cationic and anionic types are both available and a sup
phonated polytetrafluoroethene cation exchange resin, which can withstand aggres-
sive conditions, is frequently used. Arrangements for sealing this type of separator
into a laboratory scale glass ceil are also available.
,,f- Electrodes"~'~'V
Porous ~.
ill It
Figure 1,t. Ceils used for laboratoryscaleelectrochemicalpreparations:
(a) a beaker-typeceil;(b) an lt-typeceil.
General purpose laboratory scale glass cells are either of the beaker4ype (Figure
l.la) or the H-type (Figure 1. Ib). The early pioneers of organic electrochemistry
used beaker-type cells, with cylindrical symmetry, and the separator was either a
porous porcelain pot or a sintered glass disc [3]. Designs for beaker-type ceils in
more modem materials have been described [41. ~I]qe H4ype ceil can be designed
to use either one or two sintered glass separators [511. Oxygen must be excluded
from the cathode compartment during electrochemical reduction otherwise cun'ent
is consumed by the reduction of oxygen to water and the highly reactive superox-
ide anion is generated as an intermediate. A flow of into1 gas is maintained in the
cathode compartment. It is not essential to exclude oxygen during electrochemical
oxidation but usually a flow of inert gas is maintained in the anode compartment so
as to dilute any oxygen, which is evolved. A stirring device is necessary to de-
crease the thickness of the diffusion layer around the working electrode.
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