Table Of ContentThis is Volume 32 of
ORGANIC CHEMISTRY
A series of monographs
Editors: ALFRED T. BLOMQUIST and HARRY H. WASSERMAN
A complete list of the books in this series appears at the end of the volume.
Anodic Oxidation
Sidney D. Ross
Sprague Electric Company
Research and Development Center
North Adams, Massachusetts
Manuel Finkelstein
Sprague Electric Company
Research and Development Center
North Adams, Massachusetts
EricJ. Rudd
Hooker Chemical & Plastic Corporation
Research Center
Grand Island Complex
Niagara Falls, New York
ACADEMIC PRESS AW]
New York San Francisco London 1975
A Subsidiary of Harcourt Brace Jovanovich, Publishers
COPYRIGHT © 1975, BY ACADEMIC PRESS, INC.
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Library of Congress Cataloging in Publication Data
Ross, Sidney D Date
Anodic oxidation.
(Organic chemistry series)
Includes bibliographical references and index.
1. Oxidation, Electrolytic. 2. Chemistry, Physical
organic. I. Finkelstein, Manuel, joint author.
II. Rudd, Eric, joint author. III. Title.
QD273.R67 541'.393 74-17963
ISBN0-12-597650-X
PRINTED IN THE UNITED STATES OF AMERICA
Preface
The fundamental laws of electrochemical action were first enunciated
by Faraday in 1834 and applied to organic synthesis by Kolbe in 1854. After
an initial burst of activity, organic electrochemistry received only spasmodic
and dilatory attention. The development of the dropping mercury electrode
and the polarographic method by Heyrovsky in 1922 generated sustained
interest in cathodic reactions, but anodic reactions were not investigated
intensively until the middle of the present century.
The reasons for this long period of relative neglect are difficult to pin
point. The reluctance of organic chemists to introduce electrochemical
methods and equipment into the organic laboratory was certainly involved,
and this reluctance is a barrier even now. The absence of effective communi
cation between organic electrochemists and physical electrochemists did not
help. It was not uncommon for organic chemists to believe that the proper
construction and assembly of electrode systems were a domain of black magic
to which only the physical electrochemist was privy, that reactions occurring
at or near a solid electrode were necessarily different from those taking place
in a homogeneous system, and that the reaction mechanisms developed by
physical-organic chemists had no validity for and could not be applied to
reactions in electrochemical systems.
This volume is directed primarily to organic chemists who suffer any
or all of the above delusions. We hope to convince them that all electro
chemical techniques are well within their range of capabilities, that the
physical-organic reaction mechanisms, with which they are familiar, apply
equally well in electrochemical systems, and that, in fact, most of the
product-forming reactions are controlled by these very mechanisms and are
not significantly influenced by the electrode. Additionally, we hope to demon
strate, for their particular benefit, that electrochemical methods can be used
to generate highly reactive chemical intermediates, cations, radicals, cation-
ix
X Preface
radicals, etc., in a controlled manner and to determine their fundamental
properties.
This book is also directed to physical electrochemists. We recognize
the importance of the electrode in the heterogeneous electron transfer steps
and the crucial role these steps play in defining the relationships between
current and voltage during electrochemical processes. Nevertheless, many
of the purely chemical steps are only minimally impacted by the presence of
the solid electrode surface, and here the theoretical constructs and mechan
isms of physical organic chemistry hold and are useful. We would hope that
this treatment will prove convincing to the physical electrochemist and will
help to generate a more fruitful dialogue between physical and organic
electrochemists.
In the first part of this book, Chapters 1-4, we have surveyed the theory
and methods of electrochemistry as applied to organic reactions. The
treatment, though brief, should adequately answer most questions that arise,
particularly if judiciously supplemented by use of the cited references.
The second section, Chapters 5-12, surveys the anodic oxidation of
organic substrates by the functional group initially attacked. The emphasis
is on the kinds of intermediates generated and the mechanisms leading to
final products. The treatment is in no sense all inclusive. Nevertheless, we
hope that the reader will quickly see that the mechanisms follow recognizably
discrete patterns, that certain types of intermediates commonly occur, and
that the overall discussion affords some basis for predicting the course of a
new anodic oxidation.
Sidney D. Ross
Manuel Finkelstein
Eric J. Rudd
1
An Introduction to the
Electrochemical Reaction
I. Electrolysis and Its Terminology
In a general way one may describe electrochemistry as the study of the
chemical effects of electricity. The phenomenon of electrolysis is perhaps
the main manifestation of electrochemistry, being a chemical reaction which
proceeds under the direct influence of an electric current. Early in the nine
teenth century the oxidation of acetic acid by Faradayλ demonstrated the
power of an electric current to decompose organic substances and by the end
of that century Gatterman, Kolbe, Haber, and others had widely extended
the field of electroorganic chemistry. An account of the many electroorganic
reactions studied prior to 1926 has been given by Brockman.2
The assembly shown as Fig. 1.1 is representative of the experimental
conditions used in many electroorganic syntheses. The switch (S) is closed
to complete the circuit and an electric current which is detected and measured
by the ammeter (A) flows through the system. Although a detailed explana
tion is not offered at this time, it is important to realize that neither the
solvent (be it water or an organic solvent) nor the solute alone is able to
conduct electricity to any significant degree, whereas the solution has this
property. Using electrochemical terminology the conducting solution is
known as the electrolyte and the positive and negative metal conductors are
the anode and cathode, respectively. The processes which occur at the elec
trodes may be referred to as anodic and cathodic reactions and involve
electron transfer to or from an electroactive species. The overall system—
electrodes, electrolyte, and electroactive compound—constitutes an electro
lytic cell Organic electrode processes are therefore those anodic and cathodic
reactions in which an organic molecule or ion is the electroactive species.
3
4 1. An Introduction to the Electrochemical Reaction
ELECTRON FLOW
ELECTROLYTE
ΘΘ IONS OF THE
SUPPORTING
ELECTROLYTE
<0> NEUTRAL ORGANIC
MOLECULE
© SOLVENT
MOLECULE
REDUCTION AT CATHODE OXIDATION AT ANODE
Fig. 1.1. Experimental assembly typically used in electrochemical syntheses.
Π. Faraday and the Laws of Electrolysis
An important consequence of the passage of an electric current during
an electrolysis was demonstrated by the early studies of Faraday, the results
of which may be considered in the form of two laws of electrolysis. (1) The
amount of chemical change produced by an electric current, i.e., the weight
of any substance that undergoes reaction at an electrode, is proportional to
the quantity of electricity passed; and (2) the weights of different substances
reacting during electrolyses in which the same quantity of electricity is
passed are proportional to their chemical equivalent weights.
The quantity of electricity that is passed in an electrolysis is equal to the
product of the current strength and the time for which that current passes.
It is thus possible to write the equation
ItW
(1.1)
w
where w is the weight in grams of the electroactive compound that has
reacted, /is the current strength expressed in amperes, t is the time of passage
///. How Is the Electric Current Carried through the Cell? 5
of the current in seconds, and W\s the equivalent weight of the reactant. The
constant & is the Faraday, defined as the quantity of electricity required to
effect the change of one gram equivalent of an electroactive compound.
The value of the Faraday has been determined experimentally from the
electrolyses of compounds for which W, the equivalent weights, are accu
rately known. For most purposes the value used for the Faraday is 96,500
coulombs. From Eq. (1.1) it can be seen that the electrochemical equivalent
of a particular compound, defined as the weight in grams which reacts when
1 coulomb of electricity is passed, is simply the fraction W\^'. The coulomb,
the practical unit used to express the quantity of electricity, is 1 ampere-sec.
The amounts of primary products which form in an electrolysis are thus
quantitatively described, yet, in practice, few electrolyses are actually free of
extraneous influences which make it appear that Faraday's laws are not
obeyed. Causes of these deviations can be (1) chemical or physical reactions
of the products of the electron-transfer process with the electrodes or the
electrolyte, (2) simultaneous electrode reactions involving impurities in the
electrolyte; and (3) alternative reaction pathways for the primary product,
e.g., in organic electrode processes a free radical may be formed, and this
radical may be stabilized by possible dimerization, disproportionation, or
abstraction reactions.
It therefore becomes necessary to introduce a quantity of practical
importance, the current yield, current efficiency, or coulombic efficiency. This
quantity is defined as that percentage of the total quantity of electricity
passing through the cell which is used to produce the final yield of product in
the electrochemical reaction.
ΙΠ. How Is the Electric Current Carried through
the Cell?
The current flow through a system during an electrolysis involves (1)
electronic conduction through the external connectors and the metal elec
trodes, (2) ionic conduction across the solution between the electrodes, and
(3) the transfer of electrons across the metal-solution interfaces.
A simplified picture of electronic conduction is the movement of elec
trons through a relatively rigid lattice of metal ions and, as such, involves
little movement of matter. Dissolution of a solute in a solvent results in the
dissociation of the salt into ions; electric conduction in the solution is the
movement of these ions, the cations (positively charged species) moving
toward the cathode and the anions (negatively charged species) moving
toward the anode. Ionic conduction therefore involves a net transfer of
matter. At the electrode-solution interface there is a transfer of electrons (1)
6 1. An Introduction to the Electrochemical Reaction
from the cathode to the electroactive species which is then reduced and (2)
to the anode from the electroactive species which is therefore oxidized.
IV. The Electrochemical Reaction—A Simple Picture
The overall electrochemical reaction is, in fact, a heterogeneous redox
reaction—reduction at the cathode and oxidation at the anode. The inorganic
redox process may be conveniently represented as an electron transfer be
tween the two reactants, but in organic chemistry the situation is seldom so
clearly defined. However, one considers organic reactions to involve intra
molecular and intermolecular electron movements, accompanied by the
making and breaking of covalent bonds. Electrochemistry introduces a
"third body" into the reaction, namely, the electrode, which acts as an elec
tron source for a reduction process and an electron sink for an oxidation
process.
Electron transfer to or from the organic molecule occurs at the electrode
surface and at this point the electrochemical reaction may be considered
complete. The products of the reaction, probably the most important factor
to the organic chemist, are a consequence of the chemical reactions of the
intermediates generated at the electrode surface. However, the electro
chemical and chemical aspects of an electrode process are complementary,
and the appreciation of both must be emphasized. The electron-transfer
step, the orientation, adsorption, and desorption of reactants, intermediates,
and products are typically the concern of the electrochemist. Much of the
chemistry of interest comprises the various rearrangement, disproportiona-
tion, dimerization, and abstraction reactions which follow the electron-
transfer step. In some cases the chemistry of reactions preceding electron
transfer is important, e.g., enolization or protonation steps.
V. A Most Important Factor—The Electrode Potential
The discussion thus far has centered on the electric current, its passage
through an electrolyte, and the nature of an electrochemical reaction. As
early as 1898, however, Haber3 indicated the several steps involved in the
electroreduction of nitrobenzene to aniline. Furthermore, despite the
/ V-N0 ► / Y-NO ► / V-NHOH ► / \-NH
2 2
limited electrical equipment then available, Haber was perhaps the first
to realize the importance of the electrode potential in electrochemical reac-
VI. The Origin and Significance of the Electrode Potential 7
tions. An electrode process is a heterogeneous transfer of electrons between
electrode and organic molecule. It may be useful to consider the electrons as
a reagent, a controllable reagent, and it is the potential of the electrode that
controls the availability of this reagent. It is unfortunate that this factor has
been ignored in many electroorganic syntheses, because certain chemical
reductions are, in fact, electrochemical in nature and proceed effectively
under conditions of controlled potential, e.g., the Clemmensen reduction
(Zn/Hg) and reductions with Na/Hg or Mg/Hg. Here a mixed potential is
established which corresponds to both the corrosion (oxidation) of the
amalgam and reduction of the organic compound (itself adsorbed at the
surface of the amalgam) occurring at equal rates. It is true that controlled
potential electrolyses may be slow and this may have been the reason for the
persistent use of a large, albeit controlled, current in electroorganic
syntheses.4
VI. The Origin and Significance of the Electrode
Potential
When a metal is immersed in an electrolyte, there is a tendency for the
metal atoms to ionize, and the resulting cations move into a layer of the
solution adjacent to the metal. The loss of the positively charged ions leaves
an excess of electrons at the metal surface, these electrons opposing the layer
of cations on the solution side of the interface. This is shown pictorially as
Fig. 1.2 and represents the formation of the so-called electrical double layer.
An important property of this double layer is a steady difference of electric
potential across the interface between the metal and the solution.
Thus, the solution acts as the medium whereby the equilibrium is realized.
M , M2+ + ze (1.2)
M = metal
[Note that equilibrium is also established between the interfacial region and
the bulk solution, i.e., in the so-called diffuse double-layer region (Fig. 1.2c)].
To establish an equilibrium is an inherent characteristic of all chemical
reactions. The driving forces leading to equilibrium conditions, of course,
depend on the type of reaction, but they are always governed in the thermo-
dynamic sense by the tendency of the free energy system to approach zero.
The tendency toward electron loss or electron gain in the redox system
expressed in Eq. (1.2) can be measured as an electrical driving force and
expressed as the electrode potential.
It is clear from experience that some metals, notably the alkali and
alkaline earth metals, cannot attain equilibrium with their ions in aqueous
