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Cathodic Dimerization.

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clubs, etc. In the opinion of many experts we are entering
an “age of inorganic fibers”.
Received : October 8, 1971 [A 900 IE]
German version: Angew. Chem. 84, 866 (1972)
Translated by Express Translation Service, London
[l] P. H. Selden I Glasfaserverstarkte Kunststoffe. Springer-Verlag,
Berlin 1967, p. 181.
[2] I. E. Campbell and E. M . Sherwood: High-Temperatur Materials
and Technology. Wiley, New York 1967, pp. 256ff.
[3] L. Piatri: Werkstoffe der chemischen Technik, Vol. 3. Verlag
Sauerlander, Aarau 1955, p. 76.
[4] H. W Rauch, W H. Sutton, and L. R. McGreight: Ceramic Fibers
and Fibrous Composite Materials. Academic Press, New York 1968
[5] C. Z. Carroll-Porczynski: Advanced Materials. Chemical
Publishing, New York 1969.
[6] Chem. Week 108 (Feb. 24). 51 (1971).
[7] H. P. Blakelock, N . A. Hill, N . Aubrin. S. A . Lee, and C. Goatcher,
Proc. Brit. Ceram. SOC.1970, No. 15.6Y
[8] A. H. Frazer: High Temperature Resistant Fibers. Wiley, New York
1967, pp. lOSff, 183ff, 267ff.
[9] W: L. Lachmann and J . P. Srerry, Chem. Eng. Progr. 58 (lo), 37
[lo] B. H . Hamling, A. W Naumann, and W H . Dreshrr, Polymer
Reprints, Atlantic City Meeting 9 (2), 1449 (1968).
[ll} W I: Gunston, Sci. 5, 39 (1969).
[I21 H. M . Hawthorne, Int. Conf. Carbon Fibres, Composites and
Applications, London 1971, Paper No. 13.
[I31 J . Economy, R. V Anderson, and K I . Matkooich, Polymer Reprints,
Atlantic City Meeting 9 (2), 1392 (1968).
[14] H . W Rauch, Mater. Eng. 66, 74 (1967).
[15] J . Nixdorf; Metal1 23, 887 (1969).
[16] H. J . Schladitz, 2. Metallk. 59, 18 (1968).
[I71 M . Polanyi, Z. Phys. 7, 323 (1921).
[IS] E. Orowan, 2. Phys. 86,195 (1933).
[I91 F . P. Knudsen, J. Amer. Ceram. SOC.42, 376 (1959).
[20] W Kleber: Einfiihrung in die Kristallographie. VEB Verlag
Technik, Berlin 1961, pp. 114fT.
[21] H. Griinewald, DEW (Deut. Edelstahlwerke) Techn. Ber. 8 (4),
271 (1968).
[22] J . Fray, Int. Conf Carbon Fibres, Composites and Applications,
London 1971, Paper No. 46.
Cathodic Dimerization[**]
By Fritz Beck“’
The technical, electrochemical, and preparative aspects of cathodic dimerization, which
leads to bifunctional compounds. are reviewed. With the hydrodimerization of acrvlonitrile
as an example, the effects of the reaction parameters and the mechanism are discussed in
detail. In addition to the hydrodimerization of activated compounds, the coupling can
also proceed via elimination of halide or via the discharge of cations. Processes of special
preparative interest are those in which two different molecules are coupled, which can yield e.g.
esters, alcohols, or ketones with cyano groups, as well as asymmetric diols. The reductive
dimerizations of acrylonitrile, p-chloropropionitrile, acetone, acetylpyridine, nitrobenzene
(+ benzidine), and pyridinium salts have already found industrial use.
1. Introduction
The field of cathodic dimerization has developed particularly rapidly in recent years. This is no accident, since this
area of organic electrosynthesis involves aspects of great
preparative, technical, and mechanistic interest. The object of this progress report is to present a comprehensive
survey, with special emphasis on industrial and electrochemical possibilities. Only recently, Baizer (together with
Petrowitsch), who played an extremely important part in
the development of the field, published a brilliant survey
of the synthetic and mechanical aspects[’’. This topic is
also discussed at length in a recent review on organic
eIectrosynthesisr2! Overlapping with these publications
cannot be entirely avoided, but will be kept to a minimum.
[*] Dr. F. Beck
Badische Anilin- & Soda-Fabrik AG, Hauptlaboratorium
67 Ludwigshafen (Germany)
[**I Based, in part, on a lecture at the 1st EUCHEM Conference
on Organic Electrochemistry at Ronneby Brunn, Sweden, on June 9,
In cathodic dimerization, the organic molecule takes up
an electron directly at the electrode. It thus passes through
the intermediate stage of a radical-ion. The negative charge
is compensated in accordance with Scheme 1 by uptake of
protons (“hydrodimerization”), by elimination of halide
ions, or by the positive charge on the substrate when the
latter is a cation. Each of these cases can in principle be
further subdivided into the dimerization of two identical
molecules, which yields a symmetrical product, the
coupling of two different molecules, and intramolecular
reactions leading to cyclic products.
If the monomer contains one or more functional groups,
bifunctional or polyfunctional dimers are formed. Cathodic
dimerization is therefore a valuable method for the synthesis of such compounds. Since oniy one electron is taken
up per monomer in every case, the energy consumption is
comparatively low. This is extremely favorable in industrial uses.
The formation of a symmetrical dimer is only one of several
possible reactions. In principle, the molecule could also be
Angew. Chem. internot. Edit. Voi. I 1 (19721 I No. 9
hydrogenated (possibly in several stages), add on an acceptor molecule instead of the second substrate molecule, form
oligomers by addition of a third molecule, a fourth etc., or
finally form organometallic compounds with the cathode
metal. The fact that cathodic dimerization nevertheless
often occurs, with extremely high yields in many cases,
shows that this reaction is often favored in electrochemistry.
An article by Knunyants and G a r n b ~ r i a non
[ ~ hydrodimeri~
zation clearly shows that electrochemical reduction (or
the equivalent reduction with Na amalgam, Zn/glacial
acetic acid, or the like) generally leads to hydrodimers,
2X + Z H m + 7 0 HX-XH
hydrodimerization of t w o molecules of the same type
X + X + 2Hm + 2 0 +
hydrodimerization of two different molecules
2H@+ 2 0 HXEX'H
cyclizing hydrodimerization
the solution, where it can be detected e. 8. by ESR spectros ~ o p y [ ~In] .general, however, it will react where it is formed,
i. e. in the electrochemical double layer in front of the electrode. In this case, it reacts in a strong .electric field
( zl o 7V/cm); the subsequent reactions are therefore
generally different from those of the same radical-ion
formed, e. g. by a homogeneous redox process. However,
it should be noted that the electric field strengths in the
atomic range, e. g. in the vicinity of a (small) ion or a dipole,
could quite easily be of the order of magnitude mentioned
According to Scheme 2, the second reaction step (proceeding toward the hydrodimer) may be the uptake of a
proton or combination with a second substrate molecule,
i. e. the radical-anion reacts as a nucleophile. The freeradical nature of the primary product becomes evident in
the uptake of a second electron or in the (electrostatically
improbable) reaction with a second radical anion, which
2X-Hal - 2 H a I 0 + 2 0 X-X
dimerization with elimination of halogen
2 x a + '0- x-x
dimerization of cations
Scheme 1 Survey of cathodic dimerization. (Strictly speaking the
second and third variants are not dimerizations.)
whereas catalytic hydrogenation yields such products
only in exceptional cases. This may be connected with the
strong adsorption of the intermediate products on the
catalyst surface, which is unfavorable for the dimerization
step. In the electrochemical method, on the other hand, the
intermediate products generally interact only weakly with
cathodes having high hydrogen overvoltages (exceptions :
formation of organolead, organomercury, or organozinc
Scheme 2. Reaction paths in cathodic dimerization. For the figures
under the reaction arrows see also Scheme 3.
2. General Mechanistic Considerations
What intermediates are involved in cathodic hydrodimerization? Scheme 2 shows all the possibiiities that are found
on application of the following conditions :
a) The primary step is the transfer of an electron to the
substrate X with formation of the radical-anion. This reaction is reversible in most cases; surprisingly high exchange
current densities are found for this primary process, e. g.
for aromatic compounds[41.In exceptional cases, this primary electron transfer may be preceded by a chemical
reaction, such as the protonation in strongly acidic solution
that has been proposed for
or for acrylonitrile[']
The unusual increase in basicity is attributed to a field
effect ['I.
b) The second protonation can occur only after the dimerization step, since the hydrogenated monomer would otherwise result.
c) A total of two electrons and two protons is taken up.
The initially formed radical-anion, in (rare) cases in which
its stability is high, will diffuse in part into the interior of
Angew. Chem. internal. Edir.
1 Vol. 11
(1972) 1 N o . 9
can also lead to disproportionation. The possibility that
the free-radical function directly causes the abstraction of
a hydrogen atom, e . g . from the solvent, is not shown in
Scheme 2.
The secondary intermediates formed can react further as
shown in Scheme 2. The intermediate product HX. (or
HX@)cannot e. g. be protonated since this would be contrary to rule b). The intermediate product 'X-X'
eX-X. cannot take up further monomer, and the intermediate XZo has already taken up both possible electrons.
In this way one obtains eleven reaction paths to the
hydrodimer, which are summarized once again in abbreviated form in Scheme 3 (in the same order as in Scheme 2).
(The actual dimerization step in practically every case is
the formation of a C-C bond.) It can be seen that the
first reaction step, as required (rule a), is an electron transfer, and the last [except in case (I)]
is a protonation ; in (1 ),
(4), and (IIthe
),dimerization follows a free-radical course.
For electrostatic reasons all mechanisms involving doubly
negatively charged dimeric and particularly monomeric
particles are relatively unlikely [(6)-(I I ) ] .Of the remaining
first five possibilities (1) and (4) are free-radical, ( 3 ) and ( 5 )
are ionic, and in (2) the monomer reacts with a free radical.
(1)occurs e. g. in the hydrodimerization of ketones, and (3)
on the way to becoming an established chemical reaction
technique. The earlier view that the use of organic electrosynthesis in industry was to be expected at most for
valuable products on a fairly small scale had thus turned
out to be wrong. An early review on the hydrodimerization
of acrylonitrile was published by Tomiloc et ul.['*].
3.2. Electrolysis Conditions
Scheme 3. Reaction path in cathodic dimerization (see Scheme 2);
abbreviated notation : e = electron transfer, p = protonation, d = dimerization.
or (5)in the hydrodimerization of activated olefins. According to
however, two radical-ions dimerize to
give the dimeric dianion in the case of dimethyl fumarate.
The choice of the optimum electrolysis conditions will be
discussed for the case of acrylonitrile, since data on this
reaction is particularly abundant because of its technical
importance, and since these considerations can be applied
to the hydrodimerization of other compounds. The problem is to optimize the numerous important parameters
for the electrosynthesis so that under acceptable industrial
conditions of temperature, pressure, etc., the desired reaction
0 d NC(CH,),CN
+ 20He
clearly predominates over electrochemical side reactions
such as
3. The Cathodic Hydrodimerization of Acrylonitrile
3.1. Industrial Importance
The cathodic dimerization of acrylonitrile (AN) leads
directly in one step to adiponitrile (ADN), which is an important intermediate for 6,6 nylon. The fact that the monomer could be obtained relatively cheaply by a new petrochemical process (Sohio process = ammonoxidation of
propylene) drew the attention of the polyamide manufacturers to this electrochemical process about a decade
ago. This process had the fundamental advantage that it
requires one step less than the conventional industrial
+ 2H20+ 2
+ 20-
+ 2Ha + 2 0 -
+ H,O + 2 0 -
+ 2H" + 2 0 +
and chemical side reactions such as the hydrolysis of the
nitrile group (in acidic media), cyanoethylation of water to
form P-hydroxypropionitrile and p, /Y-dicyanoethyl ether
(in alkaline media), and free radial initiated polymerization.
3.2.1. Cathode Material
% 1,4-dichloro-2-hutene
adipic acid
The hydrodimerization carried out by K n u n y a n t ~ " ~1 -2 ]
in the 1950's with Na amalgam initially gave the desired
product in a material yield of only 65 % and in an amalgam
yield of 35%. It was the work of Baizer at Monsanto'I3- 15],
who was able to obtain almost quantitative material and
current yields, that gave the decisive thrust that led to the
construction of an industrial plant at Monsanto. The outstanding purity of the product was a strong argument in
favor of the polyamide synthesis. Another advantage was
the absence of stoichiometric by-products.
This development helped to bring about an unexpected
renaissance in the field of organic electrochemistry, which
to be
appeared even around the turn of the
Figure 1 shows the polarographic current-voltage curve
of acrylonitrile in aqueous solution. The half-wave potential is strongly negative (- 1.96 V against the saturated
calomel electrode). This has two immediate consequences.
Cathodes with high hydrogen overvoltages are required
to suppress the simultaneous liberation of hydrogen in
accordance with reaction (14). Furthermore, the discharge
potential of the cation of the carrier electrolyte must be
as negative as possible. Quaternary ammonium salts
satisfy this condition, as has been known for some time
from polarography. Mercury and above all lead have
proved to be excellent cathode metals. Cadmium is unsuitable, since it is attacked with formation of bis(cyanoethy1)cadrni~m"~'.Carbon is also a good cathode material,
which differs in important respects from Pb and Hg (see
below). The hydrodimer is formed even on platinum,
though in poor yields[201.This is probably due to substantial inhibition of (14) by adsorbed organic products.
Angew. Chem. infernat. Edit.
Val. 11 (1972) No. 9
3.2.2. Acrylonitrile Concentration
The number of electrons found from the wave height of
the polarographic current-voltage curve (Fig. 1) with the
aid of the Ilkovii- equation is 2. At low acrylonitrile concentrations, therefore, only propionitrile is formed. Baizer
remainder AN + acetonitrile on graphite cathodes in
undivided cells (cf. Fig.
As can be seen from
Figure 2, the A N concentration has a strong influence
on the A D N yield. An optimum is found for an initial AN
concentration of 25 wt.% (the formation of oligomer R
has decreased to a minimum). At lower AN concentrations,
as in Baizer's experiments, the yield decreases because of
the formation of increasing quantities of propionitrile PN.
The current yield shows a corresponding decrease["!
The decrease in the formation of ADN at low monomer
concentrations is reasonable in itself. The point beyond
which an appreciable decrease occurs depends on the
reaction conditions. Tomilov et
obtained high yields
of ADN on an Hg cathode at A N concentrations as low
as about 2.5% when the ratio of water to dimethylformamide in the reaction mixture was adjusted to an optimum
value. It was also shown later[24.251that high yields of
ADN are obtained on lead cathodes in 2 to 5% solutions
of acrylonitrile in the presence of non-lyotropic carrier
salts such as tetramethylammonium sulfate. These carrier
salts reduce the solubility of aciylonitrile in water, which
is around 7% (at 25"C), by the salting-out effect.
- 1.6
- 1.8
- 2.0
These observations can be best explained if one considers
that the solution compositions in the bulk of the electrolyte and at the interface are quite different. Salt effects in
particular play an important part here, as is shown in
Section 3.2.8.
Fig. 1. Polarographic current-voltage curves of acrylonitrile in the
presence of 0.1 mol/l of tetramethylammonium chloride. The numbers
by the curves indicate the content of acrylonitrile (in mmol/l). For
conditions see text.
emphasized in his first reports that ADN is formed in high
yields only if the acrylonitrile concentration is higher than
The yield of ADN was almost quantitative at
an initial A N concentration of 20%, but decreased to
7% with 10%AN and to 2% with 5 X A N . A t the same time,
the yield of propionitrile increased. The reaction mixture
also contained a high concentration of quaternary ammonium salt and water. The cathode consisted of mercury.
We have reacted mixtures of 28% of isopropanol, 16%
of water, 1% of tetraethylammonium ethyl sulfate,
3.2.3. Current Density
The current density plays a minor part in the hydrodimerization of acrylonitrile. Thus constant yields of ADN were
observed in the range between 1 and 30A/dm2[261and
even in the range between 14 and 128A/dm2'271. This
suggests that the reaction does not have a free-radical
mechanism, in which the yields should decrease greatly at
low current densities. Since hydrogenation and hydrodimerization cannot be distinguished in the current-voltage
curve in the case of acrylonitrile (both processes have the
same rate-determining step), there is no prospect of increasing the selectivity by electrolysis at a constant potential (or indirectly with the aid of the current density). To
obtain a good space-time yield in industry, one should not
use excessively low current densities[''!
3.2.4. Water Concentration
A N [%I
Fig. 2. Dependence of the yield on the acrylonitrile concentration.
PN = propionitrile, R = oligomer. For conditions see text.
Angew. Chem. internat. Edit. 1 Vol. I 1 11972) 1 No. 9
If the water concentration is too high, the formation of
proprionitrile increases, while at excessively low water
concentrations, the o l i g o m e r i z a t i ~ n becomes
[ ~ ~ ~ ~ ~very
pronounced. Figure 3 shows some examples for Hg cathodes from the l i t e r a t ~ r e ~ ~together
~ , ~ ~ ] with
our own
results (obtained on vibrating lead mesh cathodes,
It can be seen that the optimum water concentration depends on the AN concentration, and increases with the
latter. The nature of the second solvent, on the other hand,
is of minor importance.
3.2.5. pH
occur in the diffusion boundary layer in front of the cathode,
particularly where convection is inadequate.
The pH should have a considerable influence on the yield
of ADN, since excessively low pH values lead to propionitrile formation and liberation of hydrogen at the cathode,
as well as to hydrolysis of the nitrile group, while excessively high pH values lead to cyanoethylation. It is therefore
recommended in the first Monsanto patents that weakly
alkaline solutions should be used.
It can be seen from Figure 4, in fact, that the ADN yield at
an Hg cathode greatly decreases even in the weakly acidic
rangecL3’.The situation changes when a graphite cathode
3.2.6. Temperature
The conductivity of the electrolyte increases with rising
temperature, and the dissipation of the Joule heat becomes
technically easier. However, the rates of chemical side
reactions and the vapor pressure also increase. The range
between 30 and 40°C may be regarded as a good compromise. As was mentioned, electrolyses have been carried
out even at 0°C[341.We have also electrolyzed acrylonitrile
on Pb and Hg cathodes (in water/McKee salt“]) at 20-50
and 80°C (boiling point of the catholyte), and found that
the yield and the current efficiency decrease rapidly with
rising temperature; increasing quantities of cyanoethylation products and propionitrile are formed[351.
3.2.7. Second Solvent
L H2 0 1[wI-%I
Fig. 3. Dependence of the ADN yield on the water concentration.
material yield, (----) current efficiency. Conditions:
AN [%I
N R A [%I
0~ 3 1
- 3
A solubilizing agent is necessary in order to obtain higher
acrylonitrile concentrations without excessively low water
concentrations. Suitable solubilizing agents include lyotropic supporting electrolyte, e.g. tetraethylammonium
ethyl ~ u l f a t e [ ~ ~ Additional
solvents such as acetonitrile (which is present in any case in crude AN from the
Sohio process), dimethylformamide, or isopropanol have
also been used. The use of acrylonitrile-in-water emulsions
has also been s ~ g g e s t e d [ ~ ~ . ~ ’ ] .
3.2.8. Supporting Electrolyte
is used, practically no change in yield being observed in
this case over a wide pH range down to pH
In conjunction with a “wick cell”, electrolysis is possible even
in up to 5 . 5 H2S0,[331.
It is also possible to work at pH
values above 14 if the cyanoethylation is sufficiently retarded by cooling to 0°C1341.
As can be seen from Table 1, optimum material yields and
current yields are obtained only with supporting electrolytes containing higher quaternary ammonium ions[”, 391.
A decrease in the yields occurs even with tetramethylammonium salts, and this decrease becomes more pronounced with the use of tertiary ammonium salts and in
particular of alkali metal salts. This cannot be due to an
Table 1. Dependence of the A D N yield on the cation of the carrier
electrolyte. Anion: tosylate. The solution contains 40% of AN, 26%
of H,O, and 34% of carrier electrolyte. The deposition potential is
based on a current density of 10mA/cm2.
Efficiency [ %]
Fig. 4. Dependence of the ADN yield on the pH. (-):
material yield,
current efticiency. Conditions: (A) 40% AN, 26% H,O, 34%
NR,X, mercury cathode. (0)55% AN, 16% H,O, 28% isopropanol,
1% NR,X, graphite cathode.
We now know that the pH can no longer be regarded as
critical in this electrosynthesis. During optimization, it
should be noted that a pronounced increase in pH may
- 1.80
[a] Calculated from experiments with low salt concentrations.
[*] In this case McKee salt is NEt, tosylate.
Anyew. Chem. internat. Edit. 1 Vol. I 1 (1972)
1 No. 9
excessively positive deposition potential of the cations,
since even Lie is reduced only at a potential about 0.5 volts
more negative than that at which acrylonitrile is reduced.
On the other hand, triphenylmethylphosphonium salts give
good material yields with only slightly depressed current
yields, though the cation is partly discharged on electrolysis.
These observations can be readily explained as "doublelayer effects". The ions of the supporting electrolyte not
only serve for the transport of current and possibly for the
solubilization of the substrate, but also produce many
important effects at the phase boundary, which are indicated schematically in Figure 5. They influence the course of
the potential in the rigid and diffuse parts of the double
layer, and hence indirectly the rate of the electrode reaction.
They can also compete as adsorbents for the substrate
molecule. As counterions to the ionic intermediates formed,
they influence the reactivity of the latter in the subsequent
reactions. Thus the ionic function of a radical-anion can
be so extensively shielded by a small cation (ion-pair
formation) that it behaves as a free radical, whereas with a
large cation the anionic function has a chance to act as
a nucleophile. Baizer recently studied these relationships
with the aid of cyclic ~ o l t a m m e t r y [ ~ ~ ' .
tration in the double layer. This in turn isdetermined by the
hydration sheath of the cations, so that larger quantities
of water reach the vicinity of the phase boundary in the case
of alkali metal ions, but practically none in the case of the
hydrophobic higher quaternary ammonium ions. This
.immediately explains the results in Table 1123,41,391.
Current-voltage measurements as a function of the concentration of quaternary ammonium salt clearly show that
the cations influence mainly the [ potential (Fig. 6a),
whereas they have practically no action as competing
ads or bent^[^^! The hydrodimerization proceeds in high
yields even at very low carrier salt concentrations; the
electrolysis can be carried out at industrial current densities with a carrier salt concentration of only 0.1% in special
cells with a short distance between the electrodesrz2* 391.
Even in these dilute electrolyte solutions, the carrier
salt cations are accumulated in the rigid part of the
double layer. However, acrylonitrile has also been electrolyzed in concentrated carrier salt solutions (e.g. 55 wt.%
of tetramethylammonium methyl sulfate)['31. In the presence of high concentrations of lyotropic salts, the separation of the components is complicated, and requires a
multi-stage extraction and distillation process[4z!
In the presence of a mixture of tetramethylammonium
and tetraethylammonium salts, the hydrodimer is formed
in higher yields than in the presence of either of the components alone[431. Hydrodimerization of acrylonitrile is
also possible in the presence of alkali metal salts such as
NaClO, or KSCN if the water concentration is kept
below 10% and the reaction is carried out in the presence
of high concentrations of aprotic solvents such as D M F or
DMS0'441. Under these conditions, the alkali metal ions
appear to be largely resolvated.
3.2.9. Convection
As in any electrolysis, convection must be so strong that
transport processes cannot become rate-determining.
Moreover, the diffusion boundary layer in front of the
electrode must not become too thick, since the phase
boundary concentration of acrylonitrile otherwise becomes
too low, and that of OHe ions too high (Fig. 6). Particularly
strong stirring is necessary in the emulsion procedure123q38],
in order to accelerate the mass transport at the AN/solution
Fig. 5. Salt effects at the phase boundary, schematlc. a) Influence
on the rate of the electrode reaction, b) adsorption, c) action of counterions, d) orientation and polarization, e), fJ action of water at the
phase boundary.
Moreover, an organic molecule with a dipole moment may
be oriented in the high electric fields at the phase boundary,
and the dipole moment may also be considerably increased
by polarization. Finally, the rate of the proton-consuming
steps is extremely strongly dependent on the water concenAngew. Chem. internat. Edit.
Vol. 11 (1972) N o . 9
Fig. 6. Diffusion layer in the hydrodimerization of acrylonitnle,
schematic. c : concentration; x: space coordinate.
and solution/electrode phase boundaries. The increased
power required for stirring in this case may be of the same
order of magnitude as the electrolysis power.
3.3. Mechanism
There has been a great deal of speculation on the mechanism of the hydrodimerization of acrylonitrile; however,
few measurements of kinetic value have so far been carried
out. Measurements at low acrylonitrile concentrations are
of limited value (polarography, cyclic voltammetry), since
only the hydrogenation occurs under these conditions.
Knunyants originally tried to explain his observations in
the reduction with amalgam by a free-radical mechanism
of the type (1) (see Schemes 2 and 3)["].
Exclusively free-radical mechanisms are also discussed in
an earlier article on electrochemical d i m e r i z a t i o n ~ [ ~ ~ ] .
Strong adsorption of the intermediates is suggested in an
attempt to explain why free-radical initiated polymerization does not occur.
Systematic current-voltage measurements with variation
of the principal parameters1261led to a result that could
not be reconciled with this free-radical mechanism, which
was also opposed by the fact that the yield of hydrodimer
is independent of the current d e n ~ i t y [ ~ ~Figure
~~'! 7
It follows from the slope of the "Tafel lines" of 120 mV,
assuming a value of 0.5 for the transfer coefficient[461,that
one electron is transferred in the rate-determining step.
Fig. 8. Reaction order v with respect to a) acrylonitrile and b) water [28].
Potential against saturated calomel electrode: - 1.750 V, 25OC
carrier salt: tetraethylammonium tosylate. a) 26% H,O, 34% carrier
salt, acetonitrile; b) 40% acrylonitrile, 34% carrier salt, acetonitrile.
These kinetic observations were later fully confirmed by
other workers1471.Since it is not possible, as was mentioned
earlier, to decide from the current-voltage curve whether
hydrogenation or hydrodimerization has occurred, the
slow step must be the same for both reaction paths. The
only possibility was
+@+ H2O
+ OH"
The initially formed radical-anion is thus simultaneously
protonated. Controversy exists regarding the site of the
protonation (or of the unpaired electron). LCAO calculations for the free radical-anion
that the protonation should occur on the p-C atom rather than on the
a-C atom; the a radical would then be stabilized by the
71 system of the nitrile group.
- 1600
Fig. 7. TafeI lines at various acrylonitrile concentrations [26].
Electrolyte (25"C, pH=O): x % AN, (40-x)% acetonitrile, 26%
H 2 0 , 343, NR, tosylate. Concentration of acrylonitrile: 0 40%,
20y0, A lo%, A 5 7 ~ I%, m 0 . 2 ~ .
shows a semilogarithmic current-voltage plot for a series
of measurements with solid cathodes, in which the acrylonitrile concentration was varied by more than two orders
of magnitude. At a constant potential, the current density,
i.e. the reaction rate at the phase boundary, increases with
increasing acrylonitrile concentration. The following relation is also valid at a constant potential in the electrochemical kinetics :
However, these considerations ignore the fact that the
reaction steps take place at the phase boundary, with the
dipole oriented as illustrated. The stabilization of the p
radical by the n-electron system of the metal should be
much more effective than the stabilization of the a radical
by the nitrile group (cf. Ref. [41]). The protonation of the
radical-anion in the a position is also indicated by the
argument that the proton migrates first to the position of
maximum basicity, with intermediate formation of
which then undergoes prototropic
rearrangement to form the p-C radical1"!
The cyanoethyl radical is rapidly reduced further at the
prevailing potential['I, so that the steady-state freeradical concentration is extremely low. This explains why
a free radical initiated polymerization does not occur.
where v is the reaction order with respect to acrylonitrile.
A corresponding evaluation (Fig. 8) gives a reaction order
vAN=Z 1. Similar measurements at various water concentrations also lead to a reaction order vHz0=Z 1.
If the cyanoethyl radical is produced by cathodic dehalogenation of
+ 0 -1" + .CH>--CH,-CN
the reduction proceeds at a much more positive potential [49].
Angew. Chem. internat. Edit.
Vol. I1 (1972) / N o . 9
The carbanion formed either combines with a second
acrylonitrile molecule to form the anion of the dimer or is
protonated to form propionitrile.
consists of dilute sulfuric acid. The industrial cells have a
cross section of about one square meter, and are of the
filter press type.
N C - C H - CHz-CHz - CII,- CN
% CH,
The anion of the hydrodimer is less reactive than the anion
of the monomer, since the negative charge is distributed
over the CL-Catom and the nitrile
are therefore formed only in systems with very low water
N C C T I - C Hz- CH, C H2- C N
Am CH, - CII, - c Il, - CN
--?2N C - CII, - CH, - CII, - C J3, - CN
- 0 1 1'
The first protonation step may even fail to occur at very
low water concentrations, an anionic polymerization being
initiated by the resulting radical aniont301.
The two other possible anionic mechanisms, starting from
the radical-anion [CH,=CH-CN]
or from the
dianion [CH2=CH-CN]2e[7* 1 3 * 2 3 , 5 2 , 5 3 1 d o not agree
with our kinetic findings. With increasing lifetime of the
radical anion, the probability of direct dimerization of this
species should increase. Some diactivated olefins form
relatively long-lived radical-anions, and appear to favor
this reaction path[541.Two possibilities are available a
free-radical mechanism of type (11) or an ionic mechanism
of type (5) or (6) (cf. Schemes 2 and 3); the free-radical
mechanism should be favored if the anionic function is
shielded by a small cation[401.Bardt2721found this mechanism for dimethyl fumarate ; in the case of acrylonitrile,
the lifetime of the intermediates is too short for the measuring methods used. In the cathodic hydrocyclization of
diactivated olefins, a "concerted" mechanism of type (5)
has been proposed, i.e. the electron transfer and the
formation of the C-C bond are closely coupledts5!
The mechanism via the dianion of the monomer, which
has beez considered by a number of authors (see above), is
relatively improbable for energetic reasons. However, the
two charges may move as far apart as possible after the
dimerization step.
3.4. Processes in Divided Cells
The first process in divided cells that allowed the direct
electrohydrodimerization of acrylonitrile to adiponitrile
in high yields was developed by M o n ~ a n t o [ ' A
~ ~diaphragm
was necessary, since lyotropic quaternary ammonium
salts (such as tetraethylammonium ethyl sulfate), which
undergo oxidative degradation at the anode, were used in
high concentrations. An example of a typical catholyte
composition is as follows: 40% of acrylonitrile, 34% of
tetraethylammonium tosylate, 26% of H, 0; the anolyte
Angew. CI.-..m.internat. Edit. 1 Vol. I 1 11972) No. 9
Figure 9 shows a diagram of a cell block. The (internally
cooled) bipolar electrodes consist of lead on the cathode
side and lead alloy on the anode side, the latter becoming
passive during operation through the formation of a
PbO, layer. An ion exchange membrane based on polystyrenesulfonic acid is used as the diaphragmt5']. Membranes of this type have been steadily improved since the
development of this industrial organic electrosynthesis,
so that they now have a satisfactory lifetimet5']. The
advantages over a porous diaphragm are less mixing of
the anolpte with the catholyte and an improved acid/base
balance in the cell. The catholyte and the anolyte are
circulated separately ; the Joule heat is eliminated internally cia the internally cooled bipolar plates. The entire
process was described in detail some years agot5'].
The plant in Decatur, Alabama (USA), has been in operation since 1965, and was expanded in 1969 (for economic
data see Ref. [59]). The industrial importance of the
process is underlined by a series of patents on technological
details such as the separation of the quaternary ammonium
salt from the material removed from the cellt421(the salt
must be recirculated as quantitatively as possible to
maintain economy). the production of the quaternary
ammonium hydroxide used for pH control in the
catholyte by electrodialysis'h"l, the preparation of quaternary ammonium saltst6'], the separation of by-products["],
the removal of traces of metal from the catholyte (these may
reduce the hydrogen overvoltage of the lead cathodes
during continuous operation)[631,the removal of oligomers
of a~rylonitrile'~~],
the extraction ofnitriles from the anolyte
by higher amines to reduce corrosion of the anode'"], the
improvement of the ion exchange membrane[661,and the
stabilization of the anode by lead alloys[671.
The Japanese company Asahi Chemical Industries, which,
like Monsanto, has large acrylonitrile plants, has also
developed a process having divided cells; a plant of this
type is soon to be put into operation["! The company's
experience in the field of ion exchange membranes proved
very useful in this d e v e l ~ p r n e n t [ ~The
~ l . main difference
from the Monsanto process is that the concentration of
dissolved acrylonitrile in the aqueous phase is kept low
( < 5 %)by nonlyotropicsaltssuch as tetramethylammonium
sulfate[24! Excess of AN forms an emulsion[z51.The
solubility of the reaction products in the organic phase is
good, while that of the salt is low, so that subsequent
treatment is relatively simple. A typical catholyte composition (aqueous phase) is as follows : 5 % of AN, 10%of salt,
85% of H20[Z41.Here again, numerous patent applications, e.g. on problems of working up[70*711
and on the
removal of metallic impurities by carbonate precipitati~n[~’],
are indicative of the industrial development.
The Badische Anilin- & Soda-Fabrik has proposed the
use of very low concentrations of quaternary ammonium
salts in homogeneous solutions[731.To be able to operate
with industrial current densities under these conditions,
special cells were developed, in which the liquid-permeable
electrodes (meshes, sieves, or expanded sheet) are m direct
contact with the ion exchange membrane. The low carrier
salt concentrations facilitate working up. An additional
solvent is used to homogenize the catholyte, a typical
composition of which is: 77% of AN, 10% of H,O, 10%
of DMF, 3% of tetraethylammonium tosylate. It would be
possible, in principle, to omit the use of carrier salts in
these cells ; however, low concentrations are favorable
because of the salt effects, which are already pronounced.
It has also been suggested[741that these salt effects should
be produced _by introduction of insoluble compounds
groupings into the pores of the graphite
CO,. The isolation of the product is greatly facilitated by
the small quantities of carrier salt.
The Belgian Company UCB has
that good
yields are also possible in the presence of Na or K salts (in
divided or undivided cells) if mixtures containing low
water concentrations are used. A typical reaction mixture,
which is treated at 20°C with graphite cathodes, has the
composition 39% of DMSO, 27% of AN, 26% of NaClO,
and 8% of H,O.
Despite the progress in the technology of ion exchange
membranes, an electrosynthesis is technically easier to
carry out in a diaphragm-free cell. The better the yields
and the qualities of the products, therefore, the more this
process will be able to compete with the processes discussed
in Section 3.4.
It has been shown that the supporting electrolyte is preferentially decomposed at the anode in the McKee system[”]. Cells with very small distances between the
electrodes ( d z 0 . 2 mm) in the form of liquid-permeable
stationary[751or vibratingI3’1 pairs of electrodes and
capillary gap cellsr2 enabled the electrolysis to be carried
out at BASF at industrial current densities, despite a
carrier salt concentration of only 0.5 wt.%. The product
was formed in yields of 90%, with current yields of 80%.
The cell voltages were so favorable that the energy consumption was less than 3 kWh per kg of product. A typical
electrolyte composition is 55 % of AN, 28.5 % of isopropanol, 16% of H,O, and 0.5% of tetraethylammonium ethyl
sulfate. The isopropanol completely protects the acrylonitrile, which is itself fairly stable, against oxidation at the
PbO, anode, and is itself oxidized to acetone, CO, and
Fig. 10. Capillary gap cell. 1) Bipolar circular electrode plates; 2 )
capillary gap; 3) bore in the center of the stack; 4) heat exchanger;
5 ] , 6 )current leads.
Figure 10 shows one form of the capillary gap cell. It
consists of a stack of circular graphite disks, which are
connected bipolarly in series. The anode side is covered
with PbO,. The distance between the disks is 0.2 mm. The
reaction mixture is pumped in a loop, and flows radially
through the capillary gaps. The Joule heat is removed in
an external heat exchanger. (The development of these
The isolation of the product is
cells is described in[z2,z71.)
simple, and consists essentially of distillation steps. Processes for the production of quaternary ammonium
hydroxide^"^], the improvement of the yields by the use
of carrier salt mixtures[431or by the addition of small
quantities of p ~ l y a c i d s [ ~and
~ ] the
, production of PbOJTi
composite electrodes[100] may be mentioned in this
Knunyants has proposed the electrolysis of AN on graphite
cathodes in aqueous sodium hydroxide s ~ I u t i o n [ ~The
cyanoethylation of water[781which, at 35 “C has a half-life
of 20 minutes at pH = 14 and of 40 hours at pH = 12‘791,is
strongly retarded by the use of an electrolysis temperature
of 0°C. The yield of ADN is less than 80%, even when a
neutral electrolyte (Na,SO,) is used[8o1.
Tornilou et aE.[381emulsified acrylonitrile in an aqueous
phosphate buffer (1 M K,HP04) containing approximately
1 % of quaternary ammonium salt. The emulsion was
pumped turbulently throvgh a cell with graphite cathodes
and magnetite-covered iron anodes. Despite the high
water concentration and the low acrylonitrile concentration, the dimer was formed in a yield of about 90%. The
process was developed further by the Belgian Company
Angew. Chem. internat. Edit.
Vol. I 1 (1972) 1 No. 9
UCB. The internal surfaces of a drilled graphite block were
used as graphite cathodes, the cylindrical magnetite anodes
being inserted coaxially in the drillings15‘1. The corrosion
of the anode was reduced by a factor of 10 on addition of
polyphosphates[8‘I. The isolation of the product appears
to be greatly simplified in this process and consists essentially of distillation of the organic phase. The electrolysis
temperature must be kept at the relatively low level of
18-20°C to minimize corrosion of the magnetite anode.
RhBne-Poulenc solved the problem of the supporting
electrolyte in the undivided cell by the use of quaternary
ammonium salts with anions that are stable at the anode[821.
Examples are sulfates, borates and carbonates.
Regarding amalgam processes in general, they have the
disadvantage with respect to the direct process that they
are two-step processes. Large quantities of dilute (0.1 %)
alkali metal amalgam must be circulated. The separation
of the alkali metal salt and of the mercury from the organic
compounds can be problematic. We shall not discuss here
whether these disadvantages can be offset by the advantage
that it is industrially easier to construct an amalgam reactor
than an electrolytic cell for a direct electrosynthesis. From
the point of view of mechanism, the direct synthesis at an
Hg cathode is substantially equivalent to the indirect
synthesis with Na amalgam.
3.7. Chemical and Catalytic Processes
3.6. Processes with Alkali Metal Amalgam
As was mentioned earlier, the industrial development i’n
this field was started by the pioneering work of
Knunyants[’o-’21,who was able to show that an approximately 10% solution of acrylonitrile in dilute mineral acid
can be hydrodimerized with Na amalgam to adiponitrile
in material yields of 65% and with “amalgam yields” of
35%. Though these results were not good enough to allow
the process to be carried out economically, they showed
the fundamental possibility of the reaction.
Katchalskii was later able to increase the yield to 75% by
the addition of polymerization inhibitors[s31.This value
was regarded for a long time as the maximum obtainable,
until it was shown in the middle 1960’s, mainly through
work carried out at ICI, that yields and current efficiencies
of more than 90% are quite feasible if the reaction with
amalgam is carried out under the following conditions:
buffering with CO, instead of with mineral acid[841,addition of quaternary ammonium or phosphonium salts[851,
of dimethyl sulfoxide, etc.[86],and of hexamethylphosph~roamide[’~].
Particularly good results were obtained
when solutions strongly diluted with acetonitrile were
used[881.It was suggested that the actual reducing agent
was not the Na amalgam but quaternary ammonium
amalgam formed as an intermediateIs9.901. The quantitative separation of the very finely divided sodium hydrogen
carbonate formed proved to be very difficult on the industrial scale. Elimination of Hg from the products was also a
problem. These investigations have not yet progressed
beyond the pilot-plant scale.
A series of further patent applications by other workers all
claim certain additives as “catalysts”. They probably all
act by producing an optimum water concentration in the
921, biuretrg3I,
double layer. Examples are dimethy1ureatg1~
and tetraalkylarsonium salts[961.It has also been suggested that
the reaction should be carried out in liquid ammonia[971
or in DMSO/H,O without buffering with acid, so that
alkali metal hydroxide is obtained at the same time198].
In a process developed by UCB to the semi-industrial
scale, a mixture of AN/H,O/formamide is allowed to
react with K amalgam[991.Dilute H,SO, is added for
buffering. The K,SO, formed appears to be easier to
separate than Na,S04, and is a commercial product.
Anyew. Chem. internai. Edit. 1 Vol. I 1 (1972) 1 N o . 9
Attempts have been made to carry out the industrially
important synthesis of ADN by catalytic hydrogenative
dimerization with the addition of molecular hydrogen.
Ru catalysts (at 100--200°C and 400-500 atm) appear
to be the most suitable, but the yields are still low at present
because of the predominant hydrogenation to form propionitrile[ioi. 1 0 2 1
The catalytic hydrogenation generally proceeds by a freeradical mechanism, and the surface concentration of the
‘04]. Its dimerization is
intermediate HX. is very
therefore unlikely. Ionic mechanisms have also been
suggested, e.g. for polar substances such as nitrobenzene
or q u i n ~ n e [ ” 1n61.
~ , Moreover, with decreasing coverage
with Had,the mechanism is reported to change into an ionic
mechanism[’071,in which dimerization should be more
The homogeneously catalyzed hydrodimerization of AN
in the presence of carbonylmetal compounds[’n8.‘ 0 9 ] , in
some cases with exposure to light[”’], also does not
proceed very selectively. The by-products include the
head-to-tail dimer 1-methyleneglutaronitrile.
The hydrogenolysis of the cyclic dimer 1,2-dicyanocyclobutane, which is formed in high yields in the thermal
dimerization of acrylonitrile to form adiponitrile has not
yet been achieved with satisfactory yields“ “I. The hydrodimerization with metals as reducing agents is of no
industrial importance, owing to the stoichiometric formation of heavy metal salts. Almost quantitative yields
have been obtained with Mn in DMF‘”’].
4. Preparative Aspects
Following the classification shown in Scheme 1, this
section presents the preparative aspects. Though it is not
possible to go into experimental details, the essential
conditions are briefly indicated, and the result is given, if
possible, as the current efficiency (quantity of product as a
fraction of the quantity to be expected according to Faraday’s law), which is more important to the electrochemist,
and the yield (based on the quantity of monomer that has
4.1. Activated Olefins[L'31
4.1.1. a,8-Unsaturated Nitriles
A number of results are shown in Table 2. Acrylonitrile
itself was discussed very fully in Section 3. Because of the
industrial importance of the reaction, this dimerization has
probably been investigated more thoroughly by many
authors than any other. It can nowadays be carried out with
almost quantitative yields, even under industrial conditions. In other reactions, which have not been so thoroughly
investigated, a poorer reported yield may be due to failure
to optimize. In comparison with the direct dimerization
on an Hg cathode investigated by Baizer and Anderson" 14],
the indirect amalgam method described by Knunyants['0- appeared to give much poorer yields. However,
these results have now been optimized and good results
are also obtained in the presence e.g. of quaternary
which gave sebacic dinitrile on hydrogenation of the two
. Polarographic
remaining C=C double bonds[1i9~1201
findings led to a different interpretation of the dimerization[i211,according to which 2,2'-dicyanobicyclobutyl
is formed.
Tetranitriles were formed by hydrodimerization of 1,4dicyano-I-butene [reaction (23)]"22. lZ3'.
1,2-Diactivated olefms exhibit a number of peculiarities.
Cinnamonitrile [reaction (24)] reversibly takes up an
electron on polarography in DMF to form a stable radicalanion['211, which, according to investigations by Baizer
et ul.[54b1,dimerizes mainly in the 1,2-position.
Table 2. Cathodic dimerization of a, @-unsaturated nitriles. The
numbers in brackets refer to the amalgam method.
Efficiency [%I
7 5 (0) 13
CH ,=C(CH ,)-CN
(CH ,),C=CH-CN
95 (65)
90 [a1
+ 1 _rbl_
+ 50 [b]
C,H ,-CH=C(C,H
15 iCI
[a] 2,2-linked product.
[b] 1,2-linked product.
[c] 15% of 2,2-linkage; main product: 2-amino-l,3,4,S-tetraphenyl-2cyclopentene-I -carbonitrile.
In the case of methacrylonitrile [reaction (18)], the dimerization does not proceed exclusively in the 2,2 position,
10% of 1,2dimerization also taking place["'! In the case
of crotononitrile [reaction (20)] this fraction is only 1%.
A patent has been granted for the hydrodimerization of
ally1 cyanide" 16];the reaction [ ( Z l ) ] is inhibited, since in
the presence of 10% of crotononitrile, only the hydrodimer
of the latter is formed[L171.
The hydrodimerization of
allylamine, which would lead directly to hexamethylenediamine, has not been achieved in H,O/tetraethylammonium tosylate on Hg cathodes" '*I. l-Cyano-l,3-butadiene,
the vinylog of acrylonitrile, has been dimerized via position 4 in good yields [reaction (2211 to form the dinitrile,
The substituents on the C=C double bond stabilize the
: C6H,
radical-ion formed in the following
<COOC,H, <CN<C,H,CO, so that this intermediate
is formed at potentials that become increasingly positive
in the above order. However, the species containing nitrile
groups are much more reactive than any others in the
subsequent reactions[54a1.Benzoylacrylonitrile dimerizes
exclusively in the 1,l position [reaction (26)]. I-Phenylcinnamonitrile gives the 2,2dimer (2,3,4, Stetraphenyladiponitrile) only as a by-product, while the main product
is the cyclization product, which occurs in the enamine
form [reaction (25)][1241.The course of the dimerization
again depends on the size of the c o ~ n t e r i o n [ ~ ~ ~ .
The nitrile group is generally stable to reduction; in
acrylonitrile, however, selective reduction to allylamine
has been observed under certain conditions (Pb, sulfuric
acid ~ o l u t i o n ) [1261.
' ~ ~ The
dimerization on the C=C
double bond appears to proceed best in the presence of
quaternary ammonium salts, possibly with the addition
of an auxiliary solvent.
Angew. Chem. inrernat. Edit. 1 Vol. 1 1 (1972) 1 No. 9
4.1.2. a, B-Unsaturated Carbonyl Compounds
Table 3 shows some representative examples of the hydrodimerization of a,P-unsaturated ketones, esters, acids,
and amides (for other ketones and aldehydes, see Section
4.2.). Though reduction of the C=O group cannot be
entirely ruled out here, coupling in the 4,4-position, corresponding to the normal scheme, occurs in most cases;
coupling in the 2,2 or even the 2,4 position occurs only to a
small extent. P~sternak['~~]
pointed this out at an early
date on the basis of polarographic results. The "pinacones"
found for these compounds in the earlier literature were
therefore 1,6-diketones in most cases.
Table 3. Carhodic dimerization of
Sirnonet has suggested the intermediate formation of Hg
compounds['30]. Divinyl ketone with Zn/acetic acid
yields 3,ELdecanedione as the main product, i. e. the second
vinyl group is hydrogenated['311. In the case of the analogous dibenzylideneacetone, two double bonds are
retained I 1 32.1331
&unsaturated carbonyl com-
(CH ,),C=CH-CO-CH,
up to 90
carried out in aqueous-alcoholic alkali metal acetate
buffer or with tetraethylammonium tosylate in acetonitrile/water. Under these last conditions, the diketone
partly cyclized to give a cyclopentene derivative.
In the past, the electrolysis was often carried out in acidic
solution, and followed a free-radical course. More recently,
the use of a neutral solution with quaternary ammonium
salts has again been found to be advantageous, but the
difference in the yields is much smaller than in the cases of
nitriles" 28]. Wiemann and Bouguerra, for example,
found['291that on reduction of methyl vinyl ketone on Hg
cathodes at approx. -1.4 V (against the saturated calomel
electrode), the diketone was formed as the main product
[reaction (27)], irrespective of whether the reaction was
The reductive dimerization of cyclohexenone [reaction
(29)]['34a1or of 4,4-dimethylcyclohexenone~'34b1
also proceeds predominantly by the 4,4 scheme, as does the hydrodimerization of mesityl oxide['30. 13' b- 135d1, which has
been known since the work of Law (1912)['35a1.Wiemann
and B ~ u g u e r r a [ ' ~were
' ~ ~ also able to isolate furan derivatives in large quantities. Benzylideneacetone [reaction
lZ71 and diben~oylethylene['~~~
are further examples.
Ethyl acrylate [reaction (32)](114, 136b1 and methyl
a ~ r y l a t e " ~have
~ " been hydrodimerized in high yields to
the esters of adipic acid. The dimerization of acrylic acid
has now also been achieved['37].The reaction of free cinnamic acid [reaction (34)] was achieved in good yields in
acidic aqueous alcoholic solution as early as 1943 by
Wilson and
1391. Coumarin, as an unsaturated
lactone, dimerizes almost quantitatively in the 4,4 position
[reaction (35)]1140*14'] . M t' ~ o n o [has
~ reported
~ ~ , ~on~ ~ ~
the dimerization of a,P-unsaturated aldehydes, which
proceeded relatively nonuniformly. The reaction of a,bunsaturated amides, such as diethylacrylamide [reaction
(36)]1"41, cinnamic acid dimethylamide [reaction
or cyclohexenecarboxamide['441,proceeded selectively in the 4,4 position.
Angew. Chem. mfernat. Edit.
1 Vol. I 1
(1972) 1 No. 9
An example of a vinylogous reaction is the reaction of
methyl I-naphthoate [reaction (38)]. In addition to the
1,4-dihydro compound, the 4,4 hydrodimer is formed on
reduction with Na amalgam['451.
4.1.3. a, P-Unsaturated Hydrocarbons
Unsaturated hydrocarbons are generally reduced only at
very strongly negative potentials. The radical-anions
formed are extremely strong bases, so that even when the
proton activity of the solvent is very low, protonation to
give the hydrogenation product is favored over dimerization. There are consequently only a few known examples
in this series (Table 4).
Table 4. Cathodic dimerization of a,punsaturated hydrocarbons
2 -C=C-R
Efficiency [ %]
[ 7;]
W ~ w z o n e k [ ' ~was
~ ] able to hydrodimerize stilbene in a
yield of 30% in DMF [reaction (39)] (but not in acetonitrile) on Hg cathodes in the presence of tetrabutyl-
ammonium iodide. The hydrodimerization of phenanthrene
was also achieved under similar conditions['47! B ~ i ~ e r ~ ~ ~ ~ 1
dimerized 9-benzylidenefluorenein accordance with reaction (41) in a yield of 66%; only 8% of hydrodimer could
be obtained with 2-phenyl-1,3-butadiene. 2-Vinylpyridine
and 4-vinylpyridine can again be converted into dimers in
high yields"49J. The electrolytic dimerization product of
N-vinylcarbazole is formulated as a cyclobutane derivative" "1.
4.2. Ketones and Aldehydes
The hydrodimerization of ketones and aldehydes, which
has been known for some time, leads to pinacols, i.e. derivatives of ethylene glycol. As can be seen from Table 5,
there is a considerable difference even in the yield between
the aliphatic and the aromatic series (for a, 0-unsaturated
carbonyl compounds see Section 4.1.2).
4.2.1. Aliphatic Ketones and Aldehydes
Despite many investigations, some of which were carried
out a long time ago, no clear picture has yet emerged
regarding the hydrodimerization of acetone. The reason
for this may lie in the large number of possible reaction
A free radical mechanism is certain on the basis of polarographic measurements, which have been carried out in
particular on aromatic ketones. A central intermediate is
the radical anion of the ketyl type, which is formed in
acidic solution at relatively positive potentials, and which
arises via e p or p e, depending on the pH value. A comprehensive description of the complicated situation has
Table 5. Cathodic dimerization of ketones and aldehydes
Pb/Sn, Pb/Cu, C(H,SO,)
Zn (NaOH)
Efficiency [ %]
CH ,-CH ,-CH 0
Yield [ %]
Angew. Chem. inrernat. Edit. 1 Vol. I 1 (1972) 1 N o . 9
2 I,?
2 0 7
S/otterbeck['591was able to obtain a material yield of 60%
with a current yield of 50% for short periods on copper
electrodes freshly electroplated with lead, but these values
decreased considerably after only one day. It was found['601
that with cathodes electroplated with zinc, the result
depends on the support; this may be due to orientation
been published by Zuman et a/.[61(see also[5.lZ7]).It has
been suggested that isopropanol is formed on Hg or Pb
mainly via the organometallic compound, since the quantities of the two by-products are interdependentCL5
"- 100
A characteristic feature is the extremely strong dependence
50 2n
of the results on the nature and state of the cathode material.
The most suitable materials for pinacol formation appear
10 20 30
to be Hg, Pb, Pb/Cu["'] and PbjSn alloys['521,which were
U, [ m V l
t [mtnl
recommended even in early patents['531. However, the
current efficiencies are poor, with values of 13%['51~'521;
Fig. 11. Electroreduction of acetone in aqueous sulfuric acid. Left:
the high values reported in the early publications could
current-voltage curves, right: voltage-time curves. (---) 1 N H,SO, ;
1 N H,SO, + 1 mol/l of acetone. Current density j = SO mA:cm2.
not be reproduced. Graphite is also ~ u j t a b l e [ '1551;
~ ~ * this
material has the advantage that the formation of organometallic compounds is impossible. A patent on the synthesis of pinacols with Na amalgam (80% material yield)
actually describes an electrolysis on graphite, since carbon
The electrosynthesis of pinacol was of industrial interest
is in simultaneous contact with the Na amalgam and with
for some decades, since it allowed direct access to 2,3the alkaline reaction mixture['561.A yield of 60% was also
dimethyl-l,3-butadiene, an intermediate for the profound on Hg in alkaline media['571.
duction of methyl rubber. The many early patents testify
the development work carried out. An original cell with
The strong interaction of acetone with the cathode can be
overlayered catholyte, in which no diaphragm is needed,
seen from Figure 11. The cathodic current-voltage curve
is the subject of one of these patents['"! Other aliphatic
in the base electrolyte has been recorded for several metals.
ketones such as methyl ethyl ketone [reaction (43)]['621,
Evolution of hydrogen is observed. After the addition of
ketone['631,diisopropyl ketone [reaction ((44)]
acetone at a constant current density, the potential is
[reaction (45)]['651 also give the pinacols
displaced in the negative direction in most cases, and the
evolution of H, stops. After a few minutes, a new steady-
state value is reached. This unexpected displacement of
the current-voltage curve, which has also been observed by
can be explained by strong adsorption of intermediates, which completely blocks the evolution of hydrogen and allows the further reactions of the
intermediates to proceed only at negative potentials.
Angew. Chem. internal. Edit. 1 Vol. I 1 (1972) N o . 9
This is also true of aliphatic aldehydes such as acetaldehyde
and propionaldehyde [reaction (46)]['661;
according to a
Russian patent"671,however, glyceraldehyde can be hydrodimerized in high yields to the hexitol [reaction (47)].
Glyoxylic acid (which can be obtained electrochemically
from oxalic acid) can be converted into tartaric acid['6B1.
4.2.2. Aromatic Ketones and Aldehydes
Table 6 Cathodic dimerizatlon of azomethines
In the aromatic series, the ketyls formed as intermediates
in the reduction are stabilized by the aromatic system. The
tendency to form organometallic compounds is much
smaller than in the aliphatic series, but the reactivity is just
high enough for the dimerization to proceed preferentially.
The ketyl of benzophenone can be readily detected and
followed kinetically by ESR
with the
rotating ring-disk electrode['70J, by cyclic voltammetry,
and even polarographically[17'I.
2 -C=NR
Almost quantitative yields are obtained for acetophenone
' ' ~ ] ,the
[reaction (48)][172-174J, p r ~ p i o p h e n o n e [ ' ~ ~ * and
higher alkylphenones['761. Stocker et u ~ . [ ' ~investigated
~ I
the stereoselectivity of the pinacol formation for acetophenone. In alkaline media, it was possible to increase
the DL/mCSO ratio to 3, irrespective of the cathode material.
Further examples are 2 - a ~ e t y l f u r a n ~ ' ~2-acetylthio~],
phene['78J, and 2- and 3-a~etylpyridine['~~~['],
which has
also been synthesized industrially as a pharmaceutical
intermediate['801. The same is true of p-hydroxypropiophenone[" 11,[182J,which has been dimerized industrially
to the glycol with a material yield of 50%.
Diketones, such as dimedone [reaction (51)][183J,1,3diphenyl-l,3-propanedione[reaction (5211[1841 and phenyl
g l y ~ x y l a t e [ ' ~are
~ ] also dimerized smoothly because of
the mutual activation of the CO groups. Keto groups in
crossed conjugated systems such as cyclohexadienone['861
or croconic acid [reaction (54)][1871(cf. Table 5) also react
to give the pinacol in high yields. Because of the double
activation of the CO group, the dimerization takes place in
the 2,2 position and not in the 44 position.
Benzaldehyde gives hydrobenzoin, which is also formed on
reduction of the product obtained by chemical condensation. The DL/meSo ratio can be as high as 3.25['881. With
vanillin as the starting material, the pure meso form of
hydrovanilloin is obtained for steric reasons['891. Salicylaldehyde also gives the p i n a c ~ l [ ' ~
~ ~yield
with increasing substrate concentration.
4.3. Azomethines
The cathodic hydrodimerization of azomethines leads to
derivatives of ethylenediamine (cf. Table 6). The hydrolysis
of the monomer must be suppressed by the use of low
temperatures and neutral or alkaline
NBenzylideneaniline gives medium yields on Hg cathodes
[reaction (56)]['921but almost quantitative yields on reduction with alkali
Asymmetric optical induction
was observed in the reduction of N-(wmethyl benzy1idine)benzylamine on Hg cathodes .in the presence of chiral
supporting electrolyte [reaction (57)]['941. On lead cathodes, the hydrodimer is formed above all in alkaline solut i ~ n [ ' ~ ' ]Quinazoline
is a cyclic compound with an activated C=N bond" 96J. The 4,4-dimer formed [reaction
(5811 can be readily cleaved again by oxidation. In the case
[*I For 2-acetylpyridine, unlike for acetophenone, the oL/meso
ratio is found to depend on the electrode material.
2 0
- 7I - N H R
of quinoxalones alkylated in position 3, the dimerization
occurs by formation of an N-N bond [reaction (59)],
which can also be readily broken again['97J.
4.4. Cations
4.4.1. Immonium Ions
The hydrodimerization of immonium ions is similar to the
hydrodimerization of azomethines. Andrieux and Sa~kunt['~~7
199J have investigated this field very thoroughly.
On discharge, free radicals are formed, and can be detected
by cy~lovoltammetry['~~~
and ESR spectroscopy[2001.In
most cases, e.g. in reaction (60),the primary electron transfer is irreversible, and a free radical mechanism leads to the
dimer. Cations highly substituted with aromatic groups
are reversibly discharged to form stable radicals. The
electrolysis must be carried out in acetonitrile owing to
the ease of hydrolysis of the immonium ions.
4.4.2. Pyridinium (Quinolinium) Ions
This type of cathodic dimerization is also similar to the
reaction with azomethines. Emmert was able to show that
in the reduction of pyridinium or quinolinium salts on
lead in sulfuric acid solution, tetrahydrobipyridyl or tetrahydrobiquinolyl is
the reaction proceeding
cia deeply colored radicals as intermediates. The dimerization takes place in the 4,4 and 2,2 positions, and to a
small extent in the 2,4 position. Tetrahydrobipyridyl can
be reduced further to the octahydro product, but can also
be oxidized e.g. with air to the dication; because of the
herbicidal activity of these compounds, both the reduction
of pyridinium salts with alkali metal amalgam or cathodic
Anqew. Chen?. internal. Edit. Vol. 11 (1972)
No. 9
reduction on lead in alkaline s o l ~ t i o n [ ’ ~ ~and
* ’ ~the
oxidation to the d i ~ a t i o n [ ’ ~have
~ ’ been subjects of recent
development work. By reversible uptake of an electron, the
dication can be reduced to a radical-~ation[’~~.
’ 0 6 ] . It has
also been obtained in one step, by reduction of 4-chloroor 4-cyanopyridinium salts ; the substituents are eliminated as anions[’07,’OS1.
R - NX- N
propenylium ion[218.’191 dimerize quantitatively when
discharged cathodically or with reducing agents.
4.5. Halogen Compounds
4.4.3. Other Cations
The reductive cleavage of quaternary ammonium ions was
discovered by Emmert[’091. Horner later constructed a
cleavage series for the groups removed[2101. Only the
hydrogenated product is formed in aqueous solution[’ lo].
The electrochemical reduction of halogen compounds can
lead, with elimination of halide ions, to dimeric products.
The frequently observed formation of organometakc
compounds points to a free-radical mechanism. The industrially important dimerization of p-chloropropionitrile
[reaction (67)] to form adiponitrile either with Na amalgam[220~2’11
or cathodically[”’~2231 is the subject of
several patents. One disadvantage is the chloride that is
always formed. The dimerizations of benzyl bromide[’241
and of nitrobenzyl bromide [reaction (68)][2251have also
been intensively studied. The latter reaction was carried
out in acetonitrile with tetraethylammonium perchlorate.
Table 8 Cathodic dimerization of halogen compounds
2o , R - R + 2 H a l o
Yield [“,J
In DMF” ‘‘I and hexamethylphosphoramide[2121,
on the
other hand, the formation of dimers was observed (cf.
Table 7). Ylides are also obtained with p h o ~ p h o n i u m - [ ” ~ ~
Table 7. Cathodic dimerization of ammonium, phosphonium, and
sulfonium ions.
~ _ _ _ -.
In earlier work[2261,the dimer of l,l-di-p-tolyl-2-chloroethane was obtained [reaction (70)] (cf. Table 8). P-Chloro
ketones give dike tone^[''^^.
4.6. Nitroaromatic Compounds
and sulfonium salts[”41. The cleavage of the dimethyl-pnitrobenzylsulfonium ion occurs at such positive potentials (0.7 V) that the nitro group is not attacked, and the
dimeric product p,p’-dinitrobibenzyl can be isolated” ”I.
4.4.4. Cyclic Cations
As can be seen from reactions (65) and (66), cyclic cations
such as the tropylium ion[’16s 17] and the triphenylcyclo-
Anqew. Chem. intrmat. Edit.
1 Vol. 11 (1972) 1 N o . 9
The classical reductive dimerization of aromatic nitro
compounds[’’*] to form azoxybenzene, azobenzene, and
hydrazobenzene derivatives (cf. Scheme 4) can be formally
regarded as a condensation of the intermediates. The best
yields are obtained in alkaline solution ; the intermediate
azoxybenzene can be intercepted in aqueous solution,
since it is sparingly soluble, and is therefore withdrawn
from the further reduction. However, as was shown by
Haber in his fundamental investigation[zz91,azoxybenzene
is also formed in alcoholic aqueous solution (with a current
yield of 76%) if the electro.lysis is carried out at a constant
potential ( U , =-0.84 V). Hydroxylamine reacts very
rapidly with nitrosobenzene to form azoxybenzene. An
electrochemical mechanism, e.g. in accordance with reactions (71) and (72), thus does not seem to be ruled out.
Trifluoronitrosomethane can be dimerized cathodicall^[^^^! The formation of an N-N bond via the hydrodimerization of a C=N bond was mentioned in Section 4.3.
4.7. Mixed Dimers with Identical Activated Groups
The mixed dimerization of two activated olefins or two
ketones A and B should lead preferentially to the mixed
dimer AB if the reaction is carried out close to the potential
of the more easily reducible component (A), with introduction of a large excess of the other component (B)[2331.In
most cases, an ionic mechanism appears to occur. A reacts
at the cathode to form an anionic intermediate (A",
AH' ), which reacts as a nucleophile with the acceptor
molecule B. Free-radical mechanisms cannot be ruled out,
e. g. in the case of the ketones.
Some characteristic examples are shown in Table 9. Ethyl
acrylate and acrylonitrile, whose reduction potentials
differ by only 0.1 volt, react to give the cyano ester in a
material yield of 60% [reaction (73)] at a cathodeL2". 2341,
or with sodium amalgam[2351.This electrolysis has also
In the coreduction
been carried out in undivided
of diethyl maleate and acrylonitrile, whose reduction
potentials differ considerably (0.5 V), Baizer made the
interesting observation that the yield of mixed dimer
increases rapidly as the potential becomes more negative
[reaction (74)] [2371. This observation has been confirmed
by other examples[2381.Even when the reaction was carried
out at the potential of the less easily reducible compound,
a large quantity ofmixed product was obtained. To explain
this, it is assumed that the reactivity of the acceptor increases
with increasing potential as a result of a field effect. The
concentration of the acceptor in the diffusion layer is also
much higher than that of the electroactive particle, which
can react even at the limiting current density. The specific
adsorption of the acceptor may also play some part.
C gH5 NH NH CeH5
Scheme 4. Cathodic dirnerization of nitroaromatic compounds,
with nitrobenzene as the example.
Hydrazobenzene has been produced on the industrial
scale with a current efficiency of 90% as an intermediate
for the production of benzidine. The divided cells were
fitted with steel cathodes covered with spongy lead[230-23z!
Other examples are the reaction of acrylonitrile with
and mesityl
oxide [reaction (76)][24'l; with the last of these combinations, several cyclic products are also formed[2421.With
9-benzylidenefluorene [reaction (77)][2431, acrylonitrile
acts as the donor; an interesting feature here is the double
addition, in which 9,1O-dicyanoethylbenzylidenefluorene
is formed.
Table 9. Cathodic codimerization of compounds with identical activated groups
1 I
2 H 0 2 0
x-c=c+ c=c-x'
Donor (A)
CH ,-CO-CH=C(CH,)2
Yield [ %]
Angew. Chem. internat. Edit. 1 Vol. I 1 (1972)
1 No. 9
In series of ketones, Allen mentioned a number of mixed
hydrodimerizations, e.g. of p-dimethylaminoacetophenone
and p-methoxyacetophenone [reaction (78)][2441. Nicolas
and Pallaud recently combined acetone with benzophenone [reaction (79)][2451.
4.8. Mixed Dimers with Different Activated Groups
Most investigations in this field are concerned with the
reaction ofacetone with an activated olefin (cf. Table 10).As
was shown earlier, acetone itself dimerizes only with poor
The intermediates formed on cleavage of quaternary
phosphonium ions [reaction ( 8 6 ) ] [ 2 5 "'5l , and tertiary
sulfonium ions[2571can add to acrylonitrile and similar
The mixed coupling of activated olefins with azomethines
or azo compounds leads to cyclic p r o d ~ c t s [ " ~ . ~ ~ ~ ~ .
A Japanese patent describes the mixed dimerization of
acrylonitrile and acetonitrile to form g l ~ t a r o n i t r i l e [ ~ ~ ' ~ .
Two examples that prove only on closer inspection to
belong to this section are the hydrodimerization of 1-
Table 10. Cathodic codimerization of compounds with different activated groups
Donor (A)
Acceptor (B)
Efficiency [ %]
Yield [ %]
C,H ,-CO-C,H
(CJ 1
,-CH ,-CN
yields. As early as 1965, Sugino and Nonaka reported
that acetone and acrylonitrile give the mixed hydrodimer
in good yields in sulfuric acid solution [reaction
Brown and Lister were able to detect a
free-radical mechanism by kinetic
even at U, = - 1.3 V, ketyls are formed, and these add to
the acrylonitrile.
acetylnaphthalene [reaction (87)][2601 and the hydrodimerization of methacrolein [reaction (88)][2h1!
In neutral solution in the presence of quaternary
ammonium salts, Baizer recently obtained the mixed
hydrodimer in a material yield of 85%[2381.Since unprotonated acetone is reduced only at UK= -2.4V,
the roles are reversed in this case.
Nicolas and Pallaud coelectrolyzed acetone with hydrocarbons such as styrene [reaction (Sl)], butadiene, and
cyclohexene, and obtained tertiary alcohols in moderate
yields[236,2491.Similar coelectrolyses of acetone with
and b e n ~ o p h e n o n e have
pyridine [reaction (82)][2501
been described. Reaction with ally1 alcohol leads to very
good yields of 4-methyl-l,4-pentanediol,
which is cyclized
dilute sulfuric acid
to 2,2-dimethyltetrahydo-
2 0
The mixed dimerization of acrylonitrile with halogen
compounds such as P-chloropropionitrile (+ adiponitrile)
[reaction (85)][220,2531
or chloro ketones[2541has already
been examined in detail.
A n g e n . Chem. internut. Edit.
1 Vol. I 1 (1972) 1 N o . 9
4.9. Cyclizations
In electrochemical cyclizations, any of the dimerization
reactions discussed so far can in principle proceed
intramolecularly if the steric conditions in the molecule
allow this. Some examples- are shown in Table 11; a
detailed review of this subject was published in 1969[2621.
Table 11. Cathodic cyclizations. R = alkyl
X ’I i
Efficiency [”/:I
Yield [ %]
investigation of the “heterodimerizations” is just beginning
(cf. Sections 4.7 and 4.8). The number of possible variations
is immense. The use of the principle of “coelectrolysis”,
in which at least two organic substrates are allowed to
react together in an electrolytic cell, one of these substrates
being converted at the electrode into reactive intermediates,
which react further with the second component, will lead
to many valuable preparative results in the future. Specific
electrochemical factors such as supporting electrolyte
effects, concentration profiles in the diffusion layer,
and adsorption will lead in this work to unexpected
products. Together with the undeniable technological
advantages of this reaction technique, this will help
greatly toward the advance of organic electrosynthesis.
Received: July 13, 1971 [A 897 IE]
German version: Angew. Chem. 84,798 (1972)
Translated by Express Translation Service, London
The ring closure can becarried out on some activated
bisolefins with high yields [cf. reaction (89) and
(90)][263-265! The pinacol formation between two keto
groups has also been used for cyclization [reaction
(91)][2661.It was similarly shown at a very early date that
the dinitro compound gives the cinnoline (N=N linkage)
in accordance with reaction (92)[2671.
The cathodic elimination of halogen has often been used
for ring closure reactions ; whereas dibromodimethylcyclobutane can be smoothly reduced to the bicyclic
compound (material yield 94%) in accordance with
reaction (93)[268-2701,
bicyclo[l.l .l]pentane is formed
from bromo(bromomethy1)cyclobutane [reaction (94)] in
a material yield of only 4%. The main product is 1,4pentadiene, which is formed by ring cleavage[271!
5. Concluding Remarks
Whereas the investigation of electrochemical “homodimerizations” has been brought to some sort of conclusion
through the rapid developments of the past ten years, the
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Organic Syntheses in the Plasma of Glow Discharges and Their
Preparative Application
By Harald Suhr[*]
It is only during the last few years that reactions of organic substances as a result of electron
collisions in the cold plasma of glow and corona discharges have been developed into a preparatively useful method with a wide range of possibilities. Its basic principles and development prospects are discussed in the present progress report on the basis of the research results
that are known at present.
1. Introduction
In the continuous search for new methods of syntheses, the
possibility of using the plasma[**]of spark, arc, and glow
discharges for this purpose was examined by chemists at
an early date[2][*"].However, many of the earlier efforts
met with only slight success. This is easy to understand
nowadays, since the early experimental techniques were
[*] Prof. Dr. H. Suhr
Chemisches Institut der Universitat
74 Tiibingen. Wilhelmstrdsse 33 (Germanv)
parti? ioniied gas.
[***I A large number of publications (cf.
[lg]) report successful
attempts to synthesize fairly complicated systems from very simple
starting materials and to break down or isomerize organic compounds
by plasma reactions. However, the present review deals only with those
that are closely concerned with preparative applications.
Angew. Cliem. internat. Edit. 1 Vol. I 1 (1972) 1 N o 9
unfavorable, and gave only low yields and nonuniform
reaction products. The organic compounds investigated
in this way decomposed in the plasma, largely with
formation of tarry or polymeric products. Despite the
numerous efforts to make preparative use of reactions
in plasmas, therefore, the only processes of this type
that have achieved any industrial importance are the
formation of ozone in silent discharges and the formation
of nitrogen oxides and of acetylene in an arc. Apart
from these, a few syntheses of inorganic compounds
on the laboratory scaler3"]and the modification of oils[3b1
have also been developed. It is only in the last few years
that organic syntheses on the preparative scale have
been successfully carried out in plasma. These results and
the possible applications of plasma chemistry with organic
compounds form the subject of the present report.
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