Russian Journal of General Chemistry, Vol. 73, No. 5, 2003, pp. 827 828. Translated from Zhurnal Obshchei Khimii, Vol. 73, No. 5, 2003,
pp. 873 874.
Original Russian Text Copyright
2003 by Lisitsyn, Kargin.
LETTERS
TO THE EDITOR
Ratio between Mono- and Disubstituted Products
of Electrochemical Amination of Aromatic Substrates
Yu. A. Lisitsyn and Yu. M. Kargin
Kazan State University, Kazan, Tatarstan, Russia
Received January 8, 2002
The indirect cathodic amination of aromatic sub-
strates with the system Ti(IV) NH2OH is expedient
to perform in concentrated sulfuric acid solutions
([H2SO4] > 7 M) [1 3]. In highly concentrated H2SO4
media, side reactions of aminocyclohexadienyl ra-
dicals are suppressed and the concurrent amination
reaction yielding ammonia is decelerated. The final
products of the radical-cation amination in such
conditions are aromatic diamines.
aniline (Yac) and isomeric phenylenediamines (Ypch),
on the basis of consumption one electron per one
hydroxylamine molecule, are 19.7 and 23.9%, res-
pectively (Yac/Ypch = 0.82).
Introduction of an organic solvent in the elecrolyte
much enhances the overall efficiency of the substitu-
tion reaction. At the same time, whereas the yield of
aniline increases with increasing concentration of the
organic solvent, the dependences of the total yield of
o- and p- phenylenediamines on the concentrations of
acetic acid and acetonitrile show maxima in 0.5 and
1.7 M solutions (Ypch 35.8 and 42.5%), respectively.
Therewith, as the fraction of the solvent in the
catholyte increases, m-phenylenediamine appears
among the reaction products. Thus, for instance, the
In the present work we used the example of electro-
chemical amination of benzene to demostrate a pos-
sibility for selective synthesis of not only di-, but also
monoamino derivatives in highly concentrated sulfuric
acid solutions.
The electrolysis of the Ti(IV) NH2OH C6H6
system was accomplished in a three-electrode glass
electrochemical cell with the cathodic and anodic
compartments separated by a ceramic diaphragm. The
catholyte, 11 M aqueous H2SO4 containing 0.1 M
titanium(IV) sulfate, 0.2 M hydroxylamine, and acetic
acid or acetonitrile, was 25 ml in volume. The sub-
strate was a highly dispersed emulsion of 10 ml of
benzene in the catholyte, purged with argon. Tita-
nium(III) was generated on a mercury cathode at a
current density of 6 mA/cm2 and 40 C. The quantity
of electricity consumed in the amination reaction was
250 C.
current yields of phenylenediamines in
5
M
CH3CO2H and 5.5 M CH3CN are 25.5 and 18.8%,
respectively, and the Yac/Ycph ratios attain 4.4 and 9.1
(since the electrochemical amination in these media
occurs by a chain mechanism, the current yields of
aniline and the total yields of mono- and diamino
derivatives are higher than 100%), i.e. aniline
becomes the major product of the radical-cation sub-
stitution at already moderate concentrations of the
solvents.
The reduced yields of isomeric phenylenediamines
in sulfuric acid solutions containing acetic acid or
acetonitrile are probably explained by the well-known
ability of these solvents to solvate cations and anions,
as well as by the fact that the concentration of basic
species B (B = H2O, HSO4, SO24 ) decreases as the
solvent concentration in the catholyte increases. The
presence of CH3CO2H or CH3CN in the solution
prevents formation of associates C6H5NH2 HB whose
amination gives rise to o- and p-phenylenediamines
[2].
After electrolysis, the catholyte was cooled and
neutralized by successive treatment with saturated
aqueous sodium hydroxide and NaHCO3. The amina-
tion products were extracted with benzene and ana-
lyzed on a Chrom-4 chromatograph with a flame-
ionization detector and a glass column [2500 3 mm,
5% XE-60 on Chromaton N-AW-DMCS (0.160
0.200 mm)] operated at 150 C.
In the absence of an organic solvent, the electro-
chemical amination of benzene gives aniline and o-
and p-phenylenediamines. The current yields of
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