Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

188 KBDUCTION OF NITKOBBNZBNB
manufactured on a large scale in the dye industry, as important intermediates
for the preparation of substantive azo-dyes (which dye cotton
directly, cf. in this connexion pp. 300, 302).
H2N
CH3 or OCH3
On the Mechanism of the Reduction of Nitrobenzene
The reduction of aromatic nitro-compounds is of exceptionally great
interest, not only scientifically, but also technically. The conversion
of the hydrocarbons of coal tar into useful products began with the
discovery of the nitration process; the conversion, on the technical
scale, of the nitro-group of nitrobenzene into the amino-group gave
aniline, the starting material for the preparation of innumerable dyes
and pharmaceutical products; to aniline were added the homologous
toluidines, xylidines, naphthylamines, and so on.
The production of aniline from nitrobenzene proceeds in such a way
that the reactive hydrogen is added to the nitro-group, the oxygen is
eliminated as water, and finally hydrogen is again added on. The process
is not a simple one and involves a series of intermediate stages :
CHS.N^ 4 2H
H
^OH
•^1 +2H
^ C « H - N \OH ^i£ QANH,.
/8-Phenylhydroxylamine
Under the conditions prevailing during the production of aniline
neither nitrosobenzene nor phenylhydroxylamine is encountered. The
reason for this is that the rate of reduction of these intermediate products
is much greater than that of the nitrobenzene itself (F. Haber).
In neutral or alkaline solution the conditions are altered so as to
favour the immediate precursor of the final product of hydrogenation,
namely, phenylhydroxylamine. This compound is obtained from nitrobenzene,
suspended in ammonium chloride solution, by reduction with
zinc dust. Zinc dust can decompose water with the formation of
Zn(0H)2 if a substance is present which takes up the liberated hydrogen.
Molecular, i e. ordinary, oxygen is capable of doing this and is thereby
converted into hydrogen peroxide (M. Traube) :
—>• Zn(OH)2 + H2O2.