Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

AZOXYBBNZBNB TO HYDKAZOBENZENE 189
In the case under discussion the nitrobenzene takes the place of the
oxygen. (Write the equation.) If the experiment is properly carried
out, the reduction is limited in this way to the phenylhydroxylamine
If the reduction takes place in an alkaline medium, products derived
from two molecules of nitrobenzene are formed. In these products
the unaltered portions of the nitrobenzene molecules are united by
means of nitrogen atoms. These substances are :
C6HS.N:N.C6H5 Azoxybenzene,
0
C6H5.N:N.C6H5 Azobenzene,
CgH5.NH.NH.C6H5 Hydrazobenzene.
The least powerful method of reduction, boiling nitrobenzene with
sodium methoxide in solution in methyl alcohol, provides azoxybenzene
in excellent yield (Zinin); the methoxide is converted into formate.
(Write the equation.)
Since azoxybenzene is attacked by more powerful reducing agents,
e.g. zinc dust and sodium hydroxide solution or ammonia, the use of
such agents converts nitrobenzene to azobenzene and hydrazobenzene,
by passing at once beyond the azoxybenzene stage. The three reduction
products with " paired " nitrogen atoms, therefore, stand in very
close genetic relation to each other.
Experiment. Reduction of Azoxybenzene to Hydrazobenzene.—
Azoxybenzene (1 g.) is dissolved in 5 c.c. of alcohol, the solution is
heated to boiling, and 3 c.c. of 50 per cent sodium hydroxide solution
and 2-3 g. of zinc dust are added with shaking. At first the mixture
becomes red, because of the formation of azobenzene, but on more
prolonged boiling a colourless solution is obtained just as in the
reduction of nitrobenzene. When this stage has been reached, the
mixture is filtered with suction through a small Biichner funnel and
the hydrazobenzene is finally isolated in the manner described on
p. 183 et seq.
Thus the joining of the two molecules by nitrogen takes place when
azoxybenzene is formed, and the experiment described on p. 182 shows
quite definitely that this substance is produced with extraordinary ease
from phenylhydroxylamine and nitrosobenzene in the presence of
alkali, that is to say, under the conditions which prevail during the production
of the whole series. Nitrosobenzene is the first stage, but cannot
be isolated, for in the course of the reaction it is trapped by the phenylhydroxylamine
as soon as it is formed.
An explanation is thus provided of the otherwise puzzling formation