Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

182 AZOXYBENZENE
The last phase of the reaction depends on the fact that azomethines
are easily decomposed by acids into carbonyl compounds and primary
base.
The practical effect of the condensation consists, therefore, in the
conversion of methylene into >C=O. The same result is attained in a
quite similar reaction by the action of nitrous acid on ketones (cf. the
synthesis of diacetyl from methylethyl ketone).
Finally, nitrosobenzene reacts with Grignard reagents. With phenylmagnesium
bromide, in the usual way there is produced diphenylhydroxylamine,
an exceptionally reactive substance :
C6H5.N:O + Br.Mg.C6H5.
C6H5.N.C6H5 + MgBr(OH).
OH
Diphenylhydroxylamine, like phenylhydroxylamine, can best be dehydrogenated
with silver oxide. Here only one H-atom, that from
the OH-group, can be removed and the red crystalline substance which
is thus produced contains quadrivalent nitrogen. Like nitrogen peroxide,
therefore, this dehydrogenation product reacts like a free radicle. As
its formula indicates, it is derived from nitrogen peroxide by the substitution
of two C6H5-groups for one 0.
H5C6s
^N=0 Diphenylnitrogen oxide.
HC/
Experiment.—Azoxybenzenefrom phenylhydroxylamine and nitrosobenzene.—Phenylhydroxylamine
(1 g.) is added to a solution of 1 g.
of nitrosobenzene in 10 c.c. of alcohol. The mixture is shaken
while a few drops of concentrated potassium hydroxide solution
(1 : 1) are added, and is then wanned on the water bath for a few
minutes. The yellowish-red solution thus formed deposits yellow
crystals of the reaction-product when cooled and rubbed with a
glass rod. Since azoxybenzene melts at 36°, it has a great tendency
to separate from a supersaturated solution in the form of an oil.
By recrystallisation from a little alcohol or from petrol ether (retain
a few crystals for inoculation) the compound is obtained as a pale
yellow or almost colourless solid.
The feeble colour of azoxybenzene, in contrast to the red of azobenzene,
would appear to be more readily intelligible on the basis of the
old formula, I, than on that by which Angeli has replaced it, II: