Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

CONDENSATION OF NITEOSO-COMPOUNDS 181
Aldehydes react with, primary amines to give the so-called azomethines
(Schiff's bases), by elimination of water (p. 167), e.g.
C6H5.C:O + H2N.C6H5 —> C6H5.C:N.C6H5 + H2O .
H H
Benzylideneaniline
Similarly, nitrosobenzene and aniline give azobenzene :
C6H5.NO + H2N.C6H5 —• C6H5.N:N.C6H5 + H2O.
Experiment. 1 —Nitrosobenzene (1 g. in 10 c.c. of alcohol) is added
to a solution of 1 c.c. of aniline in 3 c.c. of glacial acetic acid. On
gentle warming the colour changes to dark orange. After heating
for ten minutes longer on the boiling water bath, a few cubic centimetres
of water are added. On cooling the solution azobenzene
crystallises in orange-red platelets. Washed on the filter with 50
per cent alcohol and dried on porous plate, it melts at 68°. Azobenzene
can very readily be recrystallised from little alcohol.
In this way mixed (asymmetrical) azo-compounds can conveniently
be prepared in good yield. Prepare ji-methylazobenzene, for example,
from nitrosobenzene and y-toluidine according to the precedure given
above.
Aldehydes condense with compounds containing a reactive methylor
methylene-group to form unsaturated ketones, e.g.
C6H5.C:O + H3C.CO.CH3 —> C6H5.C:CH.CO.CH3 .
H H
Benzylideneacetone
An analogous reaction is known with aromatic nitroso-compounds, but
for it an exceptionally mobile hydrogen atom must be present in the
ketone and hence no condensation occurs with simple ketones such
Us acetone. The products of the reaction are, of course, azomethines.
This condensation has made possible the synthesis of 1 : 2 : 3-triketones
(F. Sachs), e.g.
CH3.CO.CH2.CO.CH3
Acetylacetone
CH3.CO
I ,—,
.N(CH3)2 + H2O
CH3.CO.CO.CO.CH3 + H2N./ \N(CH3)2 .
Triketopentane —
1 A. Baeyer, Ber., 1874, 7, 1638.