Azobenzene from aniline and nitrosobenzene

In this way mixed (asymmetrical) azo-compounds can conveniently be prepared in good yield. Prepare p-methylazobenzene, for example, from nitrosobenzene and p-toluidine according to the procedure given above. 

Aldehydes condense with compounds containing a reactive methyl-or methylene-group to form unsaturated ketones, e.g. 

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. 

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=0. 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). 

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