Conversion of azobenzenes into N,N'-diarylhydrazines by sodium dithionite

A number of chloro-, methyl- and methoxy-substituted azobenzenes have been reduced to the corresponding hydrazines by using an aqueous solution of Na2S2O2. The yield is generally excellent. but two compounds, viz. 4,4-dimethoxyazobenzene and 2,2,4,4,6,6-hexamethylazobenzene, gave no hydrazine at all.