With diazotized benzidine it gives Congo red.
See Benzidine for diparadiaminodiphenyl.
This reaction is known as the benzidine transformation.
On the constitution of benzidine see G.
The Benzidine and Semidine Change.-Aromatic hydrazo compounds which contain free para positions are readily converted by the action of acids, acid chlorides and anhydrides into diphenyl derivatives; thus, as mentioned above, hydrazobenzene is converted into benzidine, a small quantity of diphenylin being formed at the same time.
In this latter case hydrazo-benzene C 6 H 5 NH NH C 6 H 5 is first formed and then undergoes a peculiar re-arrangement into benzidine (see H.