Benzidine is the trivial name for 4,4'-diaminobiphenyl, a carcinogenic aromatic amine which has been used as part of a test for cyanide and also in the synthesis of dyes. It has been linked to bladder cancer and pancreatic cancer.

In common with benzidine some other aromatic amines such as 2-aminonaphthalene have been withdrawn from use in almost all industries because they are so carcinogenic.

In the past a common test for blood used benzidine but this has largely been replaced by tests using phenolphthalein / hydrogen peroxide and luminol. An enzyme in blood causes the benzidine to be oxidized to a polymer which is blue coloured. The test for cyanide uses similar chemistry to give the blue colour.

Benzidine rearrangement

In a classic rearrangement reaction called the benzidine rearrangement benzidine is formed by reaction of the 1,2-diphenylhydrazine (PhNHNHPh) with acids. One plausible reaction mechanism has both hydrazine nitrogen atoms protonated after which a sigmatropic reaction takes place.

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