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the conversion of hydrazobenzene (I) by the action of dilute mineral acids into 4,4’-diaminodiphenyl, or benzidine (II); one of the types of intramolecular rearrangement. A small amount of 4,2’-diaminodiphenyl, or diphenyline (III), is formed simultaneously:
In the naphthalene series benzidine rearrangements can take place in the absence of acids. The benzidine rearrangement is realized in that case if the hydrazobenzene contains various substituents in the benzene nuclei. However, if there is a substituent in the para position, the result is a semibenzidine (or semidine) rearrangement with the formation of aminodiphenylamine:
The benzidine rearrangement is widely used in the production of azo dyes and to obtain tolidine, dianisidine, and so on.