The benzidine rearrangement hydroazoaromatics

It has been long known23 that hydrazobenzene (VII) is converted in acid solution into benzidine (VIII), which is the main product, and diphenyline (IX), viz. 

Later it was shown24 that ortho and para semidines (X and XI) could also be formed in this reaction together with oxidation and disproportionation products 

Similar experiments on the analysis of the products from the three isomeric hy-drazonaphthalenes30 and substituted hydrazobenzenes31 confirm the intramolecu-larity of the reaction. In addition, products resulting from attack on the solvent of fragments from a hydrazo-compound during rearrangement have never been detected30. 

Subsequently many other substituted hydrazobenzenes were shown to have a non-integral acidity dependence between 1 and 2, for example with 4-chloro 4 -methylhydrazobenzene (XXII) the order was 1.6. Further, the order in acid was found to vary for any one substituted hydrazobenzene substrate, increasing 

It is important for acid-catalysed reactions to determine whether the reaction is specifically catalysed by hydrogen ions or whether general acid catalysis takes place. Specific acid catalysis has been conclusively demonstrated for the benzidine rearrangement by three different sorts of kinetic experiments. In the first, it has been shown41 by the standard test for general acid catalysis (by measuring the rate of reaction in a buffered solution at constant pH over a range of concentration 

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