Mechanism of benzidine and semidine rearrangements. Part XXVI. Solvent isotope effects and conclusions concerning the nature of the proton transfers

Abstract

The solvent isotope effect (k_D2O/k_H2O) for the rearrangement of 2,2′-dimethoxyhydrazobenzene that is of the first order in acid at convenient acidities is 1·9. This suggests, in contrast to conclusions drawn from volumes of activation, that pre-equilibrium protonation occurs. The value for the one-proton catalysed rearrangement of 2,2′-di-iodohydrazobenzene is 2·2.

The two-proton catalysed reactions of 2,2′-dichloro-substituted hydrazobenzenes give values (2·0, 2·3) considerably lower than those previously reported for rearrangements of this order in acid, whereas the value for the 4-chloro-compound (3·0) is intermediate. Evidence that the second proton is transferred in an equilibrium step is not clear-cut in these examples.