Mechanism of benzidine and semidine rearrangements. Part XXI. Kinetics and products of the acid conversion of 2,2′-dibromo- and of 4,4′-dibromo-hydrazobenzene

Abstract

Over the range of concentration of perchloric acid in which the acid conversion of 2,2′-dichlorohydrazobenzene is strictly of second order in hydrogen ions, that of 2,2′-dibromohydrazobenzene shows transitional kinetics. It has an order which rises from about 1·2 to 1·9 as the acid concentration is raised from 0·2 to 2·0N. Analysis of the rate curves shows that the rate is additively composed from rates of concurrent independent reactions, one of first order, and one of second, in hydrogen ion. The two-proton rate constant is smaller by a factor of 50,000 than that for hydrazobenzene. The one-proton constant is smaller by a factor of several thousands than the smallest comparable one-proton rate constant of rearrangement, that of 2,2′-dimethylhydrazobenzene. This is the first demonstration of the promotion of the one-proton mechanism of rearrangement by deactivating aromatic substituents. The phenomenon apparently depends on the 2,2′-orientation of the substituents, for the acid-conversion of 4,4′-dibromohydrazobenzene in similar conditions is strictly of second order in hydrogen ions. The compositions of the products of the acid conversions of 2,2′- and 4,4′-dibromohydrazobenzene are closely similar to those of the 2,2′- and 4,4′-dichloro-analogues respectively.