The effect of pressure on the rate of the benzidine rearrangement V : 2-chloro-2'-methylhydrazobenzene

Abstract

The effect of pressure on the rate of the acid-catalyzed rearrangement of 2-chloro-2'-methylhydrazobenzene in 85 vol% aqueous ethanolic solution was studied under the conditions of 15～30℃ and 1～1, 500kg/cm^2. As a result, it was observed that two different reactions concurred ; one was of first order with respect to [HCl] (one-proton mechanism) and the other was of second order (two-proton mechanism), and the former was fairly, while the latter was slightly accelerated by pressure. Based on the activation parameters obtained, i.e. E≠=22.2kcal/mole, ΔS≠=-1.6 e.u., ΔV≠=-6.8cm^3/mole for one-proton mechanism and E≠=18.0kcal/mole, ΔS≠=-12.4 e.u., ΔV≠=-3.2cm^3/mole for two-proton mechanism, the reaction mechanisms and the transition states were discussed. The existence of a linear relationship between the logarithms of the rate constants of the rearrangement and pKa values of ortho-substituted anilinium ions was suggested, by correcting pKa, for the steric effect of ortho-substituents.