Kinetics and mechanism of the formation of hydrazides from hydrazidic halides

Abstract

In aqueous dioxan or acetone, N-aryl-hydrazidic bromides (α-bromoarylidenehydrazines) are converted into the corresponding benzhydrazides. The kinetics of the conversion have been studied in dioxan–water (4 : 1) at 25° and a Hammett ρ value of –0·63 was obtained for variation of the N-aryl substituent. The observation of a primary salt effect, a large common-ion effect, and an m value of 0·89 for solvent variation support a mechanism involving rate-determining ionization (azocarbonium ion formation). The solvolysis of an NN-disubstituted hydrazidic bromide (which unlike the mono-substituted analogues cannot form a 1,3-dipolar ion) was studied in buffered solution; its reactivity paralleled that of the mono-substituted compounds. Steric inhibition of azocarbonium ion formation is marked since cyclic hydrazidic bromides (e.g. 3-bromo-2-pyrazoline) failed to react in aqueous solvents even under forcing conditions.