Kinetics of geometric isomerisation about azomethine bonds. Part II. Bromination of NN-disubstituted hydrazones

Abstract

The bromination of an NN-disubstituted N′-arylmethylenehydrazines (in 70% acetic acid at 20°) exhibits zero-order kinetics, the rate-determining step being a syn–anti isomerisation of the hydrazone. The kinetics of bromination are independent both of the concentration and of the nature (Br₂ or Br₃^–) of the brominating species. Substituents (m- and p-) in the aryl ring have a small electronic effect on the rate of isomerisation (ρ– 0·29), while large substituents ortho in this ring retard the rate (δ 0·145). The electron-withdrawing acetyl and benzoyl groups as substituents on the amino-nitrogen atom cause eleven- and twenty-eight-fold increases in rate (relative to the methyl-substituted hydrazone). Contrary to earlier observations, methine bromination does occur and several novel hydrazonyl bromides have been isolated and characterised by conversion into hydrazides.