The kinetics of nitrilimine formation in base-catalysed hydrolysis of hydrazonyl halides

Abstract

The formation of 1,3-dipolar ion and carbonium ion intermediates in the solvolysis of hydrazonyl halides is competitive, and the major reaction pathway followed depends on the pH of the medium. The kinetics of hydrolysis of alkyl- and aryl-substituted hydrazonyl bromides to hydrazides has been studied in 70% dioxan at 25° as a function of pH. At high [H⁺], the pH-independent reaction was identified as unimolecular dissociation to form a carbonium ion intermediate. At higher pH, the base-catalysed process was loss of bromide ion from the hydrazonyl bromide anion to give a 1,3-dipolar ion. Electron-donating substituents, attached either to the hydrazonyl carbon atom or the hydrazone nitrogen atom, aid carbonium ion formation. On the other hand, opposing effects are operative in 1,3-dipolar ion formation, which is favoured by electron-withdrawing substituents attached to the nitrogen atom and by electron-donating substituents attached to the carbon atom. With a strongly withdrawing aromatic N-substituent [e.g. 2,4-(NO₂)₂·C₆H₃] the base-catalysed pathway is dominant even at pH 3. With poorer leaving groups than bromide ion (such as ethoxide or phenoxide) the hydrazonyl anion is relatively stable, but with leaving groups (such as NO₂^– and N₃^–) of intermediate leaving ability the rate of elimination from the anion could be measured. The pH–rate profile of α-nitro-N-(p-nitrophenyl)benzaldehyde hydrazone (leaving group NO₂^–) was studied, giving k₁(rate of decomposition of the anion)= 1·35 × 10^–2 s^–1 and K_a(acidity of the α-nitro-hydrazone)= 10^–10.