The mechanism of bromination of heterocyclic hydrazones. syn–anti-Isomerisation of 5-(arymethylenehydrazino)-1- and -2-benzyltetrazoles

The rates of bromination (in 70% acetic acid at 25°) of a series of 5-(arylmethylenehydrazino)tetrazoles, benzylated in the heteroaromatic ring, are independent of the bromine concentration. The slow step is a syn–anti-isomerisation of the hydrazone. A mechanism involving rotation about the CN bond is favoured in the light of data on the effect of substituents on the rate of isomerisation. The products formed on bromination are hydrazonyl bromides and the replacement of the labile bromine by external and internal nucleophiles is reported.