A change from rate-determining bromination to geometric isomerisation of pyridylhydrazones

Abstract

The rate-determining step in the bromination of aldehydic 4-pyridylhydrazones has been identified as isomerisation of the predominant E-isomer (in which the large groups are trans) to the more reactive Z-isomer. Substituent and temperature effects favour a transition state with considerable rotational character. The Z-isomer was isolated in one case (pyridine-2-carbaldehyde 4-pyridylhydrazone) and shown to brominate rapidly as expected. The change in rate-determining step from bromination to isomerisation in closely related hydrazones is discussed. Representative novel 4-pyridylhydrozonyl bromides, which were formed on bromination, were isolated and characterised.