Halogenation of Hydrazones

The halogenation of the arylhydrazones of arylaldehydes is an excellent method of synthesis of hydrazidoyl halides. However, halogenation of the ring attached to nitrogen can not be avoided. For example, reaction of benzaldehyde phenylhydrazone (XI) with bromine in glacial acetic acid 

The mechanism of the bromination of arylhydrazones has been recently investigated by Hegarty and Scott ( ). 

Likewise, chlorination of benzaldehyde o-tolylhydrazone yields the hydrazidoyl chloride XIV and finally the N-chloro derivative XV, which rearranges upon heating in glacial acetic acid to afford the hydrazidoyl chloride XVI (i9.28.s4) 

The Chattaway-Adamson rearrangement of the N-chloro derivatives was recently investigated by Gibson 

If the phenyl ring attached to nitrogen is deactivated, bromination to the hydrazidoyl bromides without ring bromination can be achieved For example, benzaldehyde 4-nitrophenylhydrazone (XVII) is brominated in glacial acetic acid to afford the corresponding hydrazidoyl bromide (XVIII) 

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