The Hell-Volhard-Zelinsky reaction produces only bromocarboxylic acids. However, modifi ed procedures have been developed to convert acyl chlorides into a-chloro- and a-bromoacyl chlorides by reaction with N-chloro- and N-bromobutanimide (N-chloro- and N-bromosuccinimide, NCS and NBS), respectively. Reaction with I₂ gives α-iodoacyl chlorides. (SOCl₂ is used as the solvent to maintain the acyl chloride functional group.) Suggest a mechanism for any one of these processes.

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NCS, HCI, SOCI, 70°C C,H5CH,CHCICOCI 84% NBS, HBr, SOCI, 70°C C,H;CH,CH,COCI C,H;CH,CHBrCOCI 71% I3, HI, SOCI, 85°C → C,H;CH,CHICOCI 75%