Hell-Volhard-Zelinsky Halogenation

The Hell-Volhard-Zelinsky halogenation reaction halogenates carboxlyic acids at the alpha carbon.  Treatment with bromine and a catalytic amount of phosphorus leads to the selective α-bromination of carboxylic acids. PBr₃ replaces the carboxylic OH with a bromide, resulting in a carboxylic acid bromide. The acyl bromide can then tautomerize to an enol, which will readily react with bromine to brominate a second time at the α position. 

Now that I have introduced what the HVZ halogenation reaction is, let's examine a specific example. This reaction was used in the invention of Cycloalkane Carboxaldoxime Carbamates, which are used as pesticides. The compounds can be prepared via the alpha halogenation of cyclic carboxylic acids under slightly modified Hell-Volhard-Zelinsky conditions (Step I) followed by esterification to give the cyclic alpha halocycloalkanecarboxylic acid ester. The active halogen is easily displaced with sodium alkyl mercaptide in alcohol to yield the alkylthio ester. Hydrolysis of the ester followed by reacting the hydrolyzed product with thionyl chloride gives the reactive acid chloride which is transformed into the aldehyde. Further reaction of the aldehyde with hydroxylamine hydrochloride in base gives the corresponding oxime which in turn can be carbamoylated with suitable reagents to form the final insecticidal agent.

Sources:

http://www.google.com/patentshl=en&lr=&vid=USPAT3721711&id=9yMuAAAAEBAJ&oi=fnd&dq=hell+volhard+zelinsky+halogenation&printsec=abstract#

http://en.wikipedia.org/wiki/Hell-Volhard-Zelinsky_halogenation