Hell-Volhard-Zelinsky Halogenation

A Hell-Volhard-Zelinsky Halogenation is a reaction that halogenates carboxylic acids at the alpha carbon. This reaction takes place in the absence of a halogen carrier and is initiated by the addition of a catalytic amount of PBr3. One molar equivalent of Br2 is then added. The carboxylic OH is replaced with a bromide because of the PBr3 that was added. The result is a carboxylic acid bromide. This allows the acyl bromide to tautomerize to an enol so that it can readily react with the Br2 in order to brominate a second time at the alpha position. This reaction is named after the three German chemists. The general reaction looks like this:

I found a Hell-Volhard-Zelinsky Halogenation reaction involving cycloalkane carboxaldoxime carbamates. These compounds are useful as pesticides and have very excellent miticidal activity. These compounds are prepared by the alpha halogenation of cyclic carboxylic acids under slightly modified Hell-Volhard-Zelinsky conditions. Esterification to give the cyclic alpha halocycloalkanecarboxylic acid easter follows. The active halogen is displaced with sodium alkyl mercaptide in alcohol to produce the alkylthio ester. The hydrolysis of the ester is followed by reacting the hydrolyzed product with thionyl chloride which give the reactive acid chloride. The acid chloride is changed into the aldehyde. The aldehyde then reacts with the hydroxylamine hydrochloride in a base and gives a corresponding oxime. The oxime can be carbamoylated with the suitable reagents to form the final insecticidal agent. A picture of the synthesis follows:

Sources:

• http://en.wikipedia.org/wiki/Hell-Volhard-Zelinsky_halogenation
• http://www.google.com/patents?hl=en&lr=&vid=USPAT3721711&id=9yMuAAAAEBAJ&oi=fnd&dq=Hell-Volhard-Zelinsky+halogenation&printsec=abstract#v=onepage&q=Hell-Volhard-Zelinsky%20halogenation&f=false