The Hell-Volhard-Zenlinsky Halogenation reaction causes a halogenation of carboxylic acids at their alpha-carbon, the carbon directly attached to the carbon containing the double bonded oxygen, carbonyl carbon. PBr3 initiates the reaction as a catalyst, after which one molar equivalent of Br2 is added. The PBr3 replaces the carboxylic OH with a bromide, resulting in a carboxylic acid bromide. This can then tautomerize, which switches the double bond and the hydrogen of the carboxyl group to create an enol, and will readily react with the Br2 to brominate a second time at the alpha position.(1)
(Image 1: General Reaction mechanism of the Hell-Volhard-Zenlinsky Halogenation)(2)
In one particular study, racemic [9,10-3H]-2-bromopalmitic acid was synthesized from [9,10-3H]palmitic acid by a Hell-Volhard-Zelinsky reaction. The bromopalmitate was found to be an effective tracer for assessing tissue specific plasmas free fatty acid (FFA) in vivo. Palmitic acid(image 2) was treated with excess bromine and PCl3 at 80 degrees Celsius overnight. The reaction was quenched with water and then bromine was removed by evaporation. By adding a bromide to the alpha carbon of palmitic acid, bromopalmitate was created. (3)
(Image 2: Palmitic acid)(4)
Sources:
1.
http://en.wikipedia.org/wiki/Hell-Volhard-Zelinsky_halogenation
2.
http://www.organic-chemistry.org/namedreactions/hell-volhard-zelinsky-reaction.shtm
3.
http://www.jlr.org/content/40/6/1155.full#ref-23
4.
http://en.wikipedia.org/wiki/Palmitic_acid
No comments:
Post a Comment