SELECTIVE SYNTHESIS OF ALPHA-CHLOROCARBOXYLIC ACIDS

The chlorination of butanoic, hexanoic, octanoic, decanoic, and dodecanoic acids was studied in a laboratory-scale semibatch reactor operating at temperatures ranging from 70 to 130-degrees-C at atmospheric pressure. The experiments showed that carboxylic acids can be selectively alpha-chlorinated with chlorine in the presence of molecular oxygen as a radical trapper and chlorosulfonic acid as an enolizing agent: the only products were the alpha-monochloro- and alpha,alpha-dichlorocarboxylic acids. High yields of alpha-chlorocarboxylic acids were obtained when the enolizing catalyst was added stepwise into the reaction mixture. In the case that the catalyst was introduced only at the initial stage, the reaction rate stagnated after a short period of time, which was due to the decomposition of chlorosulfonic acid. In the stepwise catalyst addition the chlorocarboxylic acid formation was autocatalytic. This effect was explained by a reaction mechanism involving the acid-catalyzed enolization of a reaction intermediate as the main rate-controlling step. The parameters of the rate equation based on the proposed reaction mechanism were determined for the carboxylic acids studied. The kinetic parameters were linearly dependent on the carboxylic acid carbon number.