SELECTIVE SYNTHESIS OF ALPHA-CHLOROPROPANOIC ACID

Propanoic acid was selectively chlorinated to 2-chloropropanoic acid i n a laboratory scale semibatch reactor operating at atmospheric pressu re and in a temperature range of 70-110 degrees C. Chlorosulfonic acid was used as a catalytic agent and molecular oxygen as a radical scave nger. 2,2- and 2,3-dichloropropanoic acids were formed as byproducts. The chlorination kinetics was autocatalytic, when the catalyst concent ration was maintained constant by addition. When all of the catalyst w as introduced in the beginning of the experiment, a decreasing chlorin ation rate was observed because of decomposition of the catalyst. The primary reaction product from catalyst decomposition was sulfopropanoi c acid, which under the reaction conditions is probably decomposed to gaseous products. The explanation of the autocatalysis is the increase of the acidity of the liquid phase during the progress of the reactio n. The autocatalytic kinetics was explained with a reaction mechanism involving the acid-catalyzed enolization of the key intermediate, prop anoyl chloride, as a rate-determining step. In the absence of air a ra dical mechanism contributed to the overall kinetics, which resulted in polychlorinated products. Rate equations based on the autocatalytic i onic mechanism and radical chlorination mechanism were derived, and th e kinetic parameters included in the equations were estimated with non linear regression analysis from the semibatch reactor data.