New syntheses of aromatic acid chlorides from trichloromethylarenes. 1. Reaction with sulfur dioxide

Abstract

0C to yield aroyl chloride and thionyl chloride in high yields. l,3-Bis(trichloromethyl)benzene reacts especially well to yield isophthaloyl chlo­ ride. The reaction proceeds at a much lower temperature in the presence of antimony pentachloride or other Lewis acids, although Friedel-Crafts side reactions reduce the yield of aroyl chloride unless the ring bears strongly elec­ tron-attracting substituents. Both thermal and catalyzed reactions are interpreted in terms of a dichlorobenzyl cat­ ion intermediate, although an alternative radical mechanism was also demonstrated by adding di-tert- butyl perox­ ide. Laboratory synthesis of acid chlorides is readily achieved by treating the acid with thionyl chloride, phosphorus pen­ tachloride, or other inorganic nonmetal halide, often with a little dimethylformamide or tertiary amine as catalyst. In­ dustrial synthesis is similar, except that phosgene is usually the reagent of choice because of its low cost.1 Aromatic acids are often prepared by oxidation of methylarenes. One useful oxidant is chlorine, and some acids have been prepared commercially by side-chain chlorination of the methylarene, followed by hydrolysis. Where the acid chloride is desired, hydrolysis can be controlled, for the acid chloride is hydrolyzed more slowly under acidic conditions than is the trichloromethyl group. Alcohols and carboxylic acids react similarly, with Lewis acid catalysis, to yield alkyl chlorides and other acid chlorides, respectively. ArCH3 + 3Cl 2 —* ArCCl 3 + 3HCl