CHLORINATION .2. FREE RADICAL VS HELL-VOLHARD-ZELINSKY CHLORINATION OF CYCLOHEXANECARBOXYLIC ACID

Treatment of cyclohexanecarboxylic acid with chlorine at temperatures below ca. 100° gives comparatively nonselective chlorination, even in the dark or in the presence of phosphorus halides. In the range 100-150°, there is a competition between nearly random and selective α-chlorination in the presence of phosphorus halides, and at 170°, the product is almost exclusively the α-chloro acid when as little as 0.6 mol % of PCl3 is present. At higher temperatures formation of considerable unsaturated acid is observed. It is suggested that a free-radical mechanism is operating at low temperatures, yielding, at higher temperatures, to the Hell-Volhard-Zelinsky reaction, which presumably involves addition of chlorine to an enol and leads to the α-chloro acid. At still higher temperatures the α-chloro acid undergoes elimination of HCl to form the unsaturated acid. © 1969, American Chemical Society. All rights reserved.