Chlorination of αβ-unsaturated carbonyl compounds. Part III. Reaction of chlorine with trans-cinnamic acid and the trans-cinnamate ion
The addition of chlorine to trans-cinnamic acid in acetic acid and in acetic acid containing lithium chloride, lithium perchlorate, or perchloric acid is very similar to that observed for methyl trans-cinnamate. A similar mixed mechanism, involving a direct addition of chlorine and collapse of, and reaction of the solvent with, an ion-pair and chloronium ions, is proposed. No acid-catalysis other than a salt effect is observed. In acetic acid containing lithium acetate, the addition products are accompanied by trans-β-chlorostyrene, 1,2,2-trichloroethylbenzene, and 1-acetoxy-2,2-dichloroethylbenzene. The chlorostyrene is formed by a direct reaction of the trans-cinnamate ion with chlorine, and not by elimination of carbon dioxide from the addition products or from the intermediate ion-pairs. The ethylbenzene derivatives are formed by further chlorination of the chlorostyrene. The trans-cinnamate ion also undergoes chlorodecarboxylation in aqueous solution, whereas trans-cinnamic acid undergoes addition of hypochlorous acid (or chlorine monoxide) or of chlorine, depending upon the method of mixing the reagents.