Chlorination of αβ-unsaturated carbonyl compounds. Part V. Mechanism of chlorination of αβ-unsaturated ketones in methanol and in trifluoroacetic acid

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Abstract
The kinetics and products of the reaction of chlorine with 1-phenyl-3-arylprop-2-enones and with 4-arylbut-3-en-2-ones in methanol have been studied. The reactions involve predominant trans-addition of the elements of methyl hypochlorite to the double bond with little formation of dichlorides, and are interpreted in terms of the formation of an open or partially-bridged carbocation which is captured by the solvent mainly prior to appreciable rotation about the carbon–carbon bond. Some catalysis by hydrogen chloride is apparent in the reaction with 4-phenylbut-3-en-2-one and is believed to involve the formation of acetals prior to reaction with chlorine. In contrast the reaction of 1,3-diphenylprop-2-enone and of 4-phenylbut-3-en-2-one with chlorine in trifluoroacetic acid gives appreciable yields of the excepted dichlorides, probably through a mechanism involving ion-pairs.

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