Chlorination of αβ-unsaturated carbonyl compounds. Part IV. Kinetics and mechanism of the uncatalysed and hydrogen chloride catalysed chlorination of trans-cinnamaldehyde in acetic acid
The reaction of chlorine with trans-cinnamaldehyde in acetic acid is strongly catalysed by hydrogen chloride. In the absence of added hydrogen chloride the chlorination is autocatalysed by the hydrogen chloride liberated during the reaction. In the presence of sufficient added alkali-metal acetate, the autocatalysis is suppressed and the uncatalysed reaction takes place. The uncatalysed reaction gives both acetoxy-chlorides and dichlorides, and the mechanism is similar to that observed with methyl trans-cinnamate and trans-cinnamic acid, except that there is also ca. 40% side-chain substitution. The hydrogen chloride catalysed reaction involves the chlorination of the small equilibrium concentration of the 1,4-adduct of trans-cinnamaldehyde and hydrogen chloride, i.e., the enol of β-chloro-β-phenylpropionaldehyde. The products of this reaction are entirely dichlorides with a preponderance of the erythro-isomer, in marked contrast to the uncatalysed reaction in which the main addition products are threo-dichloride and erythro-acetoxy-chloride.