A study of the chlorination of crotonic acid and related compounds

Abstract

The chlorination of crotonic acid and of crotonaldehyde was studied under various conditions with particular reference to the products of the reactions. The reaction of crotonic acid with chlorine was carried out in water saturated with sodium chloride, in nitromethane, and in chloroform. In the first of these three solvents the products appeared to be entirely those which would he expected if the reaction proceeded through a two-stage mechanism in which the first stage involved electrophilic attach by a chlorine molecule on the alpha-carbon atom, followed by reaction of the carbonium ion so formed with any nucleophile present in the solution. In nitromethane the products were more unexpected, and appeared to consist largely of alpha-chloro-beta-hydroxybutyric acid, even when the reaction was carried out in dry solvent. A mechanism involving the intermediate formation of a lactone ring is suggested for this reaction. The chlorination which was carried out in chloroform produced, apart from ab-dichlorobutyric acid, a product which has not yet been identified conclusively, hut which, at the moment, appears to he a-chlorovinylacetic acid. A mechanism by which this could have been formed is suggested. One quantitative determination of the amounts of the two isomers of alpha,beta-dichlorobutyric acid produced by the chlorination in chloroform of crotonic acid was made, using the method of isotopic dilution. It was shown that not more than 2% of the isomer formed by cis addition of the halogen was produced under these conditions. The chlorination of crotonaldehyde was also carried out in chloroform. The reaction was faster than the corresponding reaction of the acid, and the products, after oxidation, consisted entirely of the two isomers of alpha,beta-dichlorobutyric acid, the one formed by cis addition to the aldehyde being present to an extent of about 20%. A nucleophilic mechanism is suggested for this reaction.