The kinetics and mechanisms of additions to olefinic substances. Part 16. Addition of halogens to 1,4-benzoquinone and to 1,4-napthoquinone, and dehydrohalogenation of the resulting adducts

Abstract

The products of acid-catalysed halogenation of 1,4-benzoquinone and 1,4-naphthoquinone in acetic acid have been investigated under conditions which could lead to the formation of dichloride, dibromide, and bromochloride. The variation in ratio of bromochloride to dibromide with relative availability of bromine and chlorine as electrophile and nucleophile suggests that the first entering halogen atom is attached nucleophilically as the results of ‘1,4’-(rather than ‘1,2’-) addition, and that the second halogen atom is then attached electrophilically. Acetate ion does not compete effectively with chloride ion in the nucleophilic stage of the reaction; and chlorine acetate does not add, even with sulphuric acid as catalyst, to 1,4-benzoquinone. The mechanistic significance of these results is discussed. The kinetics and products of solvolytic eliminations from the dihalides derived from 1,4-benzoquinone have been studied in 95% ethanol; the effects of structure on the rate make it probable that the reactions lie towards the E1cB end of the spectrum of elimination mechanisms.