Rearrangement of allylic bromine accompanying the addition of hypobromous acid to allyl bromide

Abstract

Hypobromous acid reacts with allyl bromide in water in the presence of perchloric acid to give 2,3-dibromopropan-1-ol (79%) and 1,3-dibromopropan-2-ol (21%). An isotope dilution experiment indicated that no other product was formed in significant amount. When the reaction was carried out with allyl [⁸²Br] bromide, the resulting 2,3-dibromopropanol contained 29 ± 1% of its radioactivity in the 2-position. When the bromine labelling was reversed, i.e., [⁸²Br]hypobromous acid was added to inactive allyl bromide, the 3-position was labelled to the extent of 30 ± 1%. This value, which is taken as a measure of the allylic bromine rearrangement, was reduced to 25% when the reaction was carried out in 40% dioxan–water. These results, together with earlier work on the corresponding chloro- and chlorobromo-systems, are discussed in terms of participation by neighbouring halogen groups in the intermediate carbonium ions.