Factors in the formation of isomerically and optically pure alkyl halides. Part VIII. Rearrangements in the reactions of branched-chain alcohols with boron trichloride and boron tribromide

Abstract

Reactions of branched-chain alcohols with boron trichloride and boron tribromide afford mixtures of alkyl halide isomers in almost all cases. Only the reaction of boron trichloride with t-butyl alcohol gives an isomerically pure product. The dichloroborinates (alkoxydichloroboranes) of primary alcohols are the only intermediates which are isolable at room temperature. Rearrangements are largely in accord with a carbonium ion mechanism, although isomerisation of t-pentyl products can yield 2-halogeno-3-methylbutanes. The unexpected formation of primary alkyl bromides in the isopropyl, t-butyl, and t-pentyl systems, is probably due to free-radical combination of alkene and hydrogen bromide which are by-products of the reaction; no rearrangement accompanies the nitrous acid deamination of the corresponding primary amines.