Factors in the formation of isomerically and optically pure alkyl halides. Part X. Reactions of saturated aliphatic alcohols with thionyl chloride

Abstract

Whereas primary alcohols such as butan-1-ol and 3-methylbutan-1-ol are converted into the corresponding chlorides without rearrangement on heating with thionyl chloride, varying degrees of alkyl rearrangement are observed for those which are branched at the 2-position (2-methylpropyl, 0.3; 2-methylbutyl, 22; 2,2-dimethylpropyl, 98%). Straight-chain secondary alcohols afford 2–10% of the isomeric s-alkyl chlorides whilst 3-methyl-butan-2-ol gives mainly the tertiary halide. The presence of pyridinium chloride (0.01–0.1 mol. equiv.) reduces the extent of rearrangement but does not eliminate it. In pyridine, 2,2-dimethylpropyl chlorosulphite gives isomerically pure 1-chloro-2,2-dimethylpropane, although the yield is poor. Reactions of alcohols with thionyl chloride in either dimethylformamide or hexamethylphosphoric triamide give isomerically pure products from all types of alcohol except the branched 3-methylbutan-2-ol. In dimethylformamide, an alkoxyformamidinium intermediate is involved.