The Walden Inversion in the Replacement of Hydroxyl by Halogen

Abstract

THE first serious study of the steric course of this replacement was that of Frankland¹, who concluded that phosphorus pentachloride always substitutes with inversion of configuration, whilst thionyl chloride does so except when a phenyl group is attached to the seat of substitution. Although this inference represented a remarkable advance, it is not quite correct, and also requires to be generalized. We have established that all the common substituting agents (PCl₅, SOCl₂, HCl, PBr₅, etc.) produce inversion unless a sufficiently powerful assembly, critical for each reagent, of electron-releasing groups at the seat of substitution reverses this result. For example, such an assembly, effective for all the reagents named, is present in the alcohols CHPhBu^α.OH and CHPhBu^iso.OH. (We have carried out an improved resolution of the former and a new resolution of the latter.)