The abnormal Finkelstein reaction: a mechanistic study

Abstract

The formation of 1,2-dichlorothetraphenylethane from dichlorodiphenylmethane with sodium iodine in acetone is a example of the abnormal course which may be followed by certain alkyl halides in lieu of the expected S(,n)2 displacement reaction (Finkelstein reaction). The reaction is kinetically second order, shows a normal salt effect with added lithium perchlorate, and-when substituted in the para-positions of the phenyls- shows the substituent effects expected of positive charge development in the transition state of the rate determining step. Benzhydryl chloride first forms benzhydryl iodide which then undergoes either photochemical or thermal dissociation to form tetraphenylethane. The reaction of tert-butyl diphenylmethyl chloride with sodium iodide in acetone yields equal amounts of tert-butyl diphenylmethane and 2,3-diphenyl-3-methyl-l-butene. When 1,8-bis-(dimethylamino) naphtalene is present only 2,3-diphenyl-3-methyl-l-butene is formed. These olefins are formed by a sequence of rearrangements of an initially formed cation. The methane formed in the first reaction arises from the hydrogen iodide reduction of an initially formed tert-butyl diphenylmethyl iodide. The overall reaction mechanism is described as an S(,n)2(C('+)) reaction followed either by a free radical dissociation of the first form iodide or direct reduction of the cation to the radical.