Electron-transfer versus nucleophilic substitution in the reactions of α-halogenated 4-nitrotoluenes with base

Abstract

A comparative study of the reactions of 4-nitrobenzyl and 4-nitrobenzylidene halides (ArCHXY; Ar = 4-NO₂C₆H₄^–; X = H or Halogen; Y = Halogen) with alkali shows that the reaction is sensitive to the nature of the side chain halogen substituent(s) as well as the reaction conditions. In alcoholic alkali, only 4-nitrobenzyl chloride (ArCH₂Cl) reacts smoothly via the electron transfer (ET)-radical pathway to give mainly dimeric products while the corresponding bromide (ArCH₂Br) and iodide (ArCH₂I) give exclusively S_N products. The ET pathway is enhanced for these substrates by the application of phase-transfer catalysis. In the case of the 4-nitrobenzylidene dihalides (ArCHXY; X = Y = Cl; X = Cl, Y = Br; X = Y = Br; X = F, Y = Br) it is also shown that although progressive bromine substitution suppresses the ET pathway, application of phase-transfer catalysis minimises competing solvolytic reactions and enhances the radical reaction.