Directive effects in benzylic hydrogen atom abstraction. Part V. Halogenation of benzyl chlorides and α-substituted toluenes

Abstract

The relative rates of halogenation of XC₆H₄CH₂Cl with N-bromosuccinimide in CCl₄ at 80° and with sulphuryl chloride in benzene at 40° are correlated by the Hammett equation to give ρ⁺–1·30 (r0·993) and ρ–0·63 (r0·997), respectively. These results are consistent with the different nature of the transition states of atomic bromine and the benzene-complexed chlorine atom with the substrates XC₆H₄CH₂Y previously observed. Benzylic halogenation of PhCH₂Y with the two reagents has also been examined. The effect of Y on the reactivity shows that CH₃∼ Ph > H > CH₂Cl > CH₂F > CH₂CN > Cl > CN towards the benzene-complexed chlorine atom, and the reactivity values correlated by the Hammett equation for σ_p⁺ and σ_p gave ρ–1·90 and ρ–2·44 (r 0·94 for both).