Relative Reactivities in the Addition of Free Trichloromethyl Radicals to Substituted Styrenes. An Attempt to Separate Polar and Resonance Effects

Abstract

Abstract Relative reactivities in dibenzoyl peroxide-catalyzed addition of bromotrichloromethane to nine nuclearsubstituted styrenes have been investigated with unsubstituted styrene. The effect of meta substituents upon the relative rates was correlated with the simple Hammett equation using either σ or σ+ constants. The relative rates of para-substituted styrenes, however, did not obey the simple Hammett relation, with the results best explained on the basis of polar (ρσ+) and resonance (ED) terms. All para substituents examined increase the reactivity. log(k⁄kH)=ρσ++ED The ED term shows a striking correlation with logQ, of the corresponding para styrene in Alfrey-Price′s Q-e scheme of free-radical copolymerization. That the polar term originates in charge-transfer interactions between the olefin and the electrophilic trichloromethyl radical and can be expressed by ρσ+ is further supported by the evidence that charge-transfer energies of substituted styrenes showed good correlation with σ+. Factors influencing the reactivity have been discussed.