Absolute Rate Constants for the Addition of the 2-(Methoxycarbonyl)propan-2-yl and the 3,3.3-Trifluoracetonyl Radicals to Alkenes in Solution

Abstract

Absolute rate constants and some of their Arrhenius parameters were obtained by time-resolved electron spin resonance (ESR) spectroscopy for the addition of the 2-(alkoxycarbonyl)propan-2-yl and 3,3,3-trifluoroacetonyl (=3,3,3-trifluoro-2-oxopropyl) radicals to a variety of mono- and 1,1-disubstituted alkenes. Their analysis shows that the addition of 2-(alkoxycarbonyl)propan-2-yl is mainly governed by the exothermicity of the reaction with slight modifications by nucleophilic and electrophilic effects giving rise to an overall ambiphilic behavior. In contrast, large electrophilic polar effects dominate the addition of the 3,3,3-trifluoroacetonyl (=3,3,3-trifluoro-2-oxopropan-2-yl) radical, as it is expected from its large electron affinity. For both radicals, the activation energies are well-predicted by analytic equations for the enthalpic and polar terms. A comparison of the rate data of 2-(alkoxycarbonyl)propan-2-yl with the homo- and copolymerization rate constants of the propagating radical of methyl methacrylate shows that the additions of these structurally related low- and high-molecular-weight radicals to alkenes are governed by very similar effects.