Absolute rate constants and their temperature dependence were determined by time-resolved electron spin resonance for the addition of the radicals 'CH,CN and .CH,CO,C(CH,), to a variety of mono-and 1,l-disubstituted and to selected 1,2-and trisubstituted alkenes in acetonitrile solution. To alkenes CH,=CXY, .CH,CN adds at the unsubstituted C-atom with rate constants ranging from 3.3.10' M-Is- ' (ethene) to 2.4.106 M-IS-'( 1 , ldiphcnylethene) at 278 K, and the frequency factors are in the narrow range of log (A/M-'s-') = 8.7 * 0.3.. CH2C02C(CH,), shows a very similar reactivity with rate constants at 296 K ranging from 1.1 lo4 M-'s-' (ethene) to lo7 M-'s-' (],I-diphenylethene) and frequency factors log (A/M-'s-') = 8.4 f 0.1. For both radicals, the rate constants and the activation energies for addition to CH,=CXY correlate well with the overall reaction enthalpy. In contrast to the expectation of an electro-or ambiphilic behavior, polar alkene-substituent effects are not clearly expressed, but some deviations from the enthalpy correlations point to a weak electrophilicity of the radicals. The rate constants for the addition to 1,2-and to trisubstituted alkenes reveal additional steric substituent effects. Self-termination rate data for the title radicals and spectral properties of their adducts to the alkenes are also given.1. Introduction. -The addition of carbon-centered radicals to C=C bonds is one of the most useful radical reactions. However, its rate constants are far from being quantitatively understood and predictable, because they reflect a complex interplay of polar, steric, and enthalpic substituent effects [l]. To provide a larger basis for their analysis, we have developed a technique of time-resolved electron spin resonance (ESR) which yields reliable rate data for large series of reactions with moderate efforts. Hitherto 121, the addition rates of the easily oxidizable radicals tert-butyl, hydroxymethyl, and 2-hydroxy-2-propyl to various alkenes CH,=CXY were found to correlate well with the alkene electron affinities. This means that these radicals are nucleophilic, and indicates a substantial stabilization of the transition state by partial electron transfer to the substrates. The variations of the reaction enthalpy had little or no effect. On the other hand, these were found to govern the rate constants for the addition of the 2-cyano-2-propyl radical [3] which exhibits no clear polar effects, and the benzyl radical shows an intermediate, weakly nucleophilic behavior [4]. In related studies, other groups demonstrated clearcut electrophilic reactivity patterns for perfluoroalkyl [5], dicyanomethyl [6], and malonyl [7] radicals.Here, we present rate data for the addition of the cyanomethyl (. CH,CN) and the
