Does the cationic or the radical character dominate the reactivity of alkene radical cations towards solvent molecules?

Abstract

The reaction of ethylene radical cations with a broad selection of uncharged nucleophiles (hydrogen fluoride, dichloromethane, trifluoroethanol, water, formaldehyde, methanol, ethanol, ethylene glycol, dimethyl formamide, acetonitrile, ammonia) has been studied at the BHandHLYP/6-31 + G(d,p) level of theory. With the exception of hydrogen fluoride and dichloromethane, all of these nucleophiles form covalent addition products with ethylene radical cations. Transition states for these addition reactions could, however, not be identified. The reaction energies calculated for the addition reactions correlate well with the proton affinities of the respective nucleophiles. Based on this correlation the reactivity of ethylene radical cations can be characterized as intrinsically cationic. The reaction of ammonia, the water dimer, and the water trimer with the radical cations of some substituted alkenes (propene, trans-but-2-ene, butadiene, styrene) have been studied at the same level of theory. In all cases the most exothermic reaction occurs for the water trimer. Comparison of the reaction energies calculated for these radical cations with those calculated for the analogously substituted carbocations shows that alkene radical cations can best be understood as strongly stabilized carbocations. The stabilizing effect of the radical center on cationic intermediates is not constant and varies between 40 kcal mol⁻¹ for the smaller systems and 20 kcal mol⁻¹ for the larger systems investigated in this study.