Electron spin resonance studies. Part XLV. Reactions of the methyl radical with some aliphatic compounds in aqueous solution

Abstract

Results are reported of an e.s.r. investigation of the reactions of the methyl radical, generated from dimethyl sulphoxide with the Ti^III–H₂O₂ couple, with a variety of aliphatic compounds in aqueous solution. For example, with carboxylic acids and nitriles, the spectra of radicals formed by hydrogen-atom abstraction are detected: it is shown that the methyl radical is more selective than hydroxy, having a relatively greater propensity for abstracting hydrogen from the carbon atom adjacent to the functional group, and rate constants for these reactions, which are in the range ca. 10²–10⁴ l mol^–1 s^–1, have been determined. Reaction with nitromethane is more complex. The spectra observed are those of the nitroethane radical-anion or its conjugate acid, depending on the pH, and it is concluded that abstraction by the methyl radical to give ·CH₂NO₂ is followed by rapid one-electron reduction by Ti^III to give the aci-compound to which methyl then adds. Nitroethane behaves analogously, and both EtNO₂H and PrⁱNO₂H radicals are estimated to have pK_a 4·4.