Reactions of hydroxymethyl radicals with some solutes in methanol. Pulse radiolysis study

Abstract

The reactions of hydroxymethyl radicals, ·CH₂OH, with a number of solutes are studied and the spectra of the products characterised. Evidence is presented for the formation of adducts between ·CH₂OH and the solute, but in some cases these decompose by an intramolecular electron-transfer to yield formaldehyde and a protonated or unprotonated radical-anion. In the case of nitrobenzene addition is to the —NO₂ group and decomposition of the adduct is shown to be acid catalysed. Galvinoxyl reacts with ·CH₂OH to form galvinoxyl anion, but the low yield of this product indicates the occurrence of a second reaction with formation of a non-absorbing product. An adduct may also be formed in the reaction with p-dinitrobenzene. The rate constant for the reaction of ·CH₂OH with benzyl radicals is evaluated from the dependence of the decay of the latter on the concentration of benzyl chlorde. The rate constants (dm³ mol^–1 s^–1) for these reactions of ·CH₂OH are: nitrobenzene, (3.9 ± 0.3)× 10⁶; p-dinitrobenzene, (1.51 ± 0.25)× 10⁸; galvinoxyl, 2.6 × 10⁸(for formation of galvinoxyl anion), 1.8 × 10⁹(for other product); benzyl radicals, (6.7 ± 0.6)× 10⁹.

The reactions of ·CH₂OH from this paper and the literature are divided into those reactions with radicals which have rate constants within an order of magnitude of the diffusion-controlled rate and with non-radical species. The rate constants, k, for addition to nitro-compounds depend on the electron affinity, E_A(eV), according to log₁₀k= 8.8 E_A+ constant.