Reactions of hydroxymethyl radicals with some solutes in methanol. Pulse radiolysis study
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, EA(eV), according to log10k= 8.8 EA+ constant.