The OH radical-induced chain reactions of methanol with hydrogen peroxide and with peroxodisulfate

Abstract

Hydroxymethyl radicals, ˙CH₂OH, were generated radiolytically in the reaction of OH radicals with methanol. In the presence of H₂O₂ they yield formaldehyde via a chain reaction which regenerates an OH radical [reaction (2)]. G(CH₂O) first increases with increasing H₂O₂ concentration and with the inverse of the square root of the dose rate, eventually reaching a plateau near G(CH₂O) ≈ 65 × 10^–7 mol J^–1. This indicates that besides the bimolecular termination of the CH₂OH radicals there must be an additional termination reaction of (pseudo-)first-order kinetics which is attributed to an H-abstraction from H₂O₂ by CH₂OH [reaction (12)].The data have been fitted using k₂ = 6 × 10⁴ dm³ mol^–1 s^–1 and k₁₂ = 2.75 × 10³ dm³ mol^–1 s^–1. In basic solution the chain length first becomes longer because the anion of the ˙CH₂OH radical, ˙CH₂O^– [pK_a(˙CH₂OH) = 10.7] rapidly transfers an electron to H₂O₂ (k = 4 × 10⁵ dm³ mol^–1 s^–1). Upon further increasing the pH, i.e. when the anion of H₂O₂ starts to become of importance [pK_a(H₂O₂) = 11.6] the chain length drops again. The data can be fitted assuming that ˙CH₂O^– is not capable of transferring an electron to HO₂^– at an appreciable rate and that the H-abstraction reaction from HO₂^– is considerably faster (k = 2.9 × 10⁴ dm³ mol^–1 s^–1) than from H₂O₂.