The OH radical-induced chain reactions of methanol with hydrogen peroxide and with peroxodisulfate
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– [pKa(˙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 [pKa(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.