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

Hydroxymethyl radicals, (CH2OH)-C-., were generated radiolytically in the reaction of OH radicals with methanol. In the presence of H2O2 they yield formaldehyde via 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 leaching a plateau near G(CH2O) approximate to 65 x 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 k(2) = 6 x 10(4) dm(3) mol(-1) s(-1) and k(12) = 2.75 x 10(3) dm(3) mol(-1) s(-1). In basic solution the chain length first becomes longer because the anion of the (CH2OH)-C-. radical, (CH2O-)-C-. [pK(a)((CH2OH)-C-.) = 10.7] rapidly transfers an electron to H2O2 (k = 4 x 10(5) dm(3) 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-)-C-. 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 x 10(4) dm(3) mol(-1) s(-1)) than from H2O2.