Hydroxyl Radical-Induced Oxidation of Ethanol in Oxygenated Aqueous Solutions. A Pulse Radiolysis and Product Study

y-Radiolysis of N20-saturated water or photolysis of aqueous H2O2 provided a source of OH radicals. These radicals react with ethanol by preferentially abstracting an H atom at C-l. In the presence of oxygen these radicals are converted into the corresponding peroxyl radicals. The a-hydroxyethylperoxyl radicals decay by first order kinetics (k = ki + k2 [OH -]) acetaldehyde and H02/H+ + 01 being the products (ki (20 °C) = 50 ± 10 s"1, Ea = 66 ± 7 kJ-mol -1, k2= ( 4 ± 1) X 109M 1s 1) . In competition (favoured by low pH, low temperature and high dose rate) they also decay by second order kinetics (2k3 = (7 ^ 2) x 108 M~1s_1). The most important route in the bimolecular decay leads to acetaldehyde, acetic acid and oxygen (ca. 75%). This route might largely be concerted (Russell mechanism), but there might also be a contribution from the disproportionation of oxyl radicals within the solvent cage. There is also a concerted route that leads to two molecules of acetic acid and to hydrogen peroxide (ca. 10%). Another pathway (ca. 15%) yields two oxyl radicals and oxygen. The former may either decompose into formic acid and methyl radicals (ca. 5%) or rearrange into 1,1-dihydroxyethyl radicals (ca. 10%). These radicals add oxygen and the resulting peroxyl radicals rapidly decompose into acetic acid and HO2. The reaction of a-hydroxyethylperoxyl radicals with H02 /02 radicals appears to be slow ( k » 107M-is-i).