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

Abstract

Abstract γ-Radiolysis of N 2 O-saturated water or photolysis of aqueous H 2 O 2 provided a source of OH radicals. These radicals react with ethanol by preferentially abstracting an H atom at C-1. In the presence of oxygen these radicals are converted into the corresponding peroxyl radicals. The a-hydroxyethylperoxyl radicals decay by first order kinetic(k=k 1 + k 2 [OH - ]) acetaldehyde and HO 2 ˙ /H + + O 2 ⨪ being the products (k 1 (20 °C) = 50 ± 10 s -1 , E a = 66 ± 7 kJ·mol -1 , k2= (4± 1) X 10 9 M -1 s -1 ). In competition (favoured by low pH, low temperature and high dose rate) they also decay by second order kinetics (2k 3 = (7 ± 2) x 10 8 M -1 s -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 HO 2 ˙ . The reaction of a-hydroxyethylperoxyl radicals with HO 2 ˙ /O2⨪ radicals appears to be slow (k≈10 7 M -1 s -1 ).