HYDROXYL RADICAL INDUCED OXIDATION OF ACETALDEHYDE DIMETHYL ACETAL IN OXYGENATED AQUEOUS-SOLUTION - RAPID O2.- RELEASE FROM THE CH3C(OCH3)2O2. RADICAL

The oxidative degradation of acetaldehyde dimethyl acetal in dilute aqueous solutions has been studied kinetically by pulse radiolysis and with respect to product formation by both γ-radiolysis and pulse radiolysis. In N2O-saturated solutions, H abstraction from the substrate by the OH radical produces the radicals CH3CH(OCH3)OCH2· (1), CH3C·(OCH3)2 (2), and·CH2CH(OCH3)2 (3). Radicals 1 and 2 have reducing properties and react rapidly with tetranitromethane, yielding nitroform arilon. Radical 3 is probably inert toward this compound on the pulse radiolysis time scale. While 1 forms an adduct with an observable lifetime (k(dec) = 4.4 X 104 s-1), 2 gives rise to the immediate formation of nitroform anion. It has been estimated that the radicals are formed with G values of G(l) = 0.34, G(2) = 0.21, and G(3) ≤ 0.02 μmol J-1. In N2O/O2 (4:1, v/v) saturated solutions, the products are (G μmol J-1) in parentheses) formaldehyde (0.16), methanol (≈0.3), acetaldehyde (0.15), methyl acetate (0.21), formic acid (0.16), hydrogen peroxide (0.14), and organic hydroperoxide (0.19). As shown by pulse radiolysis, oxygen reacts with radicals 1-3 with a rate constant of k ≥ 2 × 109 dm3 mol-1 s-1, yielding (mainly) the peroxyl radicals CH3CH(OCH3)OCH2O2· (4) and CH3C(OCH3)2O2· (5). Peroxyl radical 5 cleaves off O2·- in a fast reaction (k = 6.5 × 104 s-1), which subsequently leads to the formation of methyl acetate and methanol. Besides reacting with O2·- to form the corresponding hydroperoxide, peroxyl radical 4 undergoes bimolecular termination reactions, yielding acetaldehyde, methanol, formic acid, and hydrogen peroxide. In particular, it appears that the formic acid is not produced instantly but is released from α-methoxyethyl formate upon hydrolysis whose rate constant has been determined (k0 = 0.1 s-1) and found to be remarkably fast compared to other neutral hydrolytic reactions of comparable aliphatic esters. It is shown that the formation of the product acetaldehyde is not in consequence of acetal hydrolysis mediated by the proton formed radiolytically. © 1990, American Chemical Society. All rights reserved.