The rapid hydration of the acetyl radical. A pulse radiolysis study of acetaldehyde in aqueous solution.

In aqueous solution acetaldehyde and its hydrate are in a 0.8:1 equilibrium. Hydroxyl radicals generated by the pulse radiolysis of N/sub 2/O-saturated water react with this mixture with an overall rate constant of k = 2.4 x 10/sup 9/ dm/sup 3/ mol/sup -1/ s/sup -1/, the rate of reaction with acetaldehyde being about 3 times faster than with the hydrate. The predominant radicals formed are the acetyl radical and its hydrated form, H-abstraction at methyl occurring to only about 5-10%. The acetyl radical rapidly hydrates with its rate of hydration being 2 x 10/sup 6/ times faster than that of the parent compound acetaldehyde. The hydrated acetyl radical has been monitored by its rapid reduction of tetranitromethane the formation of /sub 2//sup .-/ in the presence of oxygen, and its deprotonation at pH 11 (pK/sub a/ /<=/ 9.5). The acetyl radical does not oxidize N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) at the pulse radiolysis time scale, but the acetylperoxyl radical (formed in the presence of oxygen) reacts rapidly with TMPD, ascorbate, and O/sub 2//sup .-/ and hence is the most strongly oxidizing peroxyl radical known so far. Data on the rate of water loss from the hydrated acetyl radical are considerably less accurate, butmore » it may well be that this reaction has a rate constant of k/sub 10/ /approx equal/ 3 x 10/sup 4/ s/sup -1/. 24 references, 3 figures, 1 table.« less