RADICAL-ANIONS OF CHLORINATED BENZALDEHYDES IN AQUEOUS-SOLUTION

The hydrated electron and H-atom reduction of aqueous benzaldehyde as well as 2-, 3-, and 4-chlorobenzaldehyde has been studied by pulse rad iolysis. The electron attachment occurs on the aldehyde group. In alka line solutions the radical anions C6H5CHO.- and ClC6H5CHO.- are formed by reaction with e(aq)(-) as well as with (CH3)(2)(COH)-C-. and CO2.- radicals. In acid solution the protonated forms of these anion radica ls, C6H5-CHOH. and ClC6H5CHOH., are obtained, pK = 8.2-8.5. The rate c onstants for the reaction with e(aq)(-) have been determined to be k(e (aq)(-) + C6H5CHO) (2.4 +/- 0.2) x 10(10) dm(3) mol(-1) s(-1) and k(e( aq)(-) + ClC6H9CHO = (2.2 +/- 0.2) x 10(10) dm(3) mol(-1) s(-1) for al l three isomers. The electron transfer process from (CH3)(2)(COH)-C-. and CO2.- to the benzaldehydes is much slower, k = (0.7-2.2) x 10(8) d m(3) mol(-1) s(-1) and k = (5.4-16) x 10(8) dm(3) mol(-1) s(-1), respe ctively, depending on the position of the halide. The anion radicals o f the chlorobenzaldehydes undergo neither intramolecular electron tran sfer nor dehalogenation. This is confirmed by gamma-radiolysis experim ents, where neither chloride nor HCHO could be detected as final produ cts. The carbonyl radicals decay by bimolecular processes, 2k = (1.8-2 .6) x 10(8) dm(3) mol(-1) s(-1) at pH 10 and 2k = (0.7-1.5) x 10(9) dm (3) mol(-1) s(-1) at pH 3.5. The reactions of H atoms with benzaldehyd e and 2,- 3-, and 4-chlorobenzaldehyde, k(H+substrates) = (0.8-1.4) x 10(9) dm(3) mol(-1) s(-1), yield cyclohexadienyl radicals, which decay in bimolecular reactions, 2k = (2.5-3.0) x 10(9) dm(3) mol(-1) s(-1). The absorption spectra of the carbonyl radicals and carbonyl radical anions as well as that of the H-adducts of each individual substrate w ere measured in the range 290-500 nm and their spectroscopic character istics determined.