Radical Anions of Chlorinated Benzaldehydes in Aqueuos Solution

Article abstract of DOI:10.1021/j100023a019

The hydrated electron and H-atom reduction of aqueous benzaldehyde as well as 2-, 3-, and 4-chlorobenzaldehyde has been studied by pulse radiolysis.The electron attachment occurs on the aldehyde group.In alkaline solutions the radical anions C6H5CHO^.- and ClC6H4CHO^.- are formed by reaction with e_aq^- as well as with (CH3)2^.COH and CO2^.- radicals.In acid solution the protonated forms of these anion radicals, C6H5CHOH^. and ClC6H4CHOH^., are obtained, pK = 8.2-8.5.The rate constants for the reaction with e_aq^- have been determined to be k(e_aq^- + C6H5CHO) = (2.4 +/- 0.2)x10¹⁰ dm³ mol^-1 S^-1 and k(e_aq^- + ClC6H4CHO = (2.2 +/- 0.2)x10¹⁰ dm³ mol^-1s^-1 for all three isomers.The electron transfer process from (CH3)2^.COH and CO2^.- to the benzaldehydes is much slower, k = (0.7-2.2)x10⁸ dm³mol^-1s^-1 and k = (5.4-16)E8 dm³mol^-1s^-1, respectively, depending on the position of the halide.The anion radicals of the chlorobenzaldehydes undergo neither intramolecular electron transfer nor dehalogenation.This is confirmed by γ-radiolysis experiments, where neither chloride nor HCHO could be detected as final products.The carbonyl radicals decay by bimolecular processes, 2k = (1.8-2.6)E8 dm³ mol^-1 s^-1 at pH 10 and 2k = (0.7-1.5)10⁹ dm³mol^-1s^-1 at pH 3.5.The reactions of H atoms with benzaldehyde and 2,- 3,- and 4-chlorobenzaldehyde, k(H+substrates) = (0.8-1.4)x10⁹ dm³mol^-1s^-1, yield cyclohexadienyl radicals, which decay in dimolecular reactions, 2k = (2.5-3.0)x10⁹ dm³mol^-1s^-1.The absorption spectra of the carbonyl radicals and carbonyl radical anions as well as that of the H-adducts of each individual substrate were measured in the range 290-500 nm and their spectroscopic characteristics determined.