Radiation-induced oxidation of substituted benzaldehydes: a pulse radiolysis study

Abstract

The reactions of the oxidizing radicals hydroxyl radical (·OH), azidyl radical(N₃^•) and sulfate radical anion(SO₄^-•) with hydroxy-, methoxy-, chloro- and nitro-substituted benzaldehydes were studied by pulse radiolysis. The rate constants for the ·OH reaction [k≈(2.6-12.0)× 10⁹lmol^-1s^-1] with these compounds are higher than those for the or reactions [k≈(0.01-4.2)× 10⁹lmol^-1s^-1]. The ·OH reaction shows a significant substituent effect which is reflected in the rate constants (k≈12.0× 10⁹dm³mol^-1s^-1 with hydroxybenzaldehyde and k≈3.0× 10⁹dm³mol^-1s^-1 with nitrobenzaldehyde). The transient species produced by the reaction of ·OH with the m-substituted benzaldehydes shows an absorption maximum at 370-400nm, whereas the p-substituted isomers exhibit two absorption peaks centred around 325 and 410nm, except in the case of p-nitrobenzaldehyde, which shows only a single peak at 330nm. These spectra are identical with the spectra obtained for the (N₃^•) or SO₄^-• reactions with substituted benzaldehydes. ·OH adduct formation and subsequent decomposition to the corresponding phenoxyl radicals is the main reaction channel for the .OH-induced oxidation of substituted benzaldehydes. One-electron oxidation by (N₃^•) or SO₄^-• of substituted benzaldehydes proceeds via an electron transfer process producing the corresponding radical cations.