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

The reactions of the oxidizing radicals hydroxyl radical ( . OH), azidyl radical $({\rm N}_3^\cdot)$ and sulfate radical anion $({\rm SO}_4^{-\cdot})$ 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) × 10 9  l mol −1  s −1 ] with these compounds are higher than those for the ${\rm N}_3^\cdot$ or ${\rm SO}_4^{-\cdot}$ reactions [ k  ≈ (0.01–4.2) × 10 9  l mol −1  s −1 ]. The . OH reaction shows a significant substituent effect which is reflected in the rate constants ( k  ≈ 12.0 × 10 9  dm 3  mol −1  s −1 with hydroxybenzaldehyde and k  ≈ 3.0 × 10 9  dm 3  mol −1  s −1 with nitrobenzaldehyde). The transient species produced by the reaction of . OH with the m ‐substituted benzaldehydes shows an absorption maximum at 370–400 nm, whereas the p ‐substituted isomers exhibit two absorption peaks centred around 325 and 410 nm, except in the case of p ‐nitrobenzaldehyde, which shows only a single peak at 330 nm. These spectra are identical with the spectra obtained for the $({\rm N}_3^\cdot)$ or ${\rm SO}_4^{-\cdot}$ 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 $({\rm N}_3^\cdot)$ or ${\rm SO}_4^{-\cdot}$ of substituted benzaldehydes proceeds via an electron transfer process producing the corresponding radical cations. Copyright © 2004 John Wiley & Sons, Ltd.