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The photochemical hydroxylation of salicylic acid (SA) by hydrogen peroxide proceeds via the same intermediate independently of the excited reactant (i.e. SA or H2O2). This conclusion is supported by the similar effect of pH on the quantum yields and on the isomer ratio of formed products: 2,3-dihydroxybenzoic acid (2,3-DHB) and 2,5-dihydroxybenzoic acid (2,5-DHB). The product ratio ([2,3-DHB]/[2,5-DHB]) is not affected by H2O2 concentration.

The rate of decomposition of tert‐butyl hydroperoxide in benzene solution within the temperature range of 132 to 170°C has been investigated. At concentrations of tert‐butyl hydroperoxide lower than 0.03 mole/1. the reaction occurs via a monomolecular mechanism (activation energy Eb = 36.7 ± 3 kcal/mole). At higher concentrations the initial rate of decomposition is that of a second‐order reaction with respect to hydroperoxide (activation energy Ea = 16 ± 4 kcal/mole). The reaction mechanism has been discussed, and the activation energy of the reaction of tert‐butoxyl radicals with tert‐butyl hydroperoxide has been estimated.

The photochemically initiated oxidation of salicylic acid by molecular oxygen in the presence of [Fe(C2O4)3]3- leads to a mixture of 2,3- and 2,5-dihydroxybenzoic acids. Iron(II) generated by the photoreduction is reoxidized by dioxygen. Hydrogen peroxide formed in this reaction takes part in the Fenton reaction in the presence of Fe(II). Experiments with OH. radical scavengers document the role of the radicals in the photochemical and thermal hydroxylation of salicylic acid.

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