Methylene-blue-sensitized photochemical hydroxylation of 2-hydroxybenzoic acid by hydrogen peroxide: photocatalytic effect of ferric chloride

The methylene-blue-sensitized hydroxylation of 2-hydroxybenzoic acid by hydrogen peroxide initiated by visible radiation and catalysed by ferric chloride has been investigated. The major reaction product is 2,3-dihydroxy-benzoic acid. In the absence of iron(III), the reaction occurs extremely slowly. Iron(III) is observed to have an appreciable catalytic effect on the photosensitized reaction at concentrations as low as 10⁻⁸ M. The first step of the reaction is the photochemical reduction of iron(III) to iron(II), i.e. the generation of Fenton's reagent.