Reactivity of an iron(III) complex of gallic acid

Abstract

Gallic acid (3,4,5-trihydroxybenzoic acid) is the end product of hydrolysis of tea tannins which are believed to be responsible for the lowered iron absorption resulting from excessive tea ingestion. Gallic acid forms a 3:1 complex with iron in the pH range 4–6. This complex is reduced by ascorbate. The second-order rate constants at 30 °C at pH 4.4, 5.0 and 5.4 are 40, 21.5 and 9.4 M −1 s −1, respectively. Activation parameters are Δ H ‡ = 17.6 kcal mol −1 and Δ S ‡ = 6.4 cal deg −1 mol −1 at pH = 5.0. Preliminary studies of substitution of gallic acid by tenfold excess of other ligands gave the following results for t 1/2: EDTA, 10 h at pH = 5; acetohydroxamic acid, ∼2 days at pH = 5; desferrioxamine B, 6 s at pH=5 and 2 min at pH = 7.