Reaction of chromium(III) with 3,4-dihydroxybenzoic acid: kinetics and mechanism in weak acidic aqueous solutions.

The interactions between chromium(III) and 3,4-dihydroxybenzoic acid (3,4-DHBA) were studied resulting in the formation of oxygen-bonded complexes upon substitution of water molecules in the chromium(III) coordination sphere. The experimental results show that the reaction takes place in at least three stages, involving various intermediates. The first stage was found to be linearly dependent on ligand concentration k(1(obs))' = k(0) + k(1(obs))[3, 4-DHBA], and the corresponding activation parameters were calculated as follows: DeltaH(1(obs)) ( not equal) = 51.2 +/- 11.5 kJ mol(-1), DeltaS(1(obs)) ( not equal) = -97.3 +/- 28.9 J mol(-1) K(-1) (composite activation parameters) . The second and third stages, which are kinetically indistinguishable, do not depend on the concentrations of ligand and chromium(III), accounting for isomerization and chelation processes, respectively. The corresponding activation parameters are DeltaH(2(obs)) ( not equal) = 44.5 +/- 5.0 kJ mol(-1), DeltaS(2(obs)) ( not equal) = -175.8 +/- 70.3 J mol(-1) K(-1). The observed stages are proposed to proceed via interchange dissociative (I(d), first stage) and associative (second and third stages) mechanisms. The reactions are accompanied by proton release, as is shown by the pH decrease.