OXIDATION OF BENZENEDIOLS BY THE IRON(III) TETRACYANO BIPYRIDYL ION - A HIGH-PRESSURE STOPPED-FLOW KINETICS STUDY

The iron(III) tetracyano 2,2-bipyridyl ion, [Fe(CN)4(bpy)]-, readily oxidizes the benzenediols, catechol (1,2-dihydroxybenzene), 4-tertiary butyl catechol (1,2-dihydroxy-4-t-Bu-benzene), and methyl hydroquinone, (2-methyl-1,4-dihydroxybenzene) in aqueous solution. The kinetics have been studied by stopped-flow spectrophotometry in the pH range 2.5-6. At the lower end of this region the reactions are pH independent, and yield second order rate constants at 298.2 K of 50, 68 and 477 mol-1 dm3 s-1 for the three diols, respectively. In the pH dependent region (3-6) the rates, in buffered solution, vary due to the presence of differently protonated species of the iron(III) complex. Application of pressure, up to 1 kbar, gives rise to more rapid oxidation, and within experimental error, the volume of activation, DELTA-V* is substrate independent (DELTA-V* = -18 cm3 mol-1). The values of DELTA-S* are compatible with this finding. Kinetic and activation parameters are treated within the framework of an outer-sphere electron transfer mechanism.