KINETICS OF THE REACTION BETWEEN V(III) IONS AND SALICYLIC-ACID - MECHANISM AND ACTIVATION PARAMETERS

The kinetics of the reaction between V(III) and salicylic acid have been investigated in the temperature range between 7.5 and 45° C, at I = 1 M. Both kinetic and equilibrium measurements show that, at hydrogen ion concentrations between 0.015 and 0.8 M, two complexes are formed, namely, VHSal2+ and VSal+. The reaction proceeds by two parallel paths, involving V3+and HSal-, and V3+ and H2Sal, respectively. At 27°C, the corresponding rate constants are 1.61 × 103M-1 s-1and 5.9 M-1 s-1. No contribution of a pathway involving VOH2+could be detected. This fact, together with the ligand-dependence of the rate constants, tends to confirm the SN2 mechanism suggested by previous authors. The entropies of activation, however, are 2.2 e.u. and - within the experimental accuracy - zero, respectively. This apparent discrepancy is discussed in terms of the properties of salicylic acid. The reaction was monitored spectrophotometrically, using a stopped flow apparatus. © 1979, Taylor & Francis Group, LLC. All rights reserved.