Interaction of Pyridine-2,6-dicarboxylic Acid with Cr(VI) in the Oxidative Decarboxylation of Phenylsulfinyl Acetic Acid and Linear Free Energy Relationship

Aims: To investigate the catalytic activity of pyridine-2,6-dicarboxylic acid in the redox reaction of Cr(VI) and phenylsulfinyl acetic acid.

Study Design: The mechanism of the reaction was designed on the basis of the observed results of kinetic, spectral and substituent effect studies.

Place and Duration of Study: Laboratory of the Research Department of Chemistry, Aditanar College of Arts and Science, Tiruchendur, Tamil Nadu, India. September 2013 – January 2014.

Methodology: Phenylsulfinyl acetic acid and ten meta- and para-substituted phenylsulfinyl acetic acids essential for the present kinetic study were synthesized. The kinetic study was performed in 40% acetonitrile-60% H2O medium under pseudo-first-order conditions by maintaining [PSAA] >> [Cr(VI)] throughout the experiment. The progress of the reaction was monitored by following the rate of disappearance of Cr(VI) spectrophotometrically at 351 nm. The effect of pyridine-2,6-dicarboxylic acid on the rate of the reaction and the applicability of linear free energy relationship with different phenylsulfinyl acetic acids were tested.

Results: The reaction shows unit order dependence on Cr(VI) but follows Michalis-Menten kinetics with respect to substrate as well as catalyst. The order with respect to [H+] is between one and two. The thermodynamic parameters ∆‡S (-93.2 JK-1 mol-1) and ∆‡H (57.7 kJ mol-1) are evaluated respectively from the intercept and slope of the Eyring’s plot. The Hammett’s correlation affords a negative ρ value (-1.05).

Conclusion: Pyridine-2,6-dicarboxylic acid catalyzes the reaction and Cr(VI)-PDA complex is assumed to be the oxidizing species of the reaction. The sulfur of PSAA undergoes nucleophilic attack on Cr(VI)-PDA complex forming a ternary complex, Cr(VI)-PDA-PSAA which experiences decarboxylation, ligand coupling and further decomposition giving methylphenyl sulfone as the product. The mechanism with the associated reaction kinetics is assigned in support of substituent effect.