KINETICS AND MECHANISM OF THE OXIDATION OF AROMATIC-ALDEHYDES BY PYRIDINIUM FLUOROCHROMATE

Kinetics of oxidation of a number of ortho-, meta-, and para-substituted benzaldehydes by pyridinium fluorochromate (PFC), in dimethyl sulfoxide, were studied. The main product of oxidation is the corresponding benzoic acid. The reaction is first order with respect to PFC. Michaelis-Menten-type kinetics were observed with respect to the aldehyde. The formation constants of the intermediate PFC-aldehyde complexes and the rates of their decomposition were determined at different temperatures. The thermodynamic and activation parameters were also determined. The oxidation of [H-2]benzaldehyde (PhCDO) exhibited a substantial primary kinetic isotope effect. The rates of oxidation of para- and meta-substituted benzaldehydes were correlated with Taft's and Swain's dual substituent-parameter equations. For the para-substituted compounds, the best correlation is obtained with Taft's sigma-I and sigma-R+; the meta-substituted benzaldehydes correlated best with sigma-I and sigma-R0. The rates of oxidation of the ortho compounds correlated best with a triparametric equation involving Taft's sigma-I and sigma-R+ and Charton's steric parameter, V. An analysis of the rate of oxidation of benzaldehyde in 19 organic solvents indicated the greater importance of cation-solvating power of the solvents. A mechanism involving transfer of a hydride ion from the aldehyde to PFC has been proposed.