Abstract
The oxidation of a number of para-, meta- and ortho-substituted benzaldehydes by butyltriphenylphosphonium dichromate (BTPPD), in dimethyl sulphoxide, leads to the formation of the corresponding benzoic acid. The reaction is first-order with respect to BTPPD. Michaelis-Menten type kinetics were observed with respect to the aldehyde. The formation constants of the aldehyde-BTPPD complexes and the rates of their decomposition were determined at different temperatures. The reactions are catalysed by hydrogen ions and the dependence is of the second order. The oxidation of [2H] benzaldehyde (PhCDO) exhibited the presence of a substantial kinetic isotope effect. The rates of oxidation of meta- and para-substituted benzaldehydes were correlated in terms of Charton's triparametric LDR equation, whereas the oxidation of ortho-substituted benzaldehydes were correlated with tetraparametric LDRS equation. The oxidation of para- and ortho-substituted benzaldehydes are more susceptible to the delocalization effect than is the oxidation of meta-substituted compounds, which display a greater dependence on the field effect. The positive value of η suggests the presence of an electron-deficient reaction centre in the rate-determining step. The reaction is subjected to steric hindrance by the ortho-substituents. The rates of oxidation of benzaldehyde were determined in nineteen organic solvents. An analysis of the solvent effect by multiparametric equations indicated the greater importance of the cation-solvating power of the solvents. Suitable mechanisms have been discussed.