Structure–Reactivity Correlation in the Oxidation of Substituted Benzaldehydes by Pyridinium Bromochromate

Abstract

Oxidation of 36 monosubstituted benzaldehydes by pyridinium bromochromate (PBC) in dimethyl sulfoxide (DMSO), leads to the formation of corresponding benzoic acids. The reaction is first order with respect to both PBC and aldehydes. The reaction is promoted by hydrogen ions; the hydrogen–ion dependence has the form kobs = a + b[H+]. The oxidation of [2H]benzaldehyde (PhCDO) exhibited a substantial primary kinetic isotope effect. The reaction was studied in 19 different organic solvents and the effect of solvent was analysed using Taft's and Swain's multi-parametric equations. The rates of oxidation of para- and meta-substituted benzaldehydes showed excellent correlation in terms of Charton's triparametric LDR equation, whereas the oxidation of ortho-substituted benzaldehydes were correlated well with the tetraperametric LDRS equation. The oxidation of para-substituted benzaldehydes is more susceptible to the delocalized effect than is that of ortho- and 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 acceleration by the ortho-substituents. A suitable mechanism has been proposed.