Separation of polar and steric effects in the oxidation of ortho-substituted benzaldehydes by N-bromobenzamide

The kinetics of the oxidation of twelve ortho-substituted benzaldehydes by N-bromobenzamide (NBB) to the corresponding benzoic acids have been studied. The reaction is first order with respect to NBB, the aldehyde and hydrogen ions. The addition of benzamide has no effect on the reaction rate. (PhCONH₂Br)⁺ has been postulated as the reactive oxidising species. The correlation of rates with the single substituent-parameter equations is poor. The correlation with Charton’s equation of inductive, resonance and steric parameters is satisfactory. However, excellent correlations were obtained, when Charton’s steric parameter was used along with Taft’s σ₁; andσ _R ⁺ substituent constants. The polar reaction constants have negative values. The reaction is subject to steric hindrance by the ortho-substituents.