Correlation analysis of reactivity in the oxidation of substituted benzyl alcohols by quinolinium fluorochromate

Abstract

Oxidation of benzyl alcohol and some <i>ortho-, meta- </i>and <i>para</i>-monosubstituted ones by quinolinium fluorochromate (QFC) in dimethyl sulphoxide (DMSO) leads to the formation of corresponding banzaldehydes. The reaction is first order each in both QFC and the alcohol. The reaction is promoted by hydrogen ions; the hydrogen-ion dependence has the form <i>k</i>_obs<i>= </i>a + b [H⁺]. Oxidation of α, α-dideuteriobenzyl alcohol (PhCD₂OH) has exhibited a substantial primary kinetic isotope effect. The reaction has been studied in nineteen organic solvents and the effect of solvent analysed using Taft's and Swain's multi-parametric equations. The rates of oxidation of <i>para- </i>and <i>meta-</i>substituted<i> </i>benzyl alcohols have been correlated in terms of Charton's triparametric LDR equation, whereas the oxidation of <i>ortho-substituted </i>benzyl alcohols with tetraparametric LDRS equation. The oxidation of <i>para</i>-substituted benzyl alcohols is more susceptible to the delocalization effect than that of <i>ortho- </i>and <i>meta-</i>substituted<i> </i>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-determjning step. The reaction is subjected to steric acceleration by the <i>ortho-</i>substituents<i>. </i>A suitable mechanism has been proposed.