Kinetics and mechanism of the oxidation of aliphatic aldehydes by sodium n-bromoarylsulphonam1des in acid solution

Abstract

The oxidation of aliphatic aldehydes to the corresponding carboxylic acids by sodium N-bromoarylsulphonamides (N-bromoamines) is first order with respect to the oxidant, the aldehyde and hydrogen ions. In the oxidation of acetaldehyde at 298 K, the primary kinetic isotope effect, k H k D is 4.91 ± 0.14 and the solvent isotope effect, k(H 2O) k(D 2O) , is 0.43. Addition of the parent sulphonamide does not affect the rate. The reduction of six substituted N-bromoamines exhibited a reaction constant of 1.22 at 298 K. (ArSO 2NH 2Br) + is postulated as the reactive oxidising species. Separate rate constants for the oxidation of aldehyde hydrate and free aldehyde forms have been computed. The rates of the oxidation of the aldehyde hydrates correlate well with Taft's substituent constants with negative reaction constants. A mechanism involving hydride transfer from the aldehyde hydrate to the oxidant is proposed.