Autoxidation of N-alkyl amides. Part III. Mechanism of thermal oxidation

Abstract

From the kinetics of the liquid phase autoxidation of a number of N-alkyl amides initiated thermally (75–131°) or by radical initiators it is established that oxidation proceeds by a radical-chain mechanism. The efficiency of αα′-azodi-isobutyronitrile as an initiator, measured by consumption of 1-naphthol, is low (0·22–0·27). Oxidation in absence of added initiator is initially autocatalytic, the rate being dependent upon the one-half power of the concentration of 1-amidoalkyl hydroperoxide formed as a primary product. Thereafter oxidation continues at a pseudo-steady rate, with a kinetic chain length of about 30 for N-n-propylpropionamide at 131°. A notable feature is the self-inhibition or strong retardation which generally becomes apparent at low extents of oxidation. The efficiency of radical initiation by 1-propionamidopropyl hydroperoxide is only 0·03 but rises when cobaltous acetate is added. Extensive non-radical breakdown of the intermediate hydroperoxides of N-alkyl amides produces complex mixtures; the reaction mechanisms are discussed for N-n-alkyl amides and N-s-alkyl amides. N-s-alkyl amides are unexpectedly resistant to “uncatalysed” autoxidation, presumably owing to an exceedingly low extent of homolysis of their hydroperoxides.