The kinetics and mechanism of metal‐catalyzed autoxidation

Abstract

Abstract The autoxidation of organic compounds, RH, occurs by a radical‐catalyzed chain reaction to give hydroperoxides, RO 2 H, as primary products. The initial rate is ‐d[O 2 ]/dt = k p [RH] {k i [Cat]/k t } 1/2 , or in the presence of an inhibitor, (In), k p [RH](k i [Cat]/k I [In]), where k p is the chain propagation rate; k i [Cat], the rate of radical catalysis; k t chain termination rate; k I [In] rate of inhibitor action. As oxidation proceeds the hydroperoxides break down to give further catalytically active radicals and eventually an autoxidation may reach a maximum rate of k p 2 [RH] 2 /fk t , independent of the concentration or nature of the catalyst. Photosensitization, by forming singlet oxygen, can catalyze autoxidation by forming peroxides. Compounds of many transition metals, e.g., Co, Mn, Fe, act as secondary catalysts by promoting the rapid formation of radicals from RO 2 H molecules by a one‐electron transfer reaction RO‐OH + M 2+ →RO· + M 3+ + OH − and the M 3+ ions are then reconverted to M 2+ ions giving further radicals. The overall catalytic activity of a metallic ion is controlled by the slower step of the M 2+ ⇌M 3+ + e redox cycle and depends on the electronic structures of the two ions concerned and on the ligand groups attached to them. These effects are discussed in detail since ligand molecules for transition metal ions can be selected so as either to promote or inhibit autoxidation. Special reference is made to biological catalysts, such as the porphyrins, found in food products. Direct activation of oxygen by metallic complexes rarely seems to occur, but direct oxidation of substrates by metallic compounds is possible. This leads to another redox cycle which is utilized in copper‐containing enzymes.