Oxygen addition to free radicals

The conversion of benzaldehyde in the presence of air to benzoic acid was reported in 1832 by Wohler and Liebig, and in 1900 Baeyer and Villiger proposed perbenzoic acid as an intermediate in the reaction. The currently accepted free radical chain mechanism for the process was proposed by Backstrom in 1934 (equation 34). Bates and Spence already in 1931 had proposed that photolysis of CH3I forming CHs in the presence of O2 led to peroxyl radicals CHaOO-.  

The reaction of the triphenylmethyl radical with oxygen to form the peroxide discovered by Gomberg in 1900 (equation 1) was a strong piece of evidence for the radical structure 1, and the affinity of carbon centered radicals for oxygen remains one of their defining characteristics. Willstatter and Haber in 1931 also proposed a general role for free radicals in oxygen reactions in chemistry, which further stimulated interest in this field. 

The known reaction product of the oxidation of cyclohexene was assigned as the hydroperoxide 27 by Criegee in 1936. The oxidation of cumene to the hydroperoxide 28 proceeds by a chain mechanism (equations 35, 36), and the conversion of the hydroperoxide by acid to phenol and acetone, in what has become a commercially important process, was reported by Hock and Lang in 1944. 2s 

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