Hydrogen peroxide, chain decomposition

The result of this change in mechanism is that the major products at high temperatures are olefins and hydrogen peroxide and their secondary decomposition products, which of course include water. The relatively unstable alkyl hydroperoxide produced by the low temperature chain is replaced by the much more stable hydrogen peroxide. The result is that the secondary initiation, responsible for the cool flames, is replaced by a much slower initiation—the second-order decomposition of hydrogen peroxide (Reaction 6). 

Apart from this mathematical aspect I think that the useful concept of critical antioxidant concentration is valid for degenerate chain branching where the effect of the presence of antioxidant on hydroperoxide decomposition is relatively minor but not when it is the predominant initiation reaction. For metals reacting with hydroperoxides the number of radicals formed may even exceed unity. Kolthoff and Medalia [/. Am. Chem. Soc. 71,3777 (1949) ] demonstrated that for the reaction of ferrous ion with hydrogen peroxide as many as six ferrous atoms can be oxidized by one molecule of hydrogen peroxide as a result of this effect. I do not think, therefore, that the critical antioxidant concentration should be applied to those cases in which the so-called antioxidant is the catalyst. 

Merz and Waters (1949) showed that oxidation of organic compounds by Fenton s reagent could proceed by chain as well as non-chain mechanisms, which was later confirmed by Ingles (1972). Kremer (1962) studied the effect of ferric ions on hydrogen peroxide decomposition for Fenton s reagent. It was confirmed that once ferric ions are produced the ferric-ferric system is catalytic in nature, which accounts for relatively constant concentration of ferrous ion in solutions. 

Hiatt et a/.34a-d studied the decomposition of solutions of tert-butyl hydroperoxide in chlorobenzene at 25°C in the presence of catalytic amounts of cobalt, iron, cerium, vanadium, and lead complexes. The time required for complete decomposition of the hydroperoxide varied from a few minutes for cobalt carboxylates to several days for lead naphthenate. The products consisted of approximately 86% tert-butyl alcohol, 12% di-fe/T-butyl peroxide, and 93% oxygen, and were independent of the catalysts. A radical-induced chain decomposition of the usual type,135 initiated by a redox decomposition of the hydroperoxide, was postulated to explain these results. When reactions were carried out in alkane solvents (RH), shorter kinetic chain lengths and lower yields of oxygen and di-te/T-butyl peroxide were observed due to competing hydrogen transfer of rm-butoxy radicals with the solvent. 

Fig. 6.8 Radical chain mechanism of the photo-induced decomposition of hydrogen peroxide in pure water according to Haber and Weiss (reference see Tab. 2-3).

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