Cumene AIBN-initiated oxidation

Small but significant effects of solvent polarity were found in the autoxidation of a variety of alkenes and aralkyl hydrocarbons [216-220] (styrene [216, 218, 219], ethyl methyl ketone [217], cyclohexene [218], cumene [218, 219], tetralin [219], etc.). An extensive study on solvent effects in the azobisisobutyronitrile (AIBN)-initiated oxidation of tetralin in a great variety of solvents and binary solvent mixtures was made by Kamiya et al. [220],... 

AIBN-initiated oxidation of cumene and Tetralin in the presence of deuterated amines were unsuccessful. They proposed an alternative mechanism involving reversible formation of a complex of antioxidant with peroxy radical as the kinetically controlling process. We observed an isotope effect, dAh = 1.8, consistent with the hydrogen-donation mechanism in the retarded oxidation of SBR polymer with deuterated amines (7,8). Our results were confirmed by observation of significant isotope effects in the initial stage of oxidation of purified cfc-l,4-polyiso-prene with both hindered phenols and amines (9). Table I shows the effect of temperature and antioxidant concentration on the rates of oxidation and the observed deuterium isotope effects. 

Thus, it has been proposed that the homolytic decomposition of hydroperoxides can be induced by sulfenic acid (12,13). There is evidence that various carboxylic acids can promote radical formation from hydroperoxides at elevated temperatures (II, 14). The intermediate thiosul-furous acid (Reaction 7) itself may function as the source of radicals, since sulfinic acid is known to initiate the radical polymerization of vinyl monomers at 20°C (15). Based on the AIBN-initiated oxidation of cumene, Koelewijn and Berger (16) proposed that pro-oxidant effects arise from catalysis of the radical decomposition of hydroperoxides by intermediate compound formation between the hydroperoxide and sulfoxide. However, under our conditions hydroperoxide was stable in the presence of sulfoxide alone. 

The results of a study of the zinc diisopropyl dithiophosphate-inhib-ited oxidation of cumene at 60°C. are shown in Figures 1 to 3. The initial oxidation rate is directly proportional to the AIBN concentration, but the dependence of initial rate on the cumene concentration or the reciprocal of the zinc salt concentration, although reasonably linear, is not in direct proportion. 

Figure 3. Initial oxidation rate as a function of concentration of cumene in tert-butylbenzene at 60°G. containing 0.06M AIBN and 0.02M ZnP...

AIBN is, however, virtually unaffected by the presence of dithio-phosphates (Table II). Further, with specific reference to the oxidation of the disulfide in Table I, which has no effect on the rate of AIBN-initi-ated autoxidation of cumene (6), it is unlikely that the efficiency of radical production from AIBN increases since this would produce a prooxidant effect in cumene. Thus, the zinc salt inhibitor is being oxidized in competition with the main chain reaction. 

The experiments on emulsion cumene oxidation with AIBN as initiator proved that oxidation proceeds via the chain mechanism inside hydrocarbon drops [17]. The presence of an aqueous phase and surfactants compounds does not change the rate constants of chain propagation and termination the ratio (fcp(2fct)-1/2 = const in homogeneous and emulsion oxidation (see Chapter 2). Experiments on emulsion cumene oxidation with cumyl hydroperoxide as the single initiator evidenced that the main reason for acceleration of emulsion oxidation versus homogeneous oxidation is the rapid decomposition of hydroperoxide on the surface of the hydrocarbon and water drops. Therefore, the increase in the aqueous phase and introduction of surfactants accelerate cumene oxidation. 

The experimentally measured direct chain termination constant was found to be 5.5 X 103 Mole"1 sec."1. However, this value was not considered very accurate because there is a large correction to the measured oxidation rates for oxygen evolved in the self-reactions of COO radicals at the relatively high photo-initiation rates required to reduce the importance of thermal initiation from the added COOH. A more accurate value of 2.9 X 103 mole"1 sec."1 was calculated from the limiting value of fcpC/[2fc fdirect)]"1/2 at high [COOH] for the AIBN thermally initiated reaction at 30 °C. combined with the measured value of Jcp for neat cumene (0.18 Mole"1 sec."1). 

Figure 9.3. Effect of initiation (by AIBN and Co2+) and inhibition (by 2,6-di-f-butyl-p-cresol) on oxidation of cumene in glacial acetic acid (adapted from Moore and Pearson [77]).

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