Reaction rates peroxidation chain mechanism

Vanoppen et al. [88] have reported the gas-phase oxidation of zeolite-ad-sorbed cyclohexane to form cyclohexanone. The reaction rate was observed to increase in the order NaY < BaY < SrY < CaY. This was attributed to a Frei-type thermal oxidation process. The possibility that a free-radical chain process initiated by the intrazeolite formation of a peroxy radical, however, could not be completely excluded. On the other hand, liquid-phase auto-oxidation of cyclohexane, although still exhibiting the same rate effect (i.e., NaY < BaY < SrY < CaY), has been attributed to a homolytic peroxide decomposition mechanism [89]. Evidence for the homolytic peroxide decomposition mechanism was provided in part by the observation that the addition of cyclohexyl hydroperoxide dramatically enhanced the intrazeolite oxidation. In addition, decomposition of cyclohexyl hydroperoxide followed the same reactivity pattern (i.e., NaY < BaY... 

The alkylation of quinoline by decanoyl peroxide in acetic acid has been studied kineti-cally, and a radical chain mechanism has been proposed (Scheme 207) (72T2415). Decomposition of decanoyl peroxide yields a nonyl radical (and carbon dioxide) that attacks the quinolinium ion. Quinolinium is activated (compared with quinoline) towards attack by the nonyl radical, which has nucleophilic character. Conversely, the protonated centre has an unfavorable effect upon the propagation step, but this might be reduced by the equilibrium shown in equation (167). A kinetic study revealed that the reaction is subject to crosstermination (equation 168). The increase in the rate of decomposition of benzoyl peroxide in the phenylation of the quinolinium ion compared with quinoline is much less than for alkylation. This observation is consistent with the phenyl having less nucleophilic character than the nonyl radical, and so it is less selective. Rearomatization of the cr-complex formed by radicals generated from sources other than peroxides may take place by oxidation by metals, disproportionation, induced decomposition or hydrogen abstraction by radical intermediates. When oxidation is difficult, dimerization can take place (equation 169). 

The reaction can be conducted under thermal conditions (reflux), radical conditions (the presence of traces of benzoyl peroxide induces a fourfold increase in the thermal reaction rate and a slightly better yield), or photochemical conditions (where the reaction proceeds under UV irradiation at room temperature to give the same yield as above no reaction is observed in the dark at room temperature).° ° The mechanism of the reaction has been studied extensively, °° ° °°° °° and it has been concluded that the thermal reaction of triethyl phosphite with CCI4 involves an SnCP substitution. In the presence of UV light or free-radical chain initiators, the radical mechanism generally dominates. The ability of the trichloromethyl radical to initiate a radical chain reaction depends on the relative concentrations of the reagents. The final product mixture is the same as in the ionic casc.°°°... 

Consistent with a radical chain mechanism, the rate of O2 insertion was found to be sensitive to light, and the addition of radical initiator AIBN was required in order to observe reproducible reaction rates. Based on analysis of the kinetics of O2 insertion into the Pd-C bond of 24, a mechanism involving mononuclear Pd(III) intermediates was proposed (Fig. 16). Palladium(III) intermediate 27, formed by the combination of dimethyl Pd(II) complex 24 with peroxy radical 26 [84], generates the observed Pd(II) peroxide 25 by homolytic Pd-C cleavage to reduce Pd(III) complex 27 and generate radical chain carrier Me. ... 

Tho equation is satisfied over fairly wide limits of variation in the parameters, and the value E = -0.045 V is obtained this does not differ greatly from the tabular value of E = —0.076 V. The same authors [18] measured the decomposition rate of hydrc en peroxide at a mercury surface at pH 12-13. At low peroxide concentrations the results were fomid to be in proximate agreem t with the hypothesis of linked electrochemical reactions the rate increases sharply at c 0.15 mole/liter, which evidently indicates an increasing contribution from the chain mechanism. 

Oxidative chain reactions of organic compounds are current targets of theoretical and experimental study. The kinetic theory of collisions has influenced research on liquid-phase oxidation. This has led to determining rate constants for chain initiation, branching, extension, and rupture and to establishing the influence of solvent, vessel wall, and other factors in the mechanism of individual reactions. Research on liquid-phase oxidation has led to studies on free radical mechanisms and the role of peroxides in their formation. 

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