Ethylbenzene, bond dissociation energy

Figure 7.10. Optimum initial concentration of / ara-substituted phenol, [IhHJq vs. bond dissociation energy = resulting in maximum inhibition of ethylbenzene oxidation at T = 60 °C (1) and...

Taking into accoimt that the dissociation energy of the a-C-H bond in the ethylbenzene equals to 343 kJ/mol [51] we have... 

In fact, / ora-substituted phenols with dissociation energy of phenolic 0-H bond exceeding 375 kJ/mol do not inhibit ethylbenzene oxidation at 60 °C. 

Numerical determination of molecular structure of the efficient inhibitor based on the reaction kinetic model The molecular stmcture of /> ra-substituted phenol having a maximum inhibiting ability in the ethylbenzene oxidation was determined by the method described above. The dissociation energy of the 0-H bond in the phenols (Don) served as the characteristic reactivity indices for the molecular stmcture of the inhibitor. By this value it has been possible to describe the reactivity of appropriate phenoxyl radicals (see Table 7.2)... 

As an example a model of die liquid-phase oxidation of the ethylbenzene in the presence of inhibitors, the iora-substituted phenols and the butylated hydroxytoluene, was selected. The identified dynamics of die value contribution of steps in the reaction mechanism is complicated. The dominant steps for die different time intervals of the inhibited reaction were determined. The inhibition mechanism of die ethylbenzene oxidation by sterically unhindered phenols is conditioned by establishing equilibrium (7.24) in the reaction of the chain carrier, the peroxyl radical, with the inhibitor s molecule (within sufficiently wide interval of the inhibitor s initial concentration), followed by the reaction radical s quadratic termination with the participation of the phenoxyl radical. The value analysis has established that the efficient inhibitor with low dissociation energy of the phenolic 0-H bond promotes shifting the mentioned equilibrium from the chain carrier to the direction of the phenoxyl radical formation. 

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