Oxidation of phenols in hydrocarbon solutions

Phenols react with 02 in hydrocarbon solution by non-chain reactions [217], e.g. 

The first bimolecular step is rate-limiting. Its activation energy is close to the endothermicity of the reaction, and the pre-exponential factor is close to the collision frequency (Table 21). A linear dependence is observed between log k and a with p = —5.0 at 180°C. 

The compensation effect is observed log k0 = 8.6 + 0.352 a at 180°C. A termolecular initiation process was suggested for the oxidation of a-naphthol in rc-decane [218], but under conditions when a-naphthol was consumed by reaction with peroxy radicals as well as by reaction with 02. A bimolecular mechanism for this reaction in benzene and cyclohexane was established recently with the same rate coefficients for both solvents [217]. 

Phenols are widely used as oxidation inhibitors, but the reaction ArOH + 02 - ArO + H02  

The value of W assumed seems to be an underestimate since such values are usual for the oxidation of hydrocarbons at 110—130° C. Therefore the reaction ArOH + 02, being very slow due to its relatively high activation energy, plays a part neither in initiation of chains nor in overall consumption of the inhibitor when the latter is added to the hydrocarbon. 

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