Autoxidation of ethylbenzene: the mechanism elucidated

Using a combined experimental and theoretical approach, we elucidated the mechanism of ethylbenzene autoxidation, at about 420 K. The generally accepted literature mechanism indeed fails to explain basic experimental observations, such as the high ketone to alcohol ratio. The hitherto overlooked propagation of 1-phenyl-ethylhydroperoxide, the primary chain product, is now unambiguously identified as the source of acetophenone as well as of 1-phenylethanol via a subsequent activated cage reaction. A similar mechanism allowed rationalizing of the cyclohexanone and cyclohexanol formation in the autoxidation of cyclohexane. The primary hydroperoxide product is found to react about 10 times faster than the arylalkane substrate with the chain carrying peroxyl radicals, whereas in cyclohexane autoxidation, this reactivity ratio is as high as 55. In combination with a lower efficiency of the above-mentioned cage reaction, this results in a rather high 1-phenyl-ethylhydroperoxide yield and causes a high ketone/alcohol ratio. Radicals are shown to be predominantly generated via a concerted bimolecular reaction of the hydroperoxide with the arylalkane substrate, producing alkyl and hydrated alkoxy free radicals. In this autoxidation system, no reaction product exhibits a major initiation-enhancing autocatalytic effect, as is the case with cyclohexanone in cyclohexane autoxidation. As a result, the conversion rate increases less sharply in time compared to cyclohexane autoxidation. In fact, even some slight inhibition can be observed, due to the formation of chain-terminating HO2* radicals in the alcohol co-oxidation. At 418 K, the chain length is estimated to be about 300-500 for conversions up to 10%.