Reactions of free radicals with aromatic compounds in the gaseous phase. II. Kinetics of the reaction of methyl radicals with phenol

Abstract

Knowledge of the reactivity of phenols towards simple free radicals is needed to throw light on the behaviour of the phenolic substances involved in the pyrolysis of coal and other organic materials. In the present investigation the reaction between methyl radicals and phenol vapour has been studied a t total pressures from 0.5 to 3 cmHg and temperatures from 445 to 547°K, the concentrations of methyl radicals and phenol being varied from 2 × 10^-12 to 4 × 10^-11 and 1 × 10^-8 to 8 × 10^-7 mole cm^-3 respectively. The main products identified by gas chromatography were methane and o- and p-cresol, together with a little anisole and 2,4- and 2,6-dimethylphenol.

The cresols are produced via hydrogen abstraction

Diagram

followed by combination of a methyl radical at a ring position of the phenoxy radical either ortho or para to the oxygen atom, e.g. in the case of the para position:

Diagram

The kinetics can be explained by postulating (a) that the keto forms of the cresols (methylcyclohexadienones) formed initially by reaction (6) have a finite lifetime in the gaseous phase and (b) that these molecules, which contain a tertiary hydrogen atom α to a system of a carbonyl bond and two carbon-carbon double bonds, partly undergo hydrogen abstraction by methyl radicals before they are able to enolize:

CH₃· + (HCH₃ = C₆H₄ = O → CH₄ + CH₃C₆H₄O·

The mechanism is consistent with the kinetics of formation of methane, the distribu- tion of the free electron in the phenoxy radical, the formation of o- and p-cresols as major products, the kinetics of formation of the cresols, and the high reactivity of the intermediate product towards methyl radicals.