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.
Novel free radicals in synthetic and natural pheomelanins: distinction between dopa melanins and cysteinyldopa melanins by ESR spectroscopy.