Pyrolysis of benzene and chlorobenzene in an atmosphere of hydrogen; formation of methane

Abstract

AbstractVapours of benzene and derivatives such as chlorobenzene are thermally unstable above ca. 500°C in both nitrogen and hydrogen atmospheres. Major products range from biaryls at lower temperatures to carbon black at temperatures above 1000°C. In hydrogen, dechlorination (ArCl → ArH) is a prominent reaction of (poly)chlorinated benzenes.In hydrogen, between 500‐1000°C, τ ∼ 5‐10 s, biaryl formation is accompanied by production of comparable amounts of methane. This reaction, for the major part, is shown not to be due to hydrogenolysis of carbonaceous deposits or of the small quantities of ethane/ethene which are also formed.A mechanistic rationale is presented based on isomerization of cyclohexadienyl radicals (formed in equilibrium with arene and •H), ultimately leading to •CH3 as the precursor of methane.