o-Quinone methide as a common intermediate in the pyrolysis of o-hydroxybenzyl alcohol, chroman and 1,4-benzodioxin
The product composition in the very low pressure pyrolysis (550–1210 K) of o-hydroxybenzyl alcohol (HBA), 3,4-dihydro-2H-1-benzopyran (chroman), and 1,4-benzodioxin (BD) indicates that o-quinone methide (o-QM) is the common intermediate in each case. At complete conversion of HBA, o-QM was observed as the only product and the mass spectrum of o-QM could be obtained. At higher temperatures (>950 K), o-QM is subsequently converted into benzene and CO. The thermolysis process for chroman starts with cleavage of the phenoxy–carbon bond and proceeds with ethene elimination, yielding o-QM. The high pressure rate parameters for unimolecular decay have been determined to obey kchroman/s–1 = 1015.3 exp (–269/RT). For BD only the cleavage of the phenyl–vinoxy bond has been observed, and after rearrangement and CO elimination o-QM is formed. The Arrhenius equation for the overall rate of disappearance has been found as kBD/s–1 = 1015.6 exp (–310/RT). Ultimately (1100 K) the thermolysis of BD leads to 1 mole of benzene and 2 moles of CO.