Hydrogenolysis and hydrocracking of the carbonoxygen bond. 3. Thermolysis in tetralin of substituted anisoles

Mild pyrolysis and hydrogenolysis products of coal contain substantial amounts of pyrocatechol and resorcinol and their homologues whereas hydroquinone and its homologues are absent or present in only low amounts. In the present work the model compounds anisole and methyl-, methoxy- or hydroxy-substituted anisoles were studied to elucidate substituent effects on the carbon—oxygen bond cleavage in the presence of tetralin. The experiments were carried out at 618 K and 6 MPa (H₂). The major reaction is demethylation to the corresponding phenols. A steric effect can be seen in the ortho compounds and an electronic effect when the substituent is a strongly electron-releasing group. In compounds with oxygen substituents para to each other little or no hydroquinone can be isolated whereas the ortho and meta compounds, respectively, give pyrocatechol and resorcinol. It is suggested that the low yield or absence of hydroquinone in this work and in coal pyrolysis is due to the high reactivity of the intermediate p-hydroxyphenoxy radical, which gives rise to adducts and other compounds of high molar mass. The ortho radical is sterically hindered and the meta radical has a lower reactivity and are hence abstracting hydrogen from the hydrogen donor or coal.