Vapour phase chemistry of arenes. Part 6. Arylation and transcyanation in the pyrolysis of benzonitrile

Abstract

Pyrolysis of benzonitrile (1) above 650 °C leads not only to biphenyldicarbonitriles (2) but also to similar proportions of biphenylmonocarbonitriles (3) and benzenedicarbonitriles (4). With added free radical sources such as formaldehyde or hexachloroethane the process can be made to occur below 500 °C. A homolytic mechanism is advanced involving aryl radicals and hydrogen atoms as intermediates. Isomer distributions of the products (2)–(4) are in full accord with this interpretation. The ‘mobility’ of cyano-groups is explained in terms of a transcyanation route (Ar·+ Ar′CN → ArCN + Ar′·) competing with arylation (biaryl formation). Differences between the pyrolysis of benzonitrile (1) and that of chlorobenzene [lower rate and/or shorter chain length for (1)] are due to the greater stability of adduct radicals, such as ·C₆H₆CN from (1) and H·, in comparison with ·C₆H₇ from benzene and H·.