Gas‐phase thermolysis oftert‐butyl hydroperoxide with benzene and chlorobenzene in the temperature range 200‐300°C

Abstract

AbstractThermolysis oftert‐butyl hydroperoxide (1) in nitrogen with an excess of benzene in the temperature range 200‐300°C, leads to biphenyl (2) as the only observable benzene‐derived product. With chlorobenzene, dichlorobiphenyls (4) are formed, together with chlorobiphenyl (5),ca.12% on4, and phenol (20‐25% on4); chlorophenols are not produced. The use ofca.equimolar quantities of iodine based on1results in formation of iodobenzene (3) and chloroiodobenzenes (7), respectively, at the expense of biaryl. In air, chlorobenzene and1produce chlorophenols (8), biaryl then being a minor product.Hydroxyl radicals produced from1abstract hydrogen from benzene to give phenyl radicals (Ph•) which, in nitrogen, arylate benzene. From isomer distributions of7and of4it is inferred that hydrogen abstraction from chlorobenzene leads to chlorophenyl radicals, with a ratioo/m/p∼ 24/52/24. With chlorobenzene,ipsosubstitution, PhCl +•OH→PhOH, also takes place. In the presence of oxygen or iodine, formation of biaryls involves irreversible addition of (chloro)phenyl radical, the intermediate adduct radicals reacting with oxygen or iodine. In nitrogen, however, the first step in arylation is reversible. Apart from undergoing O‐O bond homolysis,1is attacked by•Me (formed from Me3CO•) and, to some extent, by aryl radicals. A large arene/1intake ratio (> 100), or the addition of either iodine or air suppresses ‐ or even prevents ‐ free‐radical‐induced decomposition of1.