Kinetics and mechanism of dimerisation of benzonitrile N-oxides to furazan N-oxides

Abstract

The rates of dimerisation of benzonitrile N-oxide and of some meta- and para-substituted derivatives, leading to diarylfurazan N-oxides, have been measured. The rate increases in the order m-Cl > p-Cl > H > p-Me > p-OMe, a Hammett-type relationship being obeyed (ρ=+0·86). The reaction rate is slightly affected by change of solvent; the reaction is faster by a maximum factor of ten in carbon tetrachloride than in other more polar or better solvating media, such as chlorinated hydrocarbons, alcohols, dioxan, and acetonitrile. Activation parameters are substantially unmodified in the listed solvents and are characterised by a negative value of the activation entropy (ca.–20 e.u.). No effect on rate by tertiary amines was observed. A one-step concerted mechanism is postulated as an alternative to a carbene mechanism previously suggested.