Nucleophilic displacement in polyhalogenoaromatic compounds. Part 9. Kinetics of azidodefluorination and methoxydefluorination of some polyfluorobenzonitriles

Abstract

Pentafluorobenzonitrile and 4-chlorotetrafluorobenzonitrile do not show good second-order kinetic behaviour in their reactions with sodium methoxide in methanol. The reaction rate falls rapidly until a position of apparent equilibrium is reached, after which methoxydefluorination proceeds slowly to completion. This behaviour appears to arise from competition between methoxydefluorination and a reversible addition of methoxide ion to the carbon–nitrogen triple bond. The reaction of azide ion with polyfluorobenzonitriles does not show this competition. The sensitivity of azidodefluorination in methanol at 323.2 K to para-substituent effects in C₆F₅X, as shown by the Hammett reaction constant (ρ 6.2 ± 0.3), is slightly greater than that shown by methoxydefluorination under the same conditions, which is consistent with a less reactive and more selective reagent. The azidodefluorination of 4-substituted tetrafluorobenzonitriles, although involving attack at positions ortho to the cyano-group and meta to X, shows the same sensitivity to substituent effects (ρ 6.0 ± 0.5); this, in turn, suggests that the electronic effect of the cyano-group is less perturbed by the electron density of the ring upon which it acts than is the electronic effect of the nitro-group.