Photolysis of aromatic azides. Part 4.—Lifetimes of aromatic nitrenes and absolute rates of some of their reactions

Abstract

The transients in the flash photolysis of several aromatic azides are identified as triplet nitrenes by their spectra. Their half-lives are determined over a range of concentrations in hexane, ethanol and in several polymeric matrices (polystyrene, sucrose benzoate, polymethylmethacrylate) and are interpreted in terms of three processes : nitrene recombination, hydrogen abstraction from the solvent and reaction of the nitrene with an azido group. Recombination is diffusion-controlled (rate constant ∼10⁹ 1. mole^–1 sec^–1); the same applies to the reaction with an azido group. In contrast, hydrogen abstraction by an aromatic nitrene is a comparatively slow process, corresponding to a life-time (in ethanol) of about 2 × 10^–4 sec. This value is corroborated by experiments in soft polystyrene matrices (half-life 10^–3sec) and by the rate of intramolecular hydrogen abstraction in the cyclization of 2-nitrenobiphenyl to carbazole (half-life 8 × 10^–4 sec). In hard polymeric matrices, hydrogen abstraction is considerably slower (half-lives from 0.1 to 2 sec). Here the rate-determining step is configurational diffusion of CH bonds towards the electron-deficient nitrogen.