Pentazoles: proton and carbon-13 NMR spectra of some 1-arylpentazoles: kinetics and mechanism of degradation of the arylpentazole system

Abstract

The proton and carbon-13 NMR spectra of a range of 1-(p-substituted-phenyl)pentazoles are reported. The influence of the –N₅ group on the aryl shifts is compared with that of the –NO₂ and –N₃ groups. Direct kinetic measurements of the degradation of 1-arylpentazoles in CD₃OD–CD₂Cl₂(1:1, v/v) gave a Hammett p value of + 1.25. For 1-(p-chlorophenyl)pentazole values of ΔE^‡, 88.6 kJ mol^–1, ΔH^‡ 86.3 kJ mol^–1 and ΔS^‡+ 19.9 J mol^–1 K^–1 are obtained at –10–0 °C. These data suggest a polar two-step mechanism with rate-determining cleavage of the 1–2 bond giving an unstable azido–azo (pentazene) intermediate, the nitrogen analogue of the azido–azomethine species of tetrazole ring-opening.