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

The proton and carbon-13 NMR spectra of a range of 1-(p-substituted-phenyl)pentazoles are reported. The influence of the -N-5 group on the aryl shifts is compared with that of the -NO2 and -N-3 groups. Direct kinetic measurements of the degradation of 1-arylpentazoles in CD3OD-CD2Cl2 (1:1, v/v) gave a Hammett rho value of + 1.25. For 1-(p-chlorophenyl)pentazole values of Delta E double dagger, 88.6 kJ mol(-1), Delta H double dagger 86.3 kJ mol(-1) and Delta S double dagger + 19.9 J mol(-1) K-1 are obtained at -10-0 degrees C. These data suggest a polar two-step mechanism with rate-determining cleavage of the 1-2 bond giving an unstable addo-azo (pentazene) intermediate, the nitrogen analogue of the azido-azomethine species of tetrazole ring-opening.