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 -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 CD ₃OD-CD₂Cl₂ (1:1, v/v) gave a Hammett ρ 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.