Pyrolysis of Aryl Azides. VII. Interpretation of Hammett Correlations of Rates of Pyrolysis of Substituted 2-Nitroazidobenzenes

Abstract

Rates and products of pyrolysis have been obtained for ten 2- nitroazidobenzenes with 4- or 5-substituents. Rate data at 100° are correlated with Hammett constants of the 4- or 5-substituents, with respect to both the azido (A) and nitro (N) reaction centres, and yield the equation log k = -3.23 + 1.20σ-/A -0.716σ_N. Dissection of this equation into σ_I and σ_R components was usefully attempted.

The results are consistent with an electrocyclic process in which delocalization of negative charge from the innermost azido nitrogen is important at the transition state, whereas substituent effects on bridging by nitro are less important. An interpretation is made of pyrolysis rates of 4-azido-3-nitropyridine, several 2,6- dinitroazidobenzenes, and methyl 2-azidobenzoate. The rate for 4-azido- 3-nitropyridine yields a σ^- value of 1.2 for the 4-aza 'substituent'.