Pyrolysis of aryl azides. III. Steric and electronic effects upon reaction rate

Abstract

First-order rate constants have been measured for the pyrolysis of 15 phenyl azides in decalin solution. The rate for phenyl azide is increased only slightly by all para and many ortho substituents; in these cases E_act and ΔS_act values are related linearly.     The very large rate increases when the ortho substituent is phenylazo, nitro, acetyl or benzoyl cannot be from steric or normal electronic effects and therefore identify a specific involvement of these groups in the transition state. This rate enhancement is reduced to scarcely significant levels by a 6-chloro or 6-methyl group in 2- nitrophenyl azide, but not by a 6-nitro group. These results raise doubts about recent claims¹ to establish mechanism by measuring polar effects on rates of pyrolysis of azides in which steric effects might also operate.