Comparative study of the photolysis of 2,2?-azoisobutane and azomethane

Abstract

The characteristics of the photolysis of 2,2′-azoisobutane imply that this system provides a good source for the study of the reactions of tert-butyl radicals below 100°C. The reactivity of tert-butyl radicals toward the parent azoalkane is extremely low; under typical conditions metathesis is negligible and addition to the double bond is sterically very unfavourable. The rate of generation of tert-butyl radicals is satisfactory although the quantum yield of nitrogen is dependent upon reaction conditions, implying that the collisional deactivation of electronically excited 2,2′-azoisobutane molecules is a significant but not a predominant process. The concentration of tert-butyl radicals may be estimated conveniently from the rate of formation of isobutene as the ratio of disproportionation to combination is 2.3 ± 0.3 in this system. The application of this source to kinetic studies is illustrated by the measurement of Arrhenius parameters for the metathesis between the tert-butyl radical and cyclohexadiene-1,4 over the range 27–100°. Azomethane was photolyzed under similar conditions to provide a basis for comparison and to supplement our knowledge of this system. The quantum yields of nitrogen and of intramolecular elimination of ethane were independent of temperature between 63 and 218°, implying that collisional deactivation of electronically excited molecules is insignificant for azomethane. New estimates of the Arrhenius parameters for addition and metathesis between the methyl radical and azomethane are discussed.