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 tat-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 combinationis 2-3 f0-3 in thissystem. The applicationof this source to kineticstudies is illustrated by the measurement of Arrhenius parameters for the metathesis between the tert-butyl radical and cyclohexadiene-l,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.

The photolysis of 3,3‐dimethylbutan‐2‐one (methyl t‐butyl ketone) and the decomposition of the acetyl radical

“…Blake et al [8] proposed that this ratio should be corrected for disproportionation, but application for this correction in (a'(CH3-, t-Bus) leads to an even lower value of 1.58. The mean result for A(t-Bu-, &Bus) of 2.63 f 0.17 compares quite well with 2.3 f 0.3 found by James and Suart [9] Overall, the agreement with measurements from other systems is fair, although if the expected disproportionation yields are calculated, there seems to be about 12yo more isobutane and 4% less isobutene in the present system than anticipated. The latter figure lies within experimental error, but it is more difficult to account for the extra isobutane.…”

The thermal decomposition of azomethane. III. Kinetics of the noninhibited reaction

“…is subject perhaps to a large error and therefore should not be taken too literally; but nevertheless the trend in Table VII shows unmistakably that at the higher pressures it could well be that reaction (9) is the major source of ethane, and not reaction (10). (25) and (26), respectively. The increase in R9/Rlo at 10 torr, relative to the trend at higher pressures, is probably due to a decrease of klo because of third-body restriction.…”

Photolysis of azomethane–propane mixtures. Arrhenius parameters for the hydrogen abstraction reactions of methyl with azomethane and with propane

“…there is a small molecular component in both of the processes CH3N2CH3 CzH6 + NS which, in some circumstances, invalidates the simple interpretation of the experimental results outlined above. The molecular component in the photolytic process has long been recognized [l], although there is some disagreement as to its importance, values ranging from 0.7 [2] to 1.2% [3] of the total photolysis. Appropriate correction for the molecular decomposition can be made, but the only work on hydrogen abstraction by methyl radicals generated by photolysis of azomethane seems to be the reaction with azomethane itself.…”