Activation energy azoalkanes

An inspection of the compilation of the rate data in Table 4 makes it evident that an abnormally high frequency factor and a relatively low activation energy are characteristic features of the thermal decomposition of azoalkanes. The data coming from different laboratories show considerable scatter but in spite of this it is clearly seen that the activation energy is related to the stability of the alkyl radicals formed in the decomposition and declines with the stability of the radicals. The same trend but even more conspicuously manifested can be observed in the reactions of cyano and aryl substituted azoalkanes. These kinetic features explain... 

The activation energy, 30.8 kcal.mole of the decomposition of this molecule IS much lower than that with unsubstituted azoalkanes, and some 12 kcal.mole lower than with 2,2 -azoisobutane. This drop in activation energy caused by the introduction of the cyano groups into the molecule is interpreted by the resonance stabilization of the 2-cyanopropyl radical... 

The greater thermal stability of azoxyalkanes than azoalkanes carries over to compounds that lose N2O or N2 by concerted pericyclic retrocycloaddition. Thus the activation energy for N O... 

AHT = 19 + 1 kcal/mole, AS" " = -10.5 + 1 eu. Based on differences in the activation enthalpies between trans and cis azoalkanes, Nelsen 56) predicted that the difference between the cis and trans tetrazene activation energies ought to be on the order of 6-8 kcal/mole. It is apparent now that the difference is approximately double of that seen in azoalkanes. The primary reason that our tetrazene could be isolated at all is that the entropy of activation is quite negative, suggesting that the rather loose seven-membered ring must become more rigid in the transition state. It is therefore entirely possible that Ingold and Roberts may have been correct in their assumption that the photochemical decomposition of acyclic tetrazenes proceeds by photoisomerization to the cis form and the thermal decomposition of that isomer to the amino radicals. 

From the temperature dependence of the direct and sensitized photolysis of azoalkanes 3, the activation energies were estimated to be >3.3 kcal mol and >10.5 kcal mol for a-CN-bond breakage in the singlet and triplet states, and >7.9 kcal mol for the P-CC-bond cleavage in the triplet state. ... 

In contrast, the direct 185-nm excitation of 48 (Table 7 entry 5) preferably produces NBD, which indicates an ineffective S- T intersystem crossing, as is the case in the 185-nm photolysis of 49 (Table 7 entry 10). The thermolysis of both azoalkanes 48 and 49 was reported to yield NBD as the main product (Table 7 entries 1,6). Therefore, the 185-nm photodenitrogenation of 48 and 49 results from higher excited singlet states (Scheme 11). Furthermore, fast internal conversion (ca 10" s) may lead according to Kasha s rule to the population of vibrationally excited states which exhibit enough energy to overcome the small activation barrier (12-30 kJ mol ) of the N2 elimination from Si. 

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