Carbon tetrachloride, bond dissociation energy

In so far as the rate of formation of radicals reflects their stability or reactivity the findings of Hart and Wyman are instructive. In carbon tetrachloride the rate of decomposition of benzoyl peroxide was twice as fast as that of biscyclopropanoyl peroxide. Ingold and coworkers have found that in the photodecomposition of benzoyl and biscyclopropanoyl peroxides, in carbon tetrachloride at 298 K, the phenyl radicals produced reacted faster (7.8 x 10 M s ) than the cyclopropyl radicals (1.5 X 10 M s ). These results are consistent with C-H bond dissociation energies for benzene (llOkcalmol) and cyclopropane (106kcal mol ) which implies that the cyclopropyl radical should be less reactive than the phenyl radical. In subsequent work they also showed that at ambient temperatures radical reactivities decreased along the series /c = Ph > (Me)2 C=CH > cyclopropyl > Me. Table 4 records the absolute rate constants for the reaction of these radicals with tri-n-butylgermane. 

This reaction was reinvestigated recently in carbon tetrachloride solution at 20 kHz.57 Sonolysis of the solvent was not detected and, if present, does not induce any isomerization. Of the several alkyl bromides tried, f-butyl bromide is the most efficient, a result which is explained by its higher volatility and the lower dissociation energy of the C-Br bond. It was concluded that the rate limiting step is actually the sonolysis of the carbon-halogen bond inside the bubble.58... 

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