Cyclopropane bond dissociation energy

The photochemistry of thietanes involves entirely different pathways from those encountered in azetidines the low bond dissociation energy of the C—S bond seems to be mainly responsible. The direct photolysis of thietane vapor with 213.9-228,8- and 253.7-nm light leads to ethylene and propylene, cyclopropane, and thiocyclopropene. A white polymer appeared as a constant by-product. ... 

The experimental bond dissociation energies of cyclopropane " and propane are 106.3 0.3 and 98.6 0.4 kcal mol", respectively. Since there are six C-H bonds in cyclopropane, each of which is 7.7 kcal mol stronger than the analogous bond in propane, one might conclude that cyclopropane is stabilized by... 

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. 

A molecule is strained when the sum of the dissociation energies of its bonds is smaller than the corresponding sum of an arbitrary unstrained reference molecule. For instance, the bond dissociations energies of the three CC-bonds in cyclopropane are smaller than three times the normal CC-bond dissociation energy. Strain leads to increased reactivity and decreased stability. 

FIGURE 5.7 The bond dissociation energy of a carbon-carbon bond in cyclopropane is only 65 kcal/mol. Cyclopropane is strained by about 23 kcal/mol. 

Bond homolysis of alkyl halides requires less energy than heterolytic bond dissociation however, photolytic products are formed which point to charged intermediates. Irradiation of diiodomethane in the presence of an alkene generates cyclopropanation [56, 57]. Initially, one of the C-I bonds breaks... 

In this paperthe relative stabilities of various small-ring propellanes are discussed in terms of enthalpies of hydrogenolysis of the conjoining bond and dissociation energies of this bond in the various substrates. This is perhaps the place to state that the mechanism of addition of bromine, in the dark, to the conjoining bond of several [m.n.l]propellanes, has been discussed in generaF. It is concluded that thermally initiated low temperature radical chain addition to the cyclopropane rings is involved. 

Adiabatic frequencies of cyclopropane are compared in Table 2 with those of some other hydrocarbons [28]. The adiabatic CC frequency is about 40 and the adiabatic CH stretching frequency about 130 cm i larger than the corresponding values for cyclohexane. Compared to ethene, the adiabatic CH stretching frequencies are almost identical, which is in line with the high dissociation energy of the CH bond of cyclopropane [29]. The same observation has been made... 

This is because there is a potential energy barrier for isomerization or dissociation that can only be overcome by adding energy to-the appropriate bonds. Let us consider as prototypes the isomerization of cyclopropane to propylene... 

The exchange process can be swiftly dismissed because of its small importance. Its simplest formulation involves the reversible dissociative chemisorption of the cyclopropane molecule as structure B (Figure 11.2). However, the reactivity of the C—H bond is probably not that much different from the bond in alkanes, while cyclopropane exchange appears to go best at low temperature, and has a low (and possibly even negative) activation energy. An alternative route is therefore through the undissociated structure A, where exchange occurs as a simple displacement (Scheme 11.1). There is no kinetic information to assist us. 

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