Cyclopropane Bond energies

Use your cyclopropane bond energies to calculate AH° values for the following... 

This is the case for the quadricyclane - to norbornadiene" reaction. Although the C2K reaction path provides an attractive interpretational tool for understanding the progress of this reaction, its highest point represents a conical intersection at which the two relevant states have the same energy at the same geometry. This point cannot be a transition state, so that lowering the symmetry in any direction leads to a stabilization. The result is an asynchronous reaction path in which one of the two cyclopropane bonds is broken first to form the biradical-like transition state la. The second bond can then break to form the norbornadiene radical cation 2. 

What seems intriguing under the circumstances is a detail in the interpretation of ring strain. Present results (Table 1.1) suggest that the CC bond energy in cyclopropane is practically that of ethane—which seems odd, at first sight. Note that if we constmct cyclopropane with the CC and CH bond energies of ethane, 69.63 and 106.81 kcal/mol, respectively, we get an estimated AE of 849.75 kcal/mol, which is alsmost the experimental value 851.0 kcal/mol. 

TABLE 8. Bond energies and strain energies of cyclopropane, cyclobutane and propane as calculated by the virial partitioning method"... 

Cyclopropane has a A//0 of combustion 27.7 kcal mole-1 greater than expected from bond energies, and this clearly is associated with the abnormal C-C-C bond angles in the ring. These matters will be discussed in detail in Chapter 12. For cyclohexane, which has normal bond angles, the heat of combustion is close to the calculated value. 

The heat of combustion of cyclopropane, (CH2)3, to give carbon dioxide and liquid water is 499.8 kcal mole-1. Show how this value, assuming normal C-H bond strengths, can be used to calculate the average C-C bond energy of cyclopropane. 

The cyclopropane synthesis is also suitable for the preparation of highly strained bicyclic hydrocarbons such as [2.1.0]bicyclopentanes (14) and spiropentanes (16) [14a,b]. The formation of the spiropentane 16 is particularly remarkable as it is the result of a homolytic hydrogen abstraction from a cyclopropane ring. Those processes are very rarely observed due to the relatively high C-H-bond energies of cyclopropanes (Sch. 8). 

Cremer, D. Gauss, J. Theoretical determination of molecular structure and conformation. 20. Reevaluation of the strain energies of cyclopropane and cyclobutane - CC and CH bond energies, 1,3 interactions, and o-aromaticity, J. Am. Chem. Soc. 1986, 108, 7467-7477. 

The latter occurs despite the fact that the central bond is orthogonal to the SOMO. The reason for this exceptional behavior has been attributed to a much greater strain relief in this process compared to that in which an outer cyclopropane bond is cleaved. Using the ring strain energies of the hydrocarbons as an approximation for those of the... 

In addition to these exchange reactions, a number of alkane/alkane and al-kane/arene exchange reactions could be studied as equilibria (benzene, toluene, cyclopropane, methane, ethane, neopentane, cyclohexane). Determination of equilibrium constants allowed calculation of AG° values and estimation of relative metal-carbon bond energies. Wolczanski concluded that the differences between metal-carbon bond energies and the corresponding carbon-hydrogen bond energies were essentially the same [82]. 

The stabilization energy for the cyclopropyloxide 4 was calculated to be 2.3 kcalmol . This fairly small stabilization energy, consistent with the weak 7t-acceptor properties of a cyclopropyl group, is however sufficient to allow experimental observation of the norcaradiene oxide 4. All cyclopropane bonds in 4 are elongated, so that the norcaradiene is stabilized. ... 

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