Bond dissociation enthalpies ethylene

So far, our laboratories have primarily focused on the development, testing, and implementation of ccCA, but early success has been obtained in utilizing ccCA to solve chemical problems. For instance, a prototype version of ccCA was used by our group in collaboration with the experimental group of Professor T. Brent Gunnoe (then of North Carolina State University) to compute bond-dissociation enthalpies (BDEs) of ethylene, formaldehyde, methylene imine, carbodiimide, isocyanide, and their hydrogenated counterparts in order to probe useful correlations between the free BDEs of these model substrates and the proclivity of their insertion into the Ru(II)-phenyl bond of a catalyst-active species [108]. The BDEs of these small model systems provided a useful diagnostic to explain the thermochemical preferences of certain types of bond insertions. 

On the basis of their bond-dissociation enthalpies, the C=C bond in ethylene is stronger than the C—C single bond in ethane, but it is not twice as strong. 

Values for bond lengths and bond strengths (bond dissociation enthalpies) for ethane, ethylene, and acetylene are given in Table 1.11. 

By comparison, the length of the carbon-carbon double bond in ethylene is 134 pm (1.34 A), and that of the carbon-carbon single bond in ethane is 153 pm (1.53 A). Thus, triple bonds are shorter than double bonds, which, in turn, are shorter than single bonds. The bond dissociation enthalpy of the carbon-carbon triple bond in acetylene [966 kj (231 kcal)/mol] is considerably larger than that for the carbon-carbon double bond in ethylene [727 kJ (174 kcal)/mol] and the carbon-carbon single bond in ethane [376 kJ (90 kcal)/mol]. The difference in bond dissociation enthalpies between the carbon-carbon triple bond in acetylene and the carbon-carbon double bond in ethylene is only 239 kJ (57 kcal)/mol. This difference indicates that a tt bond in an alkyne is weaker than a tt bond in an alkene. 

The reaction enthalpy and thus the RSE will be negative for all radicals, which are more stable than the methyl radical. Equation 1 describes nothing else but the difference in the bond dissociation energies (BDE) of CH3 - H and R - H, but avoids most of the technical complications involved in the determination of absolute BDEs. It can thus be expected that even moderately accurate theoretical methods give reasonable RSE values, while this is not so for the prediction of absolute BDEs. In principle, the isodesmic reaction described in Eq. 1 lends itself to all types of carbon-centered radicals. However, the error compensation responsible for the success of isodesmic equations becomes less effective with increasingly different electronic characteristics of the C - H bond in methane and the R - H bond. As a consequence the stability of a-radicals located at sp2 hybridized carbon atoms may best be described relative to the vinyl radical 3 and ethylene 4 ... 

Figure 4 Selectivity in epoxidation for a range of substrates plotted against the dissociation enthalpy of the weakest C-H bond in the olefin (m) TS-1 peroxide system (u) silver-oxygen system. 1. 1-octene, 2. 1-butane, 3. 2-butane, 4. gropene, 5. 4-unyltoluene, 6. 1-3 butadiene, 7. styrene, 8. 4-vinylpyridine, 9. ethylene.2...

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