Ethylene dissociation energy

Resonance theory can also account for the stability of the allyl radical. For example, to form an ethylene radical from ethylene requites a bond dissociation energy of 410 kj/mol (98 kcal/mol), whereas the bond dissociation energy to form an allyl radical from propylene requites 368 kj/mol (88 kcal/mol). This difference results entirely from resonance stabilization. The electron spin resonance spectmm of the allyl radical shows three, not four, types of hydrogen signals. The infrared spectmm shows one type, not two, of carbon—carbon bonds. These data imply the existence, at least on the time scale probed, of a symmetric molecule. The two equivalent resonance stmctures for the allyl radical are as follows ... 

Table 17. Bond Dissociation Energies (D ) of Ethylenes and Benzenes [78]...

One measure of the strength of a bond is its bond dissociation energy. This topic will be introduced in Section 4.16 and applied to ethylene in Section 5.2. 

One of the earliest measurements of the gas-phase equilibrium acidity of propene involved measuring the rates of reaction of propene with hydroxide ion in both directions33. The resulting equilibrium constant gave A//acid = 391 1 kcalmol-1. In the case of ethylene, the acidity and independently measured electron affinity of vinyl radical were used to determine the bond dissociation energy, a quantity difficult to obtain accurately by other means8. 

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 ... 

Ligand dissociation energies without ethylene coordination are given in parentheses. 

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. ... 

Use the bond dissociation energies in Table 7.1 to calculate an approximate AH° (in kilojoules) for the reaction of ethylene with hydrogen to yield ethane. 

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