Ethane, bond dissociation energy

As the table indicates C—H bond dissociation energies m alkanes are approxi mately 375 to 435 kJ/mol (90-105 kcal/mol) Homolysis of the H—CH3 bond m methane gives methyl radical and requires 435 kJ/mol (104 kcal/mol) The dissociation energy of the H—CH2CH3 bond m ethane which gives a primary radical is somewhat less (410 kJ/mol or 98 kcal/mol) and is consistent with the notion that ethyl radical (primary) is more stable than methyl... 

Cleavage of the carbon-carbon bond in ethane yields two methyl radicals whereas propane yields an ethyl radical and one methyl radical Ethyl radical is more stable than methyl and so less energy is required to break the carbon-carbon bond in propane than in ethane The measured carbon-carbon bond dissociation energy in ethane is 368 kJ/mol (88 kcal/mol) and that in propane is 355 kJ/mol (85 kcal/mol)... 

Table 16. Bond Dissociation Energies (D ) of Methanes, Ethanes, and Ethers [75, 76, 7S]...

Table 1.11 Bond dissociation energy of methanes, ethanes, and halogenoethers ...

Table 10. Bond-Dissociation Energies for Methane. Ethane, and Their Fluorinaied Analogs8 ...

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. 

The C—H bond dissociation energy is not the same for all compounds. It depends on the other bonds to the carbon also. For example, the C—H bond dissociation energy for methane, CH4, is 104 kcal/mol (435 kJ/mol), compared to 98 kcal/mol (410 kJ/mol) for ethane. Of course, other bond dissociation energies also depend on the other bonds to the atoms involved. 

This extra stabilization can be demonstrated by examination of bond dissociation energies. Section 2.2 lists the bond dissociation energy of a CH bond of ethane as 98 kcal/mol (410 kJ/mol). This is the energy that must be added to ethane to homolyt-ically break one of its CH bonds. 

The CH bond in propene is weaker than the CH bond of ethane because the allyl radical is stabilized by resonance. The ethyl radical has no such resonance stabilization. The difference between these bond dissociation energies provides an estimate of the resonance stabilization of the allyl radical 13 kcal/mol (54 kJ/mol). 

Fig. 3. Isomeric energy differences of ethane and derived species in terms of bond dissociation energies (D), n bond energies ( > ), and divalent state stabilization energies (DSSE).

The RSE is calculated here as the difference between the homolytic C-C bond dissociation energy in ethane (5) and a symmetric hydrocarbon 6 resulting from dimerization of the substituted radical 2. By definition the C-C bonds cleaved in this process are unpolarized and, baring some strongly repulsive steric effects in symmetric dimer 6, the complications in the interpretation of substituent effects are thus avoided. Since two substituted radicals are formed in the process, the reaction enthalpy for the process shown in Equation 5.5 contains the substituent effect on radical stability twice. The actual RSE value is therefore only half of the reaction enthalpy for reaction 5.5 as expressed in Equation 5.6. 

Bond dissociation energies of the C - C bonds in ethane (a a bond only) and ethylene (one a and one k bond) can be used to estimate the strength of the k component of the double bond. If we assume that the 0 bond in ethylene is similar in strength to the 0 bond in ethane (88 kcal/mol), then the k bond is worth 64 kcal/mol. 

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