Bond dissociation enthalpies ethane

To ensure thermochemicai consistency, the H-H and C-H bond dissociation enthaipies were aiso quoted from Ref 124. In the cases of ethane, butane, benzene, and toluene, these are not the most updated values (see, for instance, Blanksby, S. J. Ellison, G. B. Acc. Chem. Res. 2003, 36, 255), but the (upward) adjustments are smaller than the uncertainties of the metal-carbon bond dissociation enthalpies. 

As the table indicates, C—H bond dissociation enthalpies in alkanes are approximately 400 40 kJ/mol (95-105 kcal/mol). Cleaving the H—CH3 bond in methane gives methyl radical and requires 439 kJ/mol (105 kcal/mol). The dissociation enthalpy of the H—CH2CH3 bond in ethane, which gives a primary radical, is somewhat less (421 kJ/mol,... 

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 enthalpy in ethane is 375 kJ/mol (90 kcal/mol), and that in propane is 369 kJ/mol (88 kcal/mol). 

We can understand why bromination is more selective than chlorination by using bond dissociation enthalpies (Table 4.3) to calculate the energy changes for the propagation step in which each halogen atom abstracts a hydrogen from ethane. 

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. 

To predict which of the various C—H bonds in propene is most likely to break when a mixture of propene and bromine or chlorine is heated, we need to look at bond dissociation enthalpies. We find that the bond dissociation enthalpy of an allylic C—H bond in propene (Table 8.7) is approximately 92 kj (22 kcal)/mol less than that of a vinylic C—H bond and 50 kJ (12 kcal)/mol less than a C—H bond of ethane. The allyl radical is even more stable than a 3° radical this unusual stability also applies to carbocations. The reason the allylic C—H bond is so weak is discussed in Section 8.6B. Note from Table 8.7 that the benzyl radical CgH5CH2- is stabilized in exactly the same way as the allyl radical and for the same reason benzylic compounds undergo many of the same reactions as allylic compounds (Section 21.5). 

Table 7.3 The C-C bond energy of ethane by HF, MP2(fc), and DFT (B3LYP, M06, and TPSS) calculations, at 0 and 298 K. The basis set is 6-31G. Standard, tabulated bond energies are for dissociation at 298 K. Bond energy = 2(CH3 radical enthalpy) - (CH3CH3 enthalpy). For the radical the unrestricted method (UHF etc.) was used. For the 0 K dissociation enthalpy, the HF and MP2 calculations use energies corrected for ZPE, with the ZPE itself corrected by a factor of 0.9135 (HF) or 0.9670 (MP2) [77]. The 0 K dissociation enthalpy for the DFT calculations is uncorrected for ZPE, and the 298 K dissociation enthalpy is from standard statistical thermodynamics methods [79]. The experimental C-C energy of ethane has been reported as 90.1 0.1 kcal mol-1, i.e. 377 0.4 kJ mol-1 [80]. Calculations are by the author...

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. 

For example the enthalpy of formation of ethane may be calculated from bond dissociation energies e as follows. 

The activation enthalpies for these reactions are respectively 10 and 7 kcal/mol greater than the corresponding Mn-L bond strengths in heptane. Assuming that methylcyclohexane and heptane are similar as solvents and that both reactions proceed via dissociative pathways as proposed, this implies that the Mn-heptane interaction is close to 8 or 9 kcal/mol. This is close to the 10 kcal/mol Cr-heptane interaction in (CO)5Cr-heptane (2,4) and the 9.6 kcal/mol W-ethane interaction in (CO)5W-ethane (22). We can also calculate the gas phase Mn-CO bond dissociation energy in CpMn(CO)3 to be close to 55 kcal/mol. Further experiments are necessary to confirm the magnitude of the Mn-heptane interaction. 

The dissociation enthalpy of the terminal C—H bond in propane is almost the same as that of ethane. The resulting free radical is primary (RCH2) in both cases. 

The bond-dissociation energy of a bond in a molecule is the energy required to break that bond alone —that is, to split the molecule into the two parts that were previously connected by that bond. For example, the bond-dissociation energy of the C—C bond in ethane, H3C—CH3, is the enthalpy of dissociation of ethane into two methyl radicals, CH3. 

Consider the experimentally determined enthalpy changes for the breaking, or dissociation, of a C—H bond in methane, CH4, and in ethane, C2H6, in the gas phase ... 

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