Isobutane, bond energy

Local density models yield bond separation energies of similar quality to those from corresponding (same basis set) Hartree-Fock models. Bond separation energies for isobutane and for trimethylamine, which were underestimated with Hartree-Fock models, are now well described. However, local density models do an even poorer job than Hartree-Fock models with benzene and with small-ring compounds. 

The bond dissociation energy (BDE) and acidity constant (p ) of cubane (Cub—H) have been determined experimentally by Eaton and co-workers (Hare, M. Emrick, T. Eaton, P. E. Kass, S. R, J. Am. Chem. Soc., 1997, 119, 237-238). The BDE is unusually high for a tertiary C—H bond, 427 kJ/mol, about 25 kJ/mol higher than in isobutane. The C—H bond is also quite acidic, comparable to the acidity of the N—H bond in ammonia, pK = 36, indicating an unusual stability for the anion... 

In this paper, we will report the electronic and catalytic reactivities of the model VC/V(110) surface, and our attempt to extend them to VC powder catalysts. By using high-resolution electron energy loss spectroscopy (HREELS) and NEXAFS techniques, we observed that the surface properties of V(110) could be significantly modified by the formation of vanadium carbide some of the experimental results on these model surfaces were published previously.3-5 We will discuss the selective activation of the C-H bond of isobutane and the C=C bond of isobutene on V(110) and on VC/V(110) model systems. These results will be compared to the catalytic performances of vanadium and vanadium carbide powder materials in the dehydrogenation of isobutane. 

Here it should be noted that secondary C-H bond rupture is only slightly more probable than the scission of primary bonds, despite the fact that D(iso-C3H7—H) is 5-6 kcal./mole lower than D(m-C3Ht—H) (70,71). Hence, the bond-dissociation energy does not appear to be the major determining factor in the primary mode of decomposition. However, the results obtained by Palmer and Lossing (73) for the isobutane reaction do indicate that methyl substitution on the secondary position in propane causes C-H bond cleavage to occur preponderately at the tertiary site. 

This result can be explained by the following fact. The bond dissociation energies of the C-H bond in (CH3)3C-H (isobutane) and C6H5-H (benzene) are 91 kcal/mol and 112 kcal/mol, respectively. So, the bond dissociation energy of the C-H bond in benzene is 21 kcal/mol stronger than that in isobutane. This suggests that the phenyl radical is more unstable by about 21 kcal/mol than the Lbutyl radical, and therefore should be more reactive. 

Use the bond-dissociation enthalpies in Table 4-2 (page 143) to calculate the heats of reaction for the two possible first propagation steps in the chlorination of isobutane. Use this information to draw a reaction-energy diagram like Figure 4-8, comparing the activation energies for formation of the two radicals. 

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