Bond energies, in hydrocarbons

Table I.IS gives total bonding energies in kilocalories per mole for some simple molecules. The B3iyP results are comparable in accuracy to G1 and G2 results. Another comparison was done with a series of cyclic hydrocarbons as the test case. The calculations were done using an isodesmic reaction scheme. The results are given in Table 1.19. Density functional calculations have also been successfully extended to functionalized molecules. ...

Certain C—H bonds have significantly lower bond dissociation energies than do the normal C—H bonds in saturated hydrocarbons. Offer a structural rationalization of the lowered bond energy in each of the following compounds, relative to the saturated... 

This requires sufficient energy inserted into the relevant bond vibration for the bond to break or for bonding locations to move. C-C and C-H bond energies in stable alkanes are greater than 80 kcal/molc, and these processes are very infrequent. As we wiU see later, hydrocarbon decomposition, isomerization, and oxidation reactions occur primarily by chain reactions initiated by bond breaking but are propagated by much faster abstraction reactions of molecules with parent molecules. 

It is clear that proper description of the energetics of homolytic bond dissociation requires models that account for electron correlation. Are correlated models also needed for accurate descriptions of relative homolytic bond dissociation energies where the relevant reactions are expressed as isodesmic processes A single example suggests that they may not be. Table 6-15 compares calculated and measured CH bond dissociation energies in hydrocarbons, R-H, relative to the CH bond energy in methane as a standard ... 

Table 6-15 CH Bond Dissociation Energies in Hydrocarbons Relative to Methane... 

Yoshimuzi 17>, using this method with different values for m, calculated the electronic distribution produced by substituting a heteroatom for hydrogen. He found that a value of the m parameter such that ma = 0.12 was necessary in order to reproduce the dipole moments of a set of linear paraffins. Fukui et al. 18>, using the positive square root of 0.12, i.e. m— +0.35, were able to correlate the ionization potentials, heats of formation, and bond energies in linear as well as cyclic hydrocarbons and their derivatives. It was also shown that the method permits a coherent interpretation of inductive effects to be made so that a relation exists between some calculated values and the reactivity. 

Assuming BEP-type relationships to be valid, we can make a prediction of the selectivity of fhe Fischer-Tropsch reaction as a function of the M—C bond energy. In Figure 10, a schematic representation is given of the relative rates of production of particular groups of Fischer-Tropsch products as a function of fhe M—C interaction energy. Four types of reaction are compared coke or carbide formation, hydrocarbon chain growth, CH4 formation, and CO dissociation. 

Thermal Properties, Silanes have less thermal stabflity than hydrocarbon analogues. The C—H bond eneigy in methane is 414 kj / mol (98.9 kcal/mol) the Si—H bond energy in silane is 3781 /mol (90.3 kcal/mol) (10). Silane, however, is one of the most thermally stable inoiganic silanes. Decomposition occurs at 500 0 in the absence of catalytic surfaces, at 300°C in glass vessels, and at 180°C in the presence of charcoal (11). Disilanes and other members of the binary series are less stable. Halogen-substituted silanes are subject to disproportionation reactions at higher temperatures (12). 

We have successively used the Magnasco-Perico (1967) external criterion and the Boys (1960) internal criterion. The results obtained by the Magnasco-Perico procedure allowed us (Leroy and Peeters, 1975) to study the transferable properties of localized orbitals and to elaborate a simple parametric method to construct wave functions for saturated hydrocarbons (Degand et al., 1973), unsaturated hydrocarbons (Leroy and Peeters, 1974), heteroatomic aliphatic compounds (Clarisse et al., 1976), and polymers (Peeters et al., 1980). Furthermore, we have been able to analyze the concept of bond energy in terms of localized orbitals (Leroy et al., 1975). A careful review on the utilization of transferability in MO theory has been realized by O Leary et al. (1975). 

The mechanism via bromine atoms is supported by molecular bromine formation in the interaction of with Br in the absence of a hydrocarbon (Bf2 is apparently formed by bromine atom recombination). This mechanism is also consistent with the fact that bromide ions, while catalyzing the oxidation in the case of alkylaromatic compounds, are not particularly effective in the case of simple alkanes. This corresponds to the difference of bromine atom reactivity with respect to alkylaromatic and aliphatic hydrocarbons. The bond energy in the H-Br molecule (85 kcal mole ) is practically equal to the energy of the C-H bond in the n.-position to the aromatic ring, so that the reaction... 

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