Hydrocarbons, saturated, bond energies

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... 

Data are given in Table IV for heterocyclic compounds. For piperidine there is no difference between E and E, showing that the bond energies used are applicable to saturated heterocyclic molecules. Pyridine and quinoline differ from benzene and naphthalene only by the presence of one N in place of CH and, as expected, the values 1.87 v.e. and 3.01 v.e., respectively, of the resonance energy are equal to within 10 percent to the values for the corresponding hydrocarbons. 

The flexibility and internal consistency of the present theory are well illustrated by the transformations that generate the sets of parameters required for the unsamrated hydrocarbons from those of their saturated models. But most importantly they preserve the original form and great simphcity of the basic bond energy formula, Ski = Ski + akAqk + aik qu as well as its accuracy. 

Saturated hydrocarbon substituents in the molecule have little effect on the bond energies with the catalyst. On the contrary, the appearance of stabilization energy, for example, of the carboxyl groups, has a profound influence on QAK ... 

The propagation step 8.14 is of special importance. Its rate depends on the R-H bond strength the weaker the bond, the faster the reaction. This explains why autoxidation of aldehydes is much easier than that of saturated hydrocarbons. The C-H bond energies of -CHO- and -CH2- groups are approx-... 

Isomeric effect—The additivity rule for bond energies applies only to compounds in a homologous series and small changes in structure cause deviations in the heats of formation. If the additivity rule were correct, saturated hydrocarbons possessing the same number of carbon and hydrogen atoms would have identical heats of formation. This, however, is not correct as is shown in Table CIX, where the experimental data indicate that the heat of formation... 

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). 

Relative energies of saturated molecules are also calculated well semi-empirically if constant bond energies are assumed and zero point vibrations and London-Van der Waals attractions between non-bonded regions are considered. In this way, the heats of formation and isomerization of saturated hydrocarbons are obtained to within 0.01 ev or 0.2 kcal/mole. 

Empirical bond energies are transferable from molecule to molecule, for example, in saturated hydrocarbons to within 0.01 ev. Are the detailed effects of molecular environment, implicit in the equations given above, compatible with such a constancy of bond energies ... 

The rate constant k(T) = 1.1 x 10 exp[-(9.4 2) kJ morVRT] cm -mol s" for the aromatic hydrocarbon CgHgCHg was measured in an isothermal flow reactor in the temperature range 297 to 542 K and interpreted to be due to hydrogen abstraction, since the activation energy fits an Evans-Polanyi plot for saturated hydrocarbons [151]. The effect of the aromatic ring on the reactivity is mainly due to the reduction of the CH bond energy in the methyl group. 

The chemical structure of a polymer is one of the factors that determine its reactivity, see Table 1.9. That is why polymers with saturated unbranched hydrocarbon chain structure are the most oxidation resistant, reacting extremely slowly with oxygen under normal conditions and in the absence of light. Even in the course of several years of exposure or service, these products do not change their properties. Although the bond energy of C-C-bonds is clearly weaker than that of C-H bonds, C-H bonds are attacked almost exclusively radically. Here, neither degree of poly-... 

The bond energies of saturated and unsaturated homologues of hydrocarbons may be treated in some detail since they will give us some understanding of the correlation of bond energies in polyolefins. In saturated hydrocarbons with localized electrons it is reasonable to equate the energy of a bond with the energy of the... 

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