Bond energies alkanes

It has already been stated that the evaluation of the non-bonded energy is by far the most time-consuming. Consider a series of calculations of linear alkanes CH3(CH2)n-2CH3. The number of individual contributions to each energy term is given in Table 2.4. 

Dispersion forces, dipole Interactions, and hydrogen bonds all are significantly weaker than covalent Intramolecular bonds. For example, the average C—C bond energy Is 345 kJ/mol, whereas dispersion forces are just 0.1 to 5 kJ/mol for small alkanes such as propane. Dipolar Interactions between polar molecules such as ace-tone range between 5 and 20 kJ/mol, and hydrogen bonds range between 5 and 50 kJ/mol. 

Considerable interest in the subject of C-H bond activation at transition-metal centers has developed in the past several years (2), stimulated by the observation that even saturated hydrocarbons can react with little or no activation energy under appropriate conditions. Interestingly, gas phase studies of the reactions of saturated hydrocarbons at transition-metal centers were reported as early as 1973 (3). More recently, ion cyclotron resonance and ion beam experiments have provided many examples of the activation of both C-H and C-C bonds of alkanes by transition-metal ions in the gas phase (4). These gas phase studies have provided a plethora of highly speculative reaction mechanisms. Conventional mechanistic probes, such as isotopic labeling, have served mainly to indicate the complexity of "simple" processes such as the dehydrogenation of alkanes (5). More sophisticated techniques, such as multiphoton infrared laser activation (6) and the determination of kinetic energy release distributions (7), have revealed important features of the potential energy surfaces associated with the reactions of small molecules at transition metal centers. 

When ethanol was substituted for propane, fewer moles of oxygen were required, but fewer moles of carbon dioxide and water were produced. The bond energies of the reactants decreased by (6486 kJ - 4726 kJ) = 1760 kJ, but the bond energies of the products decreased even more by (8498 kJ - 5974 kJ) = 2520 kJ. Therefore, we can deduce that the combustion of ethanol is less exothermic than that of propane and the other alkanes. 

The functionalization reaction as shown in Scheme 1(A) clearly requires the breaking of a C-H bond at some point in the reaction sequence. This step is most difficult to achieve for R = alkyl as both the heterolytic and homolytic C-H bond dissociation energies are high. For example, the pKa of methane is estimated to be ca. 48 (6,7). Bond heterolysis, thus, hardly appears feasible. C-H bond homolysis also appears difficult, since the C-H bonds of alkanes are among the strongest single bonds in nature. This is particularly true for primary carbons and for methane, where the radicals which would result from homolysis are not stabilized. The bond energy (homolytic dissociation enthalpy at 25 °C) of methane is 105 kcal/mol (8). 

Consider the effect of monomer structure on the enthalpy of polymerization. The AH values for ethylene, propene, and 1-buene are very close to the difference (82-90 kJ mol 1) between the bond energies of the re-bond in an alkene and the a-bond in an alkane. The AH values for the other monomers vary considerably. The variations in AH for differently substituted ethy-lenes arise from any of the following effects ... 

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. 

This concludes the derivation of our bond energy formula and the presentation of simple examples pertaining to saturated systems, namely, the alkanes, including their numerical parameterization. 

Figure 12.2. A comparison between reorganizational energies and theoretical bond energies of alkane CH bonds (kcal/mol). The numbering refers to that shown in Table 12.3 [233].

Alkane carbon atoms satisfy the charge-NMR shift correlation [Eq. (6.8)]. With the alkylamines, things could be different because of a possible extra effect due to the presence of the nitrogen atom a-carbons should perhaps be compared only among themselves, and so should the /3- and y-carbons. The S-carbons, in contrast, which are sufficiently separated from the nitrogen center, could probably be treated as if they were part of an alkane. This point has been examined as follows for the —C 2—H2—NH2 motif, focusing on the dissociation and intrinsic bond energies, Dc Cp and sc Cp, respectively. 

Initially, we will be concerned with the physical properties of alkanes and how these properties can be correlated by the important concept of homology. This will be followed by a brief survey of the occurrence and uses of hydrocarbons, with special reference to the petroleum industry. Chemical reactions of alkanes then will be discussed, with special emphasis on combustion and substitution reactions. These reactions are employed to illustrate how we can predict and use energy changes — particularly AH, the heat evolved or absorbed by a reacting system, which often can be estimated from bond energies. Then we consider some of the problems involved in predicting reaction rates in the context of a specific reaction, the chlorination of methane. The example is complex, but it has the virtue that we are able to break the overall reaction into quite simple steps. 

One reason alkenes and alkynes react more readily than alkanes is because the carbon-carbon bonds of a multiple bond are individually weaker than normal carbon-carbon single bonds. Consider the bond energies involved. According to Table 4-3, the strengths of carbon-carbon single, double, and triple bonds are 83, 146, and 200 kcal, respectively. From these values we can calculate that cleavage of one-half of a carbon-carbon double bond should require 63 kcal and cleavage of one-third of a carbon-carbon triple bond should require 54 kcal ... 

However, this simple model does not explain the (kinetically) enhanced reactivity of arenes relative to alkanes, considering the higher (Dc-h) bond energy of arenes, i.e. Dc-h, benzene — 111 lccal mol-1 vs. Dc-h,alkane 95 lccal mol-1. Many more examples of metal-activated cleavage of C-H bonds are known for aromatic compounds than for alkanes [37]. To account for this difference, arene/metal coordination is proposed [38], although experimental evidence for such intermediate complexes and their structural features is lacking. 

The success of empirical additivity schemes [11, 12] is based on the recognition that the contributions to the enthalpy depend not only on the atoms and bonds present but also on their particular grouping, for example as CH3, CH2, or CH. Thus, popular and successful estimation schemes use parameters which are proportional to the number of groups of atoms in the molecule, implicitly or explicitly using for all series a methylene increment derived from alkanes. The enthalpy changes resulting from the presence of heteroatoms requires appropriately derived bond energy terms. If there are any real differences in the... 

For comparison, 39.025936 hartree is the bond energy methylene increment found in Ref. 13d. It was derived from the alkanes and then transferred to functionalized alkanes. 

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