Propane bond energy

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. 

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. 

Somewhat analogous reactions would be expected for the reaction of ethylene with 02 ions but the observed reaction rate is lower than for propene, suggesting that the reaction pathway may be controlled by the C—H bond energies. For reactions of propane and 1-butene with 02, oxygenated compounds of the same carbon number as the reactants were produced. The initial step is thought to involve a hydrogen atom abstraction from a secondary carbon atom. 

TABLE 8. Bond energies and strain energies of cyclopropane, cyclobutane and propane as calculated by the virial partitioning method"... 

Electron delocalization is an important factor in the reactivity (or lack of it) of organic molecules. As an example, recall from Chapter 4 that the bond energies of various types of C-H bonds differ considerably (see Table 4-6). In particular, the methyl C-H bond in propene is about 9 kcal weaker than the methyl C-H bond of ethane or propane, and this difference can be explained by the use of the resonance concept. The following bond dissociations are involved ... 

Fig. 2.9 The propane potential energy surface as the two HCCC dihedrals are varied (calculated by the AMI method, Chapter 6). Bond lengths and angles were not optimized as the dihedrals were varied, so this is not a relaxed PES however, changes in bond lengths and angles from one propane conformation to another are small, and the relaxed PES should be very similar to this one...

Fig. 2.10 The stationary points on the propane potential energy surface. Hydrogens at the end of CH bonds are omitted for clarity...

The hydrogenation of a mole of propylene, CH3 -CH CH2, to propane. CH3—CFI2—CII3, yields 28 kcal per rnoie. From the data in Table 9-1, verify the C=C bond energy of 146 kcal per gram bond. 

Equation 1 has certain desirable features. Apart from fitting the present data, it behaves well mathematically at X = 1 and thus is valuable for extrapolation to low and even zero conversions. Further, the value of Fact obtained is satisfyingly high (78 kcal/mol). The generally accepted, free radical mechanism requires a value of Eact equal to the C-C bond energy in propane (85 kcal), less a number related to one or more of the chain-carrying reactions (5-10 kcal), hence in the range of 75-80 kcal. 

Experiments show that it takes 1656 kJ/mol to break all the bonds in methane (CH4) and 4006 kJ/mol to break all the bonds in propane (C3H3). Based on these data, calculate the average bond energy of the C—C bond. 

Compare the value of-134.5 kJ/mol for the standard enthalpy change of formation of gaseous 2-methyl propane (C4Hi0) with that calculated from bond energy data (Table 3.6 and Appendix III). 

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