Bond dissociation enthalpies propane

The CH bond dissociation enthalpy DH of 1 is 106.3 kcal mol 1, which is 11.2 kcal mol 1 larger than that of the secondary CH bond of propane (95.1 kcal mol 1)8911. At the UMP2/6-31G(d) level, a dissociation energy DE of 107.7 kcal mol"1 is calculated, which can be improved by using isogyric reactions (the number of unpaired electron spins is preserved) such as equation 21 ... 

In the chlorination of propane, the secondary hydrogen atom is abstracted more often because the secondary radical and the transition state leading to it are lower in energy than the primary radical and its transition state. Using the bond-dissociation enthalpies in Table 4-2 (page 143), we can calculate AH° for each of the possible... 

Because the starting material (propane) and one of the products (H-) are the same in both processes, the difference in bond dissociation enthalpies is equal to the energy difference between an n-propyl radical (primary) and an isopropyl radical (secondary). As depicted in Figure 4.19, the secondary radical is 10 kJ/mol (2 kcaFmol) more stable than the primary radical. 

The bond dissociation enthalpies of methylene and methyl C—H bonds in propane reveal a difference in stabilities between two isomeric free radicals. The secondary radical is more stable than the primary. 

Cleavage of the carbon-carbon bond in ethane yields two methyl radicals, whereas propane yields an ethyl radical and one methyl radical. Ethyl radical is more stable than methyl, and so less energy is required to break the carbon-carbon bond in propane than in ethane. The measured carbon-carbon bond dissociation enthalpy in ethane is 375 kJ/mol (90 kcal/mol), and that in propane is 369 kJ/mol (88 kcal/mol). 

The bond dissociation enthalpies of n-propyl and isopropyl chloride are the same within experimental error (Table 4.3). However, it is incorrect to conclude that the data indicate equal stabilities of n-propyl and isopropyl radical. Why Why are the bond dissociation enthalpies of propane a better indicator of the free-radical stabilities ... 

Using the table of bond dissociation enthalpies in Appendix 3, calculate ATf° for bromination of propane to give 2-bromopropane and hydrogen bromide. 

Delocalization of the unpaired electron stabilizes allylic radicals and causes reactions that generate them to proceed more readily than those that give simple alkyl radicals. Compare, for example, the bond dissociation enthalpies of the primary C—H bonds of propane and propene ... 

The dissociation enthalpy of the terminal C—H bond in propane is almost the same as that of ethane. The resulting free radical is primary (RCH2) in both cases. 

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