Propane, bond dissociation energy

Because the starting material (propane) and one of the products (H ) are the same m both processes the difference m bond dissociation energies is equal to the energy dif ference between an n propyl radical (primary) and an isopropyl radical (secondary) As depicted m Figure 4 20 the secondary radical is 13 kJ/mol (3 kcal/mol) more stable than the primary radical... 

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 energy in ethane is 368 kJ/mol (88 kcal/mol) and that in propane is 355 kJ/mol (85 kcal/mol)... 

The degree to which allylic radicals are stabilized by delocalization of the unpaired electron causes reactions that generate them to proceed more readily than those that give simple alkyl radicals Compare for example the bond dissociation energies of the pri mary C—H bonds of propane and propene... 

FIGURE 4.20 The bond dissociation energies of methylene and methyl C—H bonds in propane reveal difference in stabilities between two isomeric free radicals. The secondary radical is more stable than the primary. 

Here it should be noted that secondary C-H bond rupture is only slightly more probable than the scission of primary bonds, despite the fact that D(iso-C3H7—H) is 5-6 kcal./mole lower than D(m-C3Ht—H) (70,71). Hence, the bond-dissociation energy does not appear to be the major determining factor in the primary mode of decomposition. However, the results obtained by Palmer and Lossing (73) for the isobutane reaction do indicate that methyl substitution on the secondary position in propane causes C-H bond cleavage to occur preponderately at the tertiary site. 

The experimental bond dissociation energies of cyclopropane " and propane are 106.3 0.3 and 98.6 0.4 kcal mol", respectively. Since there are six C-H bonds in cyclopropane, each of which is 7.7 kcal mol stronger than the analogous bond in propane, one might conclude that cyclopropane is stabilized by... 

Explain why the bond dissociation energy for the C-C o bond in propane is lower than the bond dissociation energy for the C-C o bond in propene, CH3CH = CH2. 

A bromine radical can abstract either a 1° or a 2° hydrogen from propane, generating either a 1 ° radical or a 2° radical. Calculating AH° using bond dissociation energies reveals that both reactions are endothermic, but it takes less energy to form the more stable 2° radical. 

Problem 10.S Use the bond dissociation energies listed in Table 5.3 to calculate for the reactions of Cl and Br- with a secondary hydrogen atom of propane. Which reaction would you expect to be more selective ... 

While the focus of our research is to utimately activate methane to methanol, as is readily done by methane monooxygenase, we also want to understand what types of biomimics will activate higher homologues as well (C2, C3, and cycloC ). In addition, the bond dissociation energies may play an important role in our ability to activate methane at ambient temperature, since methane has the highest C-H bond dissociation energy (kcal) of all alkanes, i.e., methane(104) ethane(98) propane(96) and cyclohexane (94). 

An oxygen or sulfur also stabilizes an adjacent radical, so a SHT mechanism with a radical intermediate might he tin alternative. An estimate of the radical stabilization may be made by comparison of the primary ( II bond dissociation energies of propane (100 heal/mol) and dimethyl... 

A radical-based homodesmotic reaction gives a value of 30.4 kcal/mol, which compares with 29.1 kcal/mol for benzene by the same approach. " The gas phase heterolytic bond dissociation energy to form cyclopropenium ion from cyclopropene is 225 kcal/mol. This compares with 256 kcal/mol for formation of the allyl cation from propene and 268 kcal/mol for the 1-propyl cation from propane. It is clear that the cyclopropenyl cation is highly stabilized. 

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