Isopentane, bond energy

These differences have been attributed to various factors caused by the introduction of new structural features. Thus isopentane has a tertiary carbon whose C—H bond does not have exactly the same amount of s character as the C—H bond in pentane, which for that matter contains secondary carbons not possessed by methane. It is known that D values, which can be measured, are not the same for primary, secondary, and tertiary C—H bonds (see Table 5.3). There is also the steric factor. Hence, it is certainly not correct to use the value of 99.5 kcal mol (416 kJ mol ) from methane as the E value for all C—H bonds. Several empirical equations have been devised that account for these factors the total energy can be computed if the proper set of parameters (one for each structural feature) is inserted. Of course these parameters are originally calculated from the known total energies of some molecules that contain the structural feature. 

All these results are readily interpreted by assuming the existence of two bond shift mechanisms. The first one, which accounts for methyl shift, may be ascribed to the metallocyclobutane mechanism responsible for the group III reactions of n-pentane and isopentane. The second one, which accounts for chain lengthening (and chain shortening) is the same as the mechanism of higher activation energy (group II) responsible for the interconversion between n-pentane and isopentane. The first is very sensitive to alkyl substitution, while the latter seems relatively insensitive to structural effects. 

If this relation is true for ketones, then the dissociation energies of a-C—H bonds in ketones may be estimated. Let DC H in cyclohexane be 89 kcal mole-1, then DC H for the cyclohexanone a-CH2 group will be 89 — AD = 85 kcal mole-1, since AD = 1.8/0.45 = 4 kcal mole-1. If DC H t in isopentane is taken as 85 kcal mole-1 (as in isobutane), then Dq h for methyl i-propyl ketone will be 85 — AQ = 85 — 4 = 81 kcal mole-1. The decrease in strength of the a-C—H bond of ketones in comparison with... 

The addition of nitric oxide and propylene to the reaction mixture produced no change in the rate of decomposition. (These substances are known to be inhibitors of chain reactions.) However, when nitric oxide was added, formaldoxime (CH2==NOH) was formed, and when propylene was added, appreciable amounts of isopentane, n-butane, and similar substances were found. The energy of the O—O bond in the peroxide has been estimated as 38 kcal/mole. 

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