Isobutane bond dissociation energy

The bond dissociation energy (BDE) and acidity constant (p ) of cubane (Cub—H) have been determined experimentally by Eaton and co-workers (Hare, M. Emrick, T. Eaton, P. E. Kass, S. R, J. Am. Chem. Soc., 1997, 119, 237-238). The BDE is unusually high for a tertiary C—H bond, 427 kJ/mol, about 25 kJ/mol higher than in isobutane. The C—H bond is also quite acidic, comparable to the acidity of the N—H bond in ammonia, pK = 36, indicating an unusual stability for the anion... 

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. 

This result can be explained by the following fact. The bond dissociation energies of the C-H bond in (CH3)3C-H (isobutane) and C6H5-H (benzene) are 91 kcal/mol and 112 kcal/mol, respectively. So, the bond dissociation energy of the C-H bond in benzene is 21 kcal/mol stronger than that in isobutane. This suggests that the phenyl radical is more unstable by about 21 kcal/mol than the Lbutyl radical, and therefore should be more reactive. 

From this value and known C—H bond dissociation energies, pK values can be calculated. The electrochemical measurements can be made on halides or on alkyl-lithium compounds. This type of approach has some significant uncertainties but nevertheless can provide a least a semiquantitative estimate of acidities of very weakly acidic hydrocarbons. The pK for isobutane obtained in this way is 71. The necessary electrochemical measurements cannot be made directly for methane, but an extrapolation from toluene and diphenylmethane leads to a range of 52-62 for the pK of methane. ... 

Use the bond-dissociation enthalpies in Table 4-2 (page 143) to calculate the heats of reaction for the two possible first propagation steps in the chlorination of isobutane. Use this information to draw a reaction-energy diagram like Figure 4-8, comparing the activation energies for formation of the two radicals. 

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... 

---