Neopentane bond energy

Neopentane adsorbs dissociatively on the Pt surface. A C-H bond is broken and a Pt-C and Pt-H bond are formed. The cleavage of the neopentane C-H bond is not influenced by the support. Moreover, as argued above, the adsorption enthalpy of H2 is > 45 kJ/mol regardless of the support, and on a basic support an additional H2 molecule is formed prior to the adsorption of neopentane. Thus, according to equation (6) the difference in Eapp between the basic and acidic support is given by the sum of the difference in the intrinsic Pt-C bond energy (16 kJ/mol lower for the basic support) and the H2 desorption enthalpy limit at the reaction temperature ... 

In addition to these exchange reactions, a number of alkane/alkane and al-kane/arene exchange reactions could be studied as equilibria (benzene, toluene, cyclopropane, methane, ethane, neopentane, cyclohexane). Determination of equilibrium constants allowed calculation of AG° values and estimation of relative metal-carbon bond energies. Wolczanski concluded that the differences between metal-carbon bond energies and the corresponding carbon-hydrogen bond energies were essentially the same [82]. 

The bond dissociation energy of neopentane was determined to be 3.9 1 kcal mol" smaller than that of methane, which is now accepted to be 105.1, making the neopentane bond 101.2. With Af//°(neo-C5Hi2)= — 40.2 we obtain AfH°(neo-C5Hjj) = 8.9 kcal mol" McMillen and Golden recommended 8.7. 

Figure 4.2. Rotational-energy barriers as a function of substitution. Tbe small barrier ( 2kcal) in ethane (a) is lowered even further ( O.Skcal) if three bonds are tied back by replacing three hydrogen atoms of a methyl group by a triple-bonded carbon, as in methylacetylene (b). The barrier is raised 4.2 kcal) when methyl groups replace the smaller hydrogen atoms, as in neopentane (c). Dipole forces raise the barrier further ( 15 kcal) in methylsuccinic acid (d) (cf. Figure 4.3). Steric hindrance is responsible for the high barrier (> 15 kcal) in the diphenyl derivative (e). (After...

The stereospecificity of these reactions is surprising in light of the large energies absorbpd by these molecules. Indeed, the major photochemical product of these photolyses was the alternate olefin isomer (1-butene was also observed). These results indicate that free rotation about the photo-excited double bond does not occur in those molecules that dimerize. This suggests the participation of ground state complexes or excimers in the photodimerization. This view is supported by the observations that dilution of cw-2-butene with neopentane (1 1) decreased the yield of dimers and a 1 4 dilution almost completely suppressed dimerization. 

Several mechanisms were proposed to interpret bond shift isomerization, each associated with some unique feature of the reacting alkane or the metal. Palladium, for example, is unreactive in the isomerization of neopentane, whereas neopentane readily undergoes isomerization on platinum and iridium. Kinetic studies also revealed that the activation energy for chain branching and the reverse process is higher than that of methyl shift and isomerization of neopentane. 

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