Bonding considerations isomeric energy differences

The selectivity toward product molecules with the same number of carbon atoms as in the reactant varies significantly for the different transition metals. The stronger the metal-carbon surface bond strength, the lower the activation energy for dissociation and the lower the selectivity becomes. For this reason platinum, which represents the transition metal with the lowest reactivity, is the preferred metal for the catalytic isomerization or aromatization of alkanes. The Group I-B metals, Cu, Ag, and Au, form weaker metal-carbon bonds, but have a reactivity that is too low. In the case of Ag and Au the interaction energies become so weak that neither H2 nor C-H bonds dissociate. On Cu such bond dissociation is possible, but with a considerable activation energy. 

The very low value of the energy of activation for this isomerization is of considerable interest. Comparison with the decomposition of cyclobutane shows a reduction of 30 kcal mole caused by the presence of the double bond. If a similar transition state were involved in both reactions, then this difference would be a measure of the extra strain energy of the cyclobutene. This is quite unrealistically high. Thus we eliminate the possibility that the reaction path is as shown below ... 

Semiempirical methods of calculation with consideration of all valence electrons have been used only recently but already have given results on the reactivities of some aromatic and heteroaromatic com-pounds. " Thus, to analyze the reactivities of thiophene and the isomeric thienothiophenes 1-3 to electrophilic substitution, the semiempirical SCF LCAO MO method CNDO/2 was used, taking into account all valence electrons.The 3s, 3p, and 3d orbitals have been taken into account for the sulfur atom. Tlie reactivities were estimated from the difference between bond energies of the initial and the protonated molecule (in a complex). ... 

The concept of conformational isomerism is central to any consideration of molecular shape. Molecules that are flexible may exist in many different shapes or conformers. Conformational isomerism is the process whereby a single molecule undergoes transitions from one shape to another the physical properties of the molecule have not changed, merely the shape. Conformational isomerism is demonstrated by compounds in which the free rotation of atoms around chemical bonds is not significantly hindered. The energy barrier to the transition between different conformations is usually very low... 

The propionyl CoA carboxylation is another reaction in which the energy required to synthesize a car-bon-to-carbon bond is provided by ATP. Thus, the free energy of the pyrophosphate bond is transferred to CO2, which forms an N-carboxy bond with the biotin enzyme complex. Some of the molecular details of the propionyl carboxylation reaction are presented in Fig. 1-27. The isomerization of methylmalonyl CoA to succinyl CoA also involves vitamin B12. At first it was demonstrated that the activity of the isomerase was considerably decreased in the liver of vitamin B 12-deficient rats. Later the stimulating effect of 5,6-dimethylbenzimidazole carbamide coenzyme was demonstrated with partially purified preparations of the liver enzyme. The methylmalonate CoA-isomerase complex purified from sheep liver has been divided into two different protein fractions, one with isomerase and the other racemase activity. The isomerase acts on only one of the enantiomorphs of methylmalonyl CoA, but the absolute formula of the enantiomorph that serves as substrate for the isomerase is unknown. A racemase catalyzes the conversion of the alternate enantiomorph to the substrate of the isomerase. 

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