Transition state energies, hydrocarbon activation

Figure 2 AAHS P + + Px) is the difference between the solvation energy of (Pn+ + Px) in hydrocarbon solvent and in a polar solvent, and AAHS is the corresponding difference for the Transition State Complex (TrStC). Since AAHS(P+ + Px) > AAPf, the activation energy AH2 in the polar solvent is greater than that (AHX) in hydrocarbon solvent...

The results of experimental studies of the sorption and diffusion of light hydrocarbons and some other simple nonpolar molecules in type-A zeolites are summarized and compared with reported data for similar molecules in H-chabazite. Henry s law constants and equilibrium isotherms for both zeolites are interpreted in terms of a simple theoretical model. Zeolitic diffusivitiesy measured over small differential concentration steps, show a pronounced increase with sorbate concentration. This effect can be accounted for by the nonlinearity of the isotherms and the intrinsic mobilities are essentially independent of concentration. Activation energies for diffusion, calculated from the temperature dependence of the intrinsic mobilitieSy show a clear correlation with critical diameter. For the simpler moleculeSy transition state theory gives a quantitative prediction of the experimental diffusivity. 

Corma and co-workers152 have performed a detailed theoretical study (B3PW91/6-31G level) of the mechanism of the reactions between carbenium ions and alkanes (ethyl cation with ethane and propane and isopropyl cation with ethane, propane, and isopentane) including complete geometry optimization and characterization of the reactants, products, reaction intermediates, and transition states involved. Reaction enthalpies and activation energies for the various elemental steps and the equilibrium constants and reaction rate constants were also calculated. It was concluded that the interaction of a carbenium ion and an alkane always results in the formation of a carbonium cation, which is the intermediate not only in alkylation but also in other hydrocarbon transformations (hydride transfer, disproportionation, dehydrogenation). 

Suppose we know the it activation energy, (<3 )0, for a given reaction of an AH. We can then calculate 8E, die activation energy for the corresponding reaction of the heteroatomic system, by using Eqs. (69)-(71). Consider for example substitution in an even heteroatomic molecule such as quinoline. The difference in it energy between the hydrocarbon and its transition state is given by ... 

In(CH3)3 does not exchange with Hg(CH3)2 at a measurable rate but does with Cd(CH3)2 in hydrocarbon solutions. Recent studies on this system show that the reaction proceeds with an activation energy of 8.3 kcal/mole with a rate somewhat less than that observed for the corresponding Ga(CH3)3-Cd(CH3)2 system (57). This may be interpreted as a lessening in the tendency for formation of metal-carbon-metal bridge-bonded transition state. 

In zeolites, this barrier is even higher. As discussed in Section II.B, the lower acid strength and the interaction between the zeolitic oxygen atoms and the hydrocarbon fragments lead to the formation of alkoxides rather than carbenium ions. Thus, extra energy is needed to transform these esters into carbonium ionlike transition states. Quantum-chemical calculations of hydride transfer between C2-C4 adsorbed alkenes and free alkanes on clusters representing zeolitic acid sites led to activation energies of approximately 200 kJ/mol for isobutane/tert-butoxide (29), 230-305 kJ/mol for propane/sec-propoxide, and 240 kJ/mol for isobutane/tert-butoxide (32), 130-150 kJ/mol for ethane/ethene (63), 95-105 kJ/mol for propane/propene, 88-109 kJ/mol for isobutane/isobutylene, and... 

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