Activation entropy hydrogenation

Both for reaction in and IV the order with respect to catalyst is 0.5. The activation enthalpies are 96.6 3.4 and 97.6 3.4 kJ mol-1 respectively when Ti(OBu)4 is used as the catalyst. This is not too far from the activation enthalpies200 for the Sn(II)-cata-lyzed esterification of B with isophthalic acid (85.1 4.9) and with 2-hydroxyethyl hydrogen isophthalate (85.8 4.2). It is also close to the Ti(OBu)4-catalyzed esterification of benzoic acid with B (85.8 2.5)49. This is probably due to the formation of analogous intermediate complexes and similar catalytic mechanisms. On the other hand, the activation entropies of reactions III and IV are less negative than those of the reaction of benzoic or isophthalic acid with B. This probably corresponds to a stronger desolvation when the intermediary complex is formed and could be due to the presence of the sodium sulfonate group. 

Table 8.6 Activation Energies and Activation Entropies for the Alkene Hydrogenation on the Supported and Imprinted Rh-Dimer Catalysts...

Table 10.5 Alkene hydrogenation activities (TOP) at 348 K, activation energies and activation entropies on the Rh-monomer pair before molecular imprinting and for the imprinted Rh dimer.

The kinetic and activation parameters for the decomposition of dimethylphenylsilyl hydrotrioxide involve large negative activation entropies, a significant substituent effect on the decomposition in ethyl acetate, dependence of the decomposition rate on the solvent polarity (acetone-rfe > methyl acetate > dimethyl ether) and no measurable effect of the radical inhibitor on the rate of decomposition. These features indicate the importance of polar decomposition pathways. Some of the mechanistic possibilities involving solvated dimeric 71 and/or polymeric hydrogen-bonded forms of the hydrotrioxide are shown in Scheme 18. 

Schuster and coworkers also demonstrated that trans olefins show distinctly lower normalized (negative) entropies of activation AS for the ene reaction than cis, and suggested that a reversible exciplex can be formed, followed by an allylic hydrogen-oxygen interaction in the rate-determining step. In the reaction of c -2-butene, the activation entropy AS is less negative by 10 e.u. than that of the trawi-2-butene, while the activation enthalpies are very similar. The considerable difference in the activation entropies was attributed to the fact that transition states in the case of cis olefins require more... 

The effect of salts on the velocity and on the energy and entropy of activation of hydrogen exchange reactions in methyl 2-naphthyl ketone has been investigated in some detail. If the rate constant in the absence of the salts at 25° is put equal to 100, then in 2-5n solutions of salts the rate constants assume the following values ... 

Both steric and electronic effects can influence the regioselectivity of ene reactions. For example, a strong preference for hydrogen abstraction from the more substituted side of the double bond of the perepoxide intermediates, generated from trisubstituted alkenes, is observed. This effect has been called the cis effect and explained on the basis of orbital interactions1423 and activation entropy differences.1457 Scheme 6.267 shows the product distribution after photooxygenation of three alkenes (561 563).1461 There is an apparent steric effect of the methyl versus 2-propyl substituents in the first two cases. The cyclopropyl moiety in the last example remains unreacted, which rules out the formation of a biradical intermediate (see also Special Topic 6.10). 

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