Kinetic product distribution activation energy

The steady state experiments showed that the two separate phases and the mixture are not very different in activity, give approximately the same product distributions, and have similar kinetic parameters. The reaction is about. 5 order in methanol, nearly zero order in oxygen, and has an apparent activation energy of 18-20 kcal/mol. These kinetic parameters are similar to those previously reported (9,10), but often ferric molybdate was regcirded to be the major catalytically active phase, with the excess molybdenum trioxide serving for mechanical properties and increased surface area (10,11,12). 

The use of sterically hindered bases raises the activation energy barrier for the pathway to the product predicted by Saytzeffs Rule. Thus, a sterically hindered base will preferentially react with the least hindered protons, and the product distribution will be controlled by kinetics. 

Because the electronic distribution and nuclear configuration of the donor and the acceptor in the (Class II) successor complexes are similar to those of the free donor/acceptor product (i.e. radical pair), it is reasonable to suggest that products can originate directly from the successor complex (pathway Pi). Such a reaction, which includes an electron-transfer step, does not necessarily proceed via a pair of free ion radicals, and the effective activation energy can be even lower than that required by pathway P2. When the follow-up reaction involves the coupling of radicals, the reaction directly proceeding from the (ET) successor complex state can be kinetically favorable (since it excludes diffusional processes). 

The data in Table III for the photochemical isomerization of 1-pentene show that photochemical activation is also a viable means of sample activation. During these reactions, CO gas is given off and it is believed from solution studies that an Fe(C0)4L complex is initially formed. The Fe(C0) L complex, where L is a bound pentene, can then undergo isomerization to the cis and trans isomers of 2-pentene. The data in Table III show that the incorporation of a zeolite not only changes the product distribution from a 2.0 ratio of the trans to the cis, as observed in solution studies, but that the photolysis time is relatively short. It should be recognized here that high energy ultraviolet radiation is used, but the photon flux is relatively low. The kinetics of this reaction are surely different from that of the solution reactions and it is not inconceivable that there are steric constraints administered by the zeolite... 

The implication of these studies is of critical importance. Chemists generally think of the product distribution of a chemical reaction being controlled by kinetics or thermodynamics. Under kinetic control, the distribution favors the product that results from crossing the lowest activation barrier. Under thermodynamic control, the distribution favors the lowest energy product. Schreiner and Allen now add... 

The kinetics of the above-mentioned reaction can frequently be altered by introducing a third substance which may form a complex with either A or B or both and thus alter the energy of activation of the reaction. In the case of oxidations that occur via chain mechanisms, catrdysts may have some influence on product distribution, and yet they frequently have little or no discernible effect on steady-state reaction kinetics. In some instances, additional catalyst may actually retard reaction rates [7-9]. 

First, it is important to note that most aromatic electrophilic substitution reactions are under kinetic, and not thermodynamic, control. This is because most of the reactions are irreversible, and the remainder are usually stopped before equilibrium is reached. In a kinetically controlled reaction, the distribution of products (or product spread), i.e. the ratio of the various products formed, is determined not by the thermodynamic stabilities of the products, but by the activation energy barrier that controls the rate determining step. In a two-step reaction, it is a reasonable assumption that the transition state of the rate determining step is close in energy to that of the intermediate, which in this case is the Wheland intermediate and so by invoking the Hammond postulate, one may assume that they have similar geometries. 

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