Reaction coordinate diagram control

A quantitative expression developed by Albery and Knowles to describe the effectiveness of a catalyst in accelerating a chemical reaction. The function, which depends on magnitude of the rate constants describing individual steps in the reaction, reaches a limiting value of unity when the reaction rate is controlled by diffusion. For the interconversion of dihydroxacetone phosphate and glyceraldehyde 3-phosphate, the efficiency function equals 2.5 x 10 for a simple carboxylate catalyst in a nonenzymic process and 0.6 for the enzyme-catalyzed process. Albery and Knowles suggest that evolution has produced a nearly perfect catalyst in the form of triose-phosphate isomerase. See Reaction Coordinate Diagram... 

Fig. 3. Reaction coordinate diagrams for electron transfer, (a) AG° = 0 (f>) AG° negative (c) diffusion-controlled limit, —AG. = X (d) abnormal free-energy region, —AG. > X...

The features that control the eventual outcome of a reaction may also be illustrated by the reaction coordinate diagram. If there are two possible reaction products with no opportunity to equilibrate between them, the product that has the lowest activation energy (and hence is formed the fastest) will be formed preferentially. The reaction is under kinetic control.. On the other hand, if the products can equilibrate so that the most stable product is formed (i.e. AG predominates), the reaction is said to be subject to thermodynamic control... 

What one feature of a reaction coordinate diagram is indicative of kinetic control of a reaction Explain. 

The diamond to graphite reaction is thermodynamically favorable but does not appear to happen during the lifetime of an engagement ring. Use the appropriate tables to determine if this reaction is endothermic or exothermic. Construct a reaction coordinate diagram that shows the endothermic or exothermic nature of the reaction and illustrates why this reaction is under kinetic control. 

These results can be rationalized by the reaction coordinate diagram shown in Figure 6.27. Because AG for conversion of 72 to 73 is smaller than the value of AG for conversion of 72 to 74, formation of 73 is faster. Therefore, more of 73 is formed during the early stages of the reaction, so the distribution of products is said to reflect kinetic control. As the reaction proceeds, the much slower processes that convert 73 first to 72 and then to 74 become more significant. Eventually, equilibrium is established between 73 and 74, and the product distribution at the end of the reaction is said to reflect thermodynamic control. 

There is one further important aspect of this system that relates to the species whose free energies control the product ratio. This aspect can be developed in terms of transition-state theory and the reaction coordinate diagram in Figure 2.1. 

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