Reaction coordinate diagram catalysis

In what is now a classical study in enzyme kinetics, W. J. Albery and J. R Knowles developed a strategy for establishing a reaction coordinate diagram (shown in Fig. 2) for triose-phosphate isomerase catalysis using solvent exchange and kinetic isotope effect data. 

Another innovation in this text is the use of three-dimensional reaction coordinate diagrams, pioneered by Thornton, More O Ferrall, and Jencks, in the discussions of nucleophilic substitutions, eliminations, and acid catalysis of carbonyl additions. We hope that the examples may lead to more widespread use of these highly informative diagrams. 

The most widely accepted mechanism for electrophilic aromatic substitution involves a change from sp2 to sps hybridization of the carbon under attack, with formation of a species (the Wheland or a complex) which is a real intermediate, i.e., a minimum in the energy-reaction coordinate diagram. In most of cases the rate-determining step is the formation of the a intermediate in other cases, depending on the structure of the substrate, the nature of the electrophile, and the reaction conditions, the decomposition of such an intermediate is kinetically significant. In such cases a positive primary kinetic isotope effect and a base catalysis are expected (as Melander43 first pointed out). 

Figure 8.54 Schematic illustration of reaction coordinate diagram for the simplest Uni Uni Kinetic Scheme (Scheme 8.1) for bio-catalysis. The reaction coordinate profile is drawn under the assumption that excess substrate is present so that fccat conditions prevail. Two types of free energy of activation exist, from E -I- S (AG ) and from ES AG. ...

The chapter commences with a general overview of catalysis in the context of reaction coordinate diagrams and a simple thermodynamic cycle. Next, the most common factors invoked to explain transition state binding are explored differential solvation, proximity, nucleophilic and electrophilic activation, and strain. We also look at covalent catalysis, which fundamentally involves a mechanism change. 

Binding of the catalyst to the substrate and product, as well as the transition state, leads to several possible reaction coordinate diagrams, a few of which are given in Figure 9.2. Let s examine the four possibilities to explore the requirements for catalysis. 

Reaction coordinate diagrams for various types of catalysts interacting with ground states and transition states. A. The substrate is bound by the catalyst, but there is no stabilization of the transition state and no catalysis occurs. The rate is actually slower than the background rate. B. The substrate is bound weakly, but there is no stabilization of the transition state. The rate is the same as the background. C. The substrate and transition state are bound to the. same extent, and the uncatalyzed rate is the same as the catalyzed rate. D. The transition state is bound better than the substrate, and catalysis occurs. 

What is the basis for the Bronsted law Figure 19.17 shows the reaction coordinate diagrams for a series of related reactions, indicating that the more stable the product, the faster the reaction. For acid catalysis, the figure implies that the more weakly bonded the proton is to the catalyst, the faster it comes off, and the faster is the catalyzed reaction. Here is a model. 

Reaction Coordinate Diagrams and Reaction Mechanisms Catalysis... 

CATALYSIS AND FREE-ENERGY REACTION COORDINATE DIAGRAM... 

Fig. I. Reaction coordinate/free enthalpy diagram for the reaction of an educt E to a product P in the absence (path A) and in the presence (path B) of a host H. In case (A), the binding has no effect on the overall rate while case (B) shows catalysis. In case (C) educt inhibition , in case (D) product inhibition is shown. In cases (C) and (D), the differences of free enthalpy between E and E H, and P and P H, respectively, are much larger than the difference between TS and TS-H...

Figure 3.16 More O Ferrall-Jencks diagram for the general acid catalysis of an acetal. The transition state position and reaction coordinate direction are those deduced for benzaldehyde alkyl acetals of acidic alcohols or phenols.

Contents Molecular Orbitals. - Chemical Reactivity Theory. - Interaction of Two Reacting Species. - Principles Governing the Reaction Pathway. - General Orientation Rule. - Reactivity Indices. - Various Examples. -Singlet-Triplet Selectivity. - Pseudoexcitation. -Three-species Interaction. - Orbital Catalysis. -Thermolytic Generation of Excited States. - Reaction Coordinate Formalism. - Correlation Diagram Approach. 

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