Generalized reaction coordinate diagrams

This type of elimination is known as an ElcbR (elimination, unimolecular, conjugate base, reversible) reaction, and a generalized reaction coordinate diagram is shown in Figure 10.14. Such reactions exhibit C s-H/D exchange and a 1° hydrogen kinetic isotope effect (ku/ko) of 1.0. An example of an ElcbR is shown in Figure 10.15. ... 

A transition structure is the molecular species that corresponds to the top of the potential energy curve in a simple, one-dimensional, reaction coordinate diagram. The energy of this species is needed in order to determine the energy barrier to reaction and thus the reaction rate. A general rule of thumb is that reactions with a barrier of 21 kcal/mol or less will proceed readily at room temperature. The geometry of a transition structure is also an important piece of information for describing the reaction mechanism. 

Fig. 1. Reaction coordinate diagram showing general locations of regions for repulsive (R), mixed (M) and attractive (A) energy release. For a mixed energy release surface the transition state will be near M, but for an attractive (repulsive) surface the transition state will be near R(A).

Energy surfaces are also used to describe chemical reactions, albeit usually in vastly simplified form, that of the energy reaction-coordinate diagram (or reaction profile for short), examples of which are to be found in every general chemistry textbook (Figure 5.6). Reaction profiles are a convenient way of summarizing many kinds of experimental data and for discussing reactions from a molecular viewpoint. 

Knowledge Required (1) The general understanding of multi-step reaction mechanisms. (2) The general shape of energy vs. reaction coordinate diagrams. (3) The meaning of the terms exothermic and endothermic. 

Let us consider the general problem of the reaction coordinate diagram of this reaction. As a simple example, consider the reaction with ammonia and formaldehyde [Reaction (1)] ... 

The parameters 0 and cp play in this relation the role of the generalized reaction coordinates, the systematic change of which allows to describe the structure of all transient species lying "inside" the corresponding More O Ferrall diagram. For the analogy with the More O Ferral diagrams becomes even more transparent it is convenient to substitute the primary spherical coordinates 0 and cp by the ordinary More O Ferrall coordinates Qi, Q2 These transformations are described by the set of equations (86). 

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. 

Fig. 2.1. Diagram illustrating the dependence of the system (substrate-product) energy, E, on the generalized reaction coordinate, R.

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