Reaction coordinate diagram Hammond postulate

We have made use above of the idea that the magnitude of a (or / ) measures the extent of proton transfer at the transition state or, equivalently, of the position of the transition state along the proton-transfer reaction coordinate. Figures 8.4 and 8.5 show, respectively, the reaction coordinate diagrams drawn according to the Hammond postulate for Reaction 8.17 in the extreme cases where HX is a... 

Hammond added that "in highly exothermic steps it will be expected that the transition states will resemble reactants closely and in endothermic steps the products will provide the best models for the transition states." The postulate is a logical consequence of the idea that the energy of a chemical entity is a function of its structure. Therefore, two species that occur consecutively during a reaction and that have very similar energies might be expected to have very similar structures as well. In this context, the phrase "similar structures" means similar coordinates on the horizontal axis of the reaction coordinate diagram. 

The Hammond postulate is illustrated by the diagrams in Figure 6.34. In an exothermic reaction, the energy of the transition state is necessarily closer to that of the reactant than to that of the product, so we draw it closer in structure also. That is, we draw the energy maximum to the left on the reaction coordinate diagram. The reverse is true for an endothermic reaction. Thus, we often say that an exothermic reaction has an "early" transition state and an endothermic reaction has a "late" transition state. In a thermoneutral reaction, the energy of the transition state is as different from that of the reactant as... 

Most organic processes of interest are not imimolecular thermolysis reactions. Usually we investigate reactions in which one bond is broken and another is formed in the same elementary step. Based on the Hammond postulate (Figure 6.34), we can envision three scenarios for a reaction in which an atom A abstracts a hydrogen atom from a carbon atom. In an exothermic reaction, the transition state occurs early (to the left on a reaction coordinate diagram). In an endothermic reaction, the transition state occurs late (to the right). In a thermoneutral reaction, the transition state occurs near the center of a reaction coordinate diagram. 

Several reaction coordinate diagrams that obey Hammond s postulate and where the activation energy has a direct correlation with the energy change of the reaction. 

Based partly upon the Hammond postulate, chemists typically write a continuum of reaction coordinate diagrams as shown in Figure 7.9 for similar reactions. The shape of each of these curves indicates a smooth shifting of the transition state structure from resembling the product to resembling the reactant as the reaction becomes increasingly exothermic. This predicts that a thermoneutral reaction has a transition state that is close to a one-to-one mixture of the structure of fhe reactants and products. 

The Hammond postulate does not predict the height of the barrier compared to the reactant and product, only its position along the reaction coordinate. For example, we are not forced to draw the continuum as shown in Figure 7.9 in order to obey the Hammond postulate. In fact. Figure 7.10 shows a different overlay of reaction coordinate diagrams, each of which also conforms to the Hammond postulate, and you may occasionally encounter a system that exhibits this kind of behavior. However, most reactions that are of similar type but vary in their thermodynamics will have reaction coordinates that resemble those drawn in Figure 7.9. 

In a reaction coordinate diagram, it is obvious that the potential energy content at a transition state is closer to that in the starting materials in an exothermal step and closer to the products in an endothermal step. Since potential energy is required to distort a molecule, the structure of the transition state will more closely resemble those molecules to which it is closer in potential energy that is, a small vertical difference in a reaction coordinate diagram corresponds to a small horizontal difference. Transition states are late in endothermal steps and early in exothermal steps. This is the Hammond postulate [4] and it is useful for predicting products where there is potentially close competition between two alternative steps. 

Hammond postulate Reactive species represented on a reaction coordinate diagram that are similar in energy are similar in strucmre. The transition state structure may resemble the structme of either reactant or product, whichever it is closer to in energy. 

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