Exothermic reactions reaction coordinate diagram

D is correct. The reaction coordinate diagram below shows the energy of activation for an endothermic reaction is greater than for an exothermic reaction. 

Figure 2 illustrates several reaction coordinate diagrams that allow exothermic oxaphosphetane formation. Option a (four-center process) and the kinetically equivalent b (transient betaine precursor of the oxaphosphetane two-step mechanism) are consistent with the observation that oxaphosphetanes are formed rapidly and decompose slowly when R = alkyl. Since the barrier AGjJgc decomposition to the alkene is smaller than AGJ y, there will be little reversal or loss of stereochemistry in option a. Reversal should become less likely if the a-substituent R is unsaturated (CH=CH2 or aryl), a situation that would decrease AG g by weakening the P—Cj bond (reaction profile c). If substituents are present that retard the rate of decomposition relative to reversal (as in options d or e), then oxaphosphetane reversal and equilibration of stereochemistry become possible, as discussed in a later section. However, this behavior has not been demonstrated for members of the Ph3P=CHR ylide family in the absence of lithium salts. 

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. 

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. 

Assume that the glycylglycine to glycine reaction is exothermic. Sketch the reaction coordinate diagram for this reaction. Modify the diagram for the presence of the enzyme. 

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. 

Typical reaction coordinate diagrams for (a) exothermic, (b) nearly thermoneutral, and (c) endothermic reactions. 

Representation of a reaction coordinate diagram as the sum of two parabolic potentiai energy surfaces for an exothermic 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. 

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. 

Draw a reaction coordinate diagram for a two-step exothermic reaction in which the second step is a rate determining. 

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. 

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