About Potential Energy Diagrams

Chapter 5 describes elimination reactions and their mechanisms. In one example, heating tert- miy bromide in ethanol gives the alkene 2-methylpropene by a two-step mechanism  

A potential energy diagram for the reaction provides additional information to complement the mechanism expressed in the equations for the two elementary steps. 

The energy relationships in the diagram are not only useful in their own right, but also aid in understanding the structural changes occurring at the transition state. Hammond s postulate tells us that if two states occur consecutively, the closer they are in energy, the more similar they are in structure. 

55 Which equation corresponds to the overall reaction for which steps 1 and 2 describe the mechanism  

57 According to the potential energy diagram, the overall reaction is  

53 Classify the elementary steps in the mechanism according to their molecularity. 

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More About Potential Energy Diagrams 182 Chapter 5... 

FIGURE 3 4 Potential energy diagram for rotation about the carbon-carbon bond in ethane Two of the hydrogens are shown in red and four in green so as to indicate more clearly the bond rotation... 

Sketch an approximate potential energy diagram similar to that shown in Figures 3 4 and 3 7 for rotation about the carbon-carbon bond in 2 2 dimethylpropane Does the form of the poten tial energy curve of 2 2 dimethylpropane more closely resemble that of ethane or that of butane" ... 

Shape of potential energy diagram is identical with that for ethane (Figure 3 4) Activation energy for rotation about the C—C bond is higher than that of ethane lower than that of butane... 

Fig. 3.3. Potential energy diagram for rotation about C(2)—C(3) bond of n-butane.

Construct a qualitative potential-energy diagram for rotation about the C-C bond of i,2-dibromoethane. Which conformation would you expect to be more stable Label the anti and gauche conformations of 1,2-dibromoethane. 

These observations remind us of Chapter 8, in which we considered the factors that determine the rate of a chemical reaction. Of course, the same ideas apply here. We can draw qualitative information about the mechanism of the reaction by applying the collision theory. With quantitative study of the effects of temperature and concentration on the rate, we should be able to construct potential energy diagrams like those shown in Figure 8-6 (p. 134). 

O Your friend is confused about the difference between the enthalpy change and the activation energy of a chemical reaction. Write a few paragraphs, in which you define each term and distinguish between them. Use potential energy diagrams to illustrate your answer. 

This reaction is about the simplest that involves intermediate radicals (adsorbed H atoms waiting to combine to form H2). A study of potential energy diagrams such as that described below can be used to comprehend why a change in the electrode potential changes the reaction rate, and thus to understand the basis of electrocatalysis. 

FIGURE 1. Schematic potential energy diagram for twisting about the stilbene central double bond. 

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