The Path Selection Process

One can view the reaction process as a wandering over an energy surface via the lowest-energy path toward the lowest accessible point on the surface. The reacting partners have a limited amount of energy only certain transition states and intermediates can be achieved, and the reaction will end up at the lowest accessible point on the surface, which may not necessarily be the lowest point on the surface. 

Armed with the tools that we have learned in the previous chapters, we can predict where the lowest-energy path may go. The electron flow pathways are the trails to guide us in this wilderness of an unfamiliar energy surface. Some trails may dead-end in high valleys that have no reasonable exit other than the one we came in. Others may lead us to a low point (side product) on the energy surface but not the lowest accessible point (major product). The reacting partners are also exploring these dead ends, and very few reactions produce only one product. 

The exploration process is really quite simple. The hard part is to stick with it when things are not going well and you are feeling lost it s easy to get discouraged and do 

How do you go about learning this method of exploration First learn the needed skills (Chapters 1-9), and then try them out on easy problems with the help of an experienced guide. On these guided tours we will explore the entire surface, including all its dead ends, so that you feel you know the territory. We will gradually explore more difficult surfaces until you feel confident enough to venture out on your own. Let s look at each step of the exploration process (Fig. 10.1) in more detail. 

Write down the Lewis structure of the reactants, complete with formal charges, and draw any major resonance forms. Look for leaving groups, polarized single and multiple bonds, acids and bases. Classify into generic sources and sinks and then rank them. The reaction usually occurs between the best source and sink. Above all, note if the medium is acidic or basic. In basic media, find the best base, and then locate any acidic hydrogen within range (not more than 10 p Ta units above the pATabH of Ihe base). In acidic media, identify the best sites for protonation. Likewise, do not create a species that is more than 10 units more acidic than your acid. Understand what bonds have been made or broken, but do not lock into an arbitrary order as to which occurred first. 

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Enthalpy is a properly of the system independent of the path selected. Processes can be conveniently represented graphically. For example, a P-V diagram can be used to illustrate the work done when a system undergoes a change in state (see Figure 2-31). In each of the cases depicted in Figure 2-.31, the work is equal to the shaded area under the P-V curve as shown. 

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