Minggu, 11 Oktober 2009

Reaction Profiles


Introduction

Chemical reactions by definition involve the making and breaking of chemical bonds. Since different types of bonds have different bond strengths, the energy of the reacting system changes as the reaction progresses. Graphs that show the change in energy as a function of the progress of the reaction are known as reaction coordinate diagrams or reaction profiles. In this topic we will look at the most common types of reaction profiles. We will also define two fundamentally different types of reactions, kinetically controlled reactions and thermodynamically controlled reactions. We'll start our survey with a look at a reaction that is familiar to everyone-combustion.

Combustion

Close your eyes and imagine sitting by a crackling fire on a cold winter night. The word chemistry probably isn't the first word that comes to mind. But a fire is an excellent example of chemistry in action. What do you know about a fire? You know that it requires oxygen. You know that when a log burns it gives off heat and light. You probably know that carbon dioxide and water are formed (in addition to ash). And if you've thought about it, you'll realize that it does not happen spontaneously. You have to put a match to the log to get it to ignite. Let's see how a chemist expresses this information. Since wood is mostly cellulose, which is a polymeric form of glucose, which has the molecular formula C6H12O6, a chemist might write Equation 1 to describe a log burning in a fireplace.

The fact that energy is released as the log burns must mean that the products of combustion contain less energy than the reactants. The fact that you have to put a match to the log to get the fire started must mean that there is some barrier that keeps the log from burning spontaneously. Figure 1 expresses these insights diagramatically.

Figure 1

Come On Baby Light My Fire

The difference in energy between the reactants and the point of maximum energy achieved during the reaction is called the activation energy. The point of maximum energy along the path from reactants to products is called a transition state. The heat from the match must raise the energy level of the reactants to the level of the transition state in order to get the fire started. Once it's going, the energy released as product is formed is available to activate other reactants. A reaction in which the products have less energy than the reactants is exothermic.

Now that we've seen the basic features of a reaction coordinate diagram, we're ready to discuss

Kinetically vs Thermodynamically Controlled Reactions

Many chemical reactions produce more than one product. For example, the 1,2-elimination of hydrogen bromide from 2-bromobutane yields a mixture of 1-butene and cis-and trans-2-butenes as shown in Equation 2.

In reactions like this, the relative amounts of the various products depend upon the reaction conditions. To be more specific, they depend upon whether the reaction is reversible or not. Reactions that are reversible are said to be thermodynamically controlled, while reactions that are irreversible are kinetically controlled. The product distribution in a thermodynamically controlled reaction reflects the relative stabilities of the products; the most stable product will be formed in greatest amount, the least stable in the lowest amount. In a kinetically controlled reaction, the product distribution is determined by the relative rates of formation of the products; the product that is formed fastest is formed in greatest amount. Figure 2 should make these ideas clearer.

Figure 2

Over Hill Over Dale

In a reversible reaction, the reactants are converted to products and the products revert to reactants many times before the experimentalist begins the work-up. Forth and back, forth and back, forth and back.... Obviously this can only happen if the amount of energy available to the system is greater than the activation energy for the reverse reaction. As an experimentalist, you don't have any control over the relative energy levels of the reactants, the transition state, and the products, but you do have a say about the amount of energy available to the system. You can increase it by heating or irradiating the reaction mixture. You can decrease it by cooling the sample.

If the energy available to the system is greater than the activation energy for the forward reaction, but less than that required for the reverse reaction, the process is irreversible and the reaction is kinetically controlled.

Figure 2 is a bit misleading since it implies that all the products have the same energy. That's not true. Each of the products in reaction 2, for example, has a different energy. Figure 2 also implies that there's only one path from reactants to products. That's not true either. The path from reactant to product is different for each product. Figure 3 presents a more complete reaction coordinate diagram for this reaction.

Figure 3

In Real Life...


Exercise 1 Ethyl and Ester both performed reaction 2. Ethyl kept her flask at 0oC, while Ester heated her sample to 80oC. After an hour Ethyl was still waiting for her reaction to finish, while Ester had already analysed her results by GC, which showed three peaks in a ratio of 1/5/2. It was the wee hours of the morning before Ethyl analysed her reaction, but when she did, the GC showed a single peak.

What is the compound that was formed in the smallest amount in Ester's reaction? P1 P2 P3

What is the compound that was formed in the greatest amount in Ester's reaction? P1 P2 P3

What is the compound that was formed in Ethyl's reaction? P1 P2 P3


As Figure 3 makes clear, each reaction has a different activation energy leading to a different transition state. Figure 4 illustrates three stages on the path from reactants to cis-2-butene. Note that the figure implies that the product arises from one particular conformation of 2-bromobutane. In the transition state the dashed red lines represent bonds that are partially made or broken.

Figure 4

The Path Less Traveled?


Exercise 2 Draw diagrams comparable to the one in Figure 4 to show the stages involved on the path from reactants to 1-butene.

Exercise 3 Draw diagrams comparable to the one in Figure 4 to show the stages involved on the path from reactants to trans-2-butene. Compare the conformation of the starting material that leads to trans-2-butene to the one that gives rise to cis-2-butene. Which conformation is more stable?

Exercise 4 In the reaction outlined in Figure 4, the hybridization at C-2 changes from to , and that at C-3 changes from to.

Exercise 5 What is the dihedral angle between the H atom and the Br atom that are eliminated from the conformation of 2-bromobutane shown in Figure 4?


The reaction profiles we have considered up to this point all depict processes in which the energy level of the reactants increases smoothly to a maximum then decreases to a minimum. There are many reactions in which the experimental evidence suggests the formation of an intermediate structure, lower in energy than the transition state but higher than the products. In such cases, the reaction profile looks like that shown in Figure 5.

Figure 5

Life Has Its Ups and Downs

We will see examples of this type of reaction profile when we consider specific examples of various reaction types.

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