Notes for Chapter 6

Chem 243

In this chapter we learn about reactions and mechanisms and how structure determines reactivity of molecules. Learn how to identify nucleophiles and leaving groups on potential substrates. When given a set of reactants and specific conditions, be able to draw the product of the reaction. Also, we begin some simple syntheses here, so be able to carry out a series of reactions which convert a given compound to another given compound.

We study nucleophiles, species which are electron rich and tend to approach electron poor atoms. We can also consider nucleophiles to be bases, that is, they react with acids in the Lewis acid sense. Here we can generalize to many reactions that we will study. If we consider nucleophiles as bases and electrophiles as acids, many reactions can be understood as acid-base reactions. This is the reason for the importance of knowing how charges are distributed on a molecule. The molecular orbital description of reactions is more correct and we will consider that as well, but initially think of acid-base reactions.

This approach is also very good for helping understand elimination reactions. We will look at these reactions in considerable detail because they are excellent examples which can be expanded to other reaction mechnisms. You should understand these concepts thoroughly because they will be valuable later in the course.

Kinetics

Kinetics is the study of reaction rates and mechanisms. We will see how experimental observations of kinetic processes can give us a great deal of information about how a chemical reaction proceeds, i.e., its mechanism.

Kinetics refers to the rate of a reaction. We find that there are relatively simple relationships between the rate of a reaction and the concentration of the reactants. This relationship is usually characterized by an experimentally determined factor called the rate constant. Its value is constant for given reactants, solvents and temperature. (That's why they call it a CONSTANT!)

When we derive the mathematical expression that describes the rate for a particular reaction, we can determine the rate by varying the values of the concentrations, and hopefully, you see the same thing in the lab. We will be working with some simple rate expressions in this course, but some reactions can be described by rather complex relationships.

Once we have the rate expression, we can see how changing the reactant concentrations effects the rate. If there are two concentrations in the rate expression, then both reactants are involved in the rate determining process. If only one reactant appears in the rate expression, then no matter how many other reactants there are, their concentrations have no effect on the rate. The only one that is involved in the rate determining process is the one that appears in the expression.

So who cares about this? Well, when you know which reactants are in the rate expression, you know that they are the ones controlling the rate. Reactions occur by a sequence of steps. Some reactions involve only one step, while others involve many steps. It is important to know these steps (the mechanism) so that we can get some control of the reactions. Now if you have a series of steps and we can assume that each step proceeds at a different rate, (it would be a tremendous coincidence if they all had the same rate) then the overall rate ccan only be as fast as the slowest step. This is what is actually measured in kinetics experiments, the rate of the slowest step, think about it!

This gives us some important information about how a chemical reaction takes place and how we can control it to some extent.

When we study the nucleophilic substitution reactions and elimination reactions, we will see how some reactions can go through the same intermediate structures. Then we get an idea of how to direct the reaction toward a particular product.