In Chapter 8, you were introduced to the 'nuts and bolts' of the nucleophilic substitution reaction, meeting the 'Three Amigos' of organic reactions (nucleophiles, electrophiles, and leaving groups), learning the difference between SN1 and SN2 mechanisms, and thinking about issues of regio- and stereoselectivity. In the interest of keeping these discussions as clear as possible, most examples were very simple and generalized.
Now that we know the basics, we're going to make things a lot more interesting by looking at real examples of biologically relevant nucleophilic substitutions. We're also going to explore a little bit about how some of the ideas from Chapter 8 were discovered: we'll talk about the data from actual experiments performed by scientists who wanted to address questions such as "is this nucleophilic substitution SN1 or SN2?", or "can I show that this reaction really proceeds with inversion of configuration?". As we do this, remember always to keep focused on the simple things you already know: for each reaction, be sure that you can strip away in your mind the unreactive parts of the molecules and focus on the interplay between the nucleophile, electrophile, and leaving group.
We'll begin by looking more closely at a biochemical SN2 reaction that we have already glimpsed briefly in chapters 6 and 8.