Introduction to synthesis
In section 7.2., we examined the different levels at which we can look at organic reactions. In chapter 8, we then studied four mechanisms in detail: SN2, SN1, E2 and E1. In this chapter, we want to zoom out somewhat, and begin to think about applying these reactions to the synthesis of particular products. In chapter 8 we looked at: “What happens when I mix A and B?” In this chapter, we ask: “How can we use this reaction to make something useful?”
So far, most reactions have been rather theoretical in nature, using “Nuc” as a nucleophile or “R-X” as an electrophile. In this chapter we will learn what Nuc, R and X are in practice. Some nucleophiles work well in practice, while others don’t; we will focus on learning some of the combinations that are effective in the lab or on the chemical plant. Especially useful are reagents and conditions that have a wide scope, i.e., they work in a similar and predictable way for a wide variety of target products.
At this point, we will just look at syntheses involving only one reaction – one synthetic step. When a target product has a certain functional group, or a particular pattern of bonds, it is important that we recognize how to make that type of product using the reactions we know. As we work through this course, you will gradually learn new reactions (“words”) add to your reaction “vocabulary.” This semester it is important to learn a few common reactions, and to know how to use each one to make a particular type of product. Next semester we will put these reactions together into a multi-step synthesis, like putting words together to make sentences.
To learn how to use reactions in synthesis, instructors various types of question. Some, such as “give the reagent” help the student to learn what is needed to turn A into B. Others, like “predict the product,” help the student to use their knowledge of mechanism and reaction specificity to work out which product will form, with what stereochemistry, etc. The last type, the open-ended synthesis question, is the hardest for beginners, even though there may be now several correct answers. To answer such questions, you need to be able to work back from the product, something that is hard at first but which becomes easy with practice.
For much of the remainder of the semester we will be focused on this type of synthesis question. This chapter and the next will teach you all the basic reactions you need in order to perform a simple synthesis of several different functional groups. Remember that you do not need to show the mechanism unless asked, though you may need to have the mechanism in the back of your mind as you draw the product.