In 1969 the molecule formaldehyde, the molecule pictured above, was discovered to be the first polyatomic organic molecule present in interstellar space by the National Radio Astronomy Observatory. The reactivity of formaldehyde gives it a wide variety of uses from embalming fluid to finger nail polish. Formaldehyde contain a C=O bond called a carbonyl, which is fundamental to study of organic chemistry. In this chapter, we will begin a multi-chapter discussion on the chemistry of the carbonyl bond starting with the functional groups aldehydes and ketones.
Nucleophilic addition reactions were described in general terms in Chapter 18. In Chapter 19, we take a more detailed look at these reactions as we make a comprehensive study of the chemistry of aldehydes and ketones. Aldehydes and ketones are discussed together because their chemistry is very similar. However, as you work through the chapter, be sure to look for specific instances where the chemistry of these two classes of compounds differs.
As usual, we begin the chapter with a discussion of nomenclature. This introduction is followed by descriptions of the methods used to prepare aldehydes and ketones in the laboratory. You will notice that a number of these reactions have already appeared in previous chapters. Note, that an important differences between aldehydes and ketones is the resistance of the latter to oxidation.
A large part of this chapter is concerned with the addition of various nucleophiles to the carbonyl group of aldehydes and ketones. In particular, we discuss the addition of a variety of nitrogen‑containing compounds, alcohols, and phosphorus ylides. Many of these reactions are important to chemists primarily interested the synthesis of new organic compounds.
We also describe the Cannizzaro reaction and conjugate additions to α, β‑unsaturated carbonyl compounds. We mention the occurrence of nucleophilic addition reactions in biological systems, and conclude the unit with a look at the use of spectroscopic techniques in the analysis of aldehydes and ketones.
When you have completed Chapter 19, you should be able to
- fulfill all of the detailed objectives listed under each individual section.
- design a multi‑step synthesis in which you may have to use any of the reactions discussed in this chapter together with any number of reactions from previous chapters.
- solve “road‑map” problems that require a knowledge of the chemistry of aldehydes and ketones.
- use evidence from any combination of infrared spectroscopy, nuclear magnetic resonance spectroscopy, mass spectroscopy and chemical reactions to determine the structure of an unknown aldehyde or ketone.
- define, and use in context, the key terms introduced in this chapter.