8: Carbonyl Alpha-Substitution Reactions
When you have completed Chapter 8, you should be able to
- fulfill all of the detailed objectives listed under each individual section.
- design multi‑step syntheses in which the reactions introduced in this unit are used in conjunction with any of the reactions described in previous units.
- solve road‑map problems that require a knowledge of carbonyl alpha‑substitution reactions.
- define, and use in context, the key terms introduced.
Alpha‑substitution reactions are the third major type of reaction that you will study in your investigation of the chemistry of carbonyl compounds. As you will see, these reactions proceed through the formation of the enol form of the carbonyl compound.
After a brief review of keto‑enol tautomerism, we begin our discussion of alpha‑substitution reactions by looking at the methods in which the enol form of the carbonyl compound is directly involved. After discussing the factors that influence the formation and stability of enolate anions, we will examine some halogenation reactions in which an enolate ion is formed as an intermediate.
The chapter concludes with a study of the alkylation of enolate anions. These reactions are of tremendous use in organic syntheses, as they provide a method of forming new carbon‑carbon bonds, and hence facilitate the laboratory preparation of increasingly complex compounds.
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- 8.6: Acidity of Alpha Hydrogen Atoms- Enolate Ion Formation
- The section on the acidity of alpha hydrogen atoms explains that these hydrogens are more acidic than other hydrogens due to the stabilization of the resulting enolate ion. When an alpha hydrogen is removed by a strong base, it forms an enolate, which serves as a key intermediate in various organic reactions, including nucleophilic additions and substitutions. This characteristic is crucial for many synthetic pathways in organic chemistry.