Addition, Condensation and Substitution Reactions of Carbonyl Compounds

Addition, Condensation and Substitution Reactions of Carbonyl Compounds 

The carbonyl group is one of the most prevalent of the functional groups and is involved in many synthetically important reactions. Reactions involving carbonyl groups are also particularly important in biological processes. Most of the reactions of aldehydes, ketones, esters, carboxamides, and the other carboxylic acid derivatives directly involve the carbonyl group. We discussed properties of enols and enolates derived from carbonyl compounds in Chapter 6. In the present chapter, the primary topic is the mechanisms of addition, condensation and substitution reactions at carbonyl centers. We deal with the use of carbonyl compounds to form carbon-carbon bonds in synthesis in Chapters 1 and 2 of Part B. 

In many reactions at carbonyl groups, a key step is the addition of a nucleophile, which generates a tetracoordinate carbon atom. The overall course of the reaction is then determined by the fate of this tetrahedral intermediate. Addition occurs when the tetrahedral intermediate goes directly on to product. Condensation occurs if the carbonyl oxygen is eliminated and a double bond is formed. Substitution results when one of the groups is eliminated from the tetrahedral intermediate to re-form a carbonyl group. 

The reaction patterns of the specific classes of carbonyl compounds are related by the decisive importance of tetrahedral intermediates, and differences in reactivity can often be traced to structural features present in those intermediates. In Section 3.4.4, we considered some of the fundamental substiment effects on the stability of both carbonyl compounds and tetrahedral intermediates. These relationships will be important as we discuss the reactions in this chapter. 

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