Nucleophilic Addition Reactions of Enolate Anions

A most important property of enolate anions, at least as far as synthesis is concerned, is their excellent nucleophilicity, which enables them to add to double bonds and to participate in nucleophilic substitution. When the addition is to a carbonyl double bond, it is called an aldol addition (Equation 17-4). Additions of enolate anions to carbon-carbon double bonds usually are classified as Michael additions (Equation 17-5), and these are discussed in Sections 17-5B and 18-9D. The principles of SN nucleophilic reactions of enolate anions (Equation 17-6) will be considered in Section 17-4, and their synthetic applications in detail in Chapter 18. 

17 Carbonyl Compounds II. Enols Enolate Anions. Unsaturated and Polycarbonyl Compounds 

The products of aldol addition are /3-hydroxy aldehydes (ald-ols) or /3-hydroxy ketones (ket-ols). A typical example is the reaction of ethanal with base and, if the conditions are reasonably mild, the product is 3-hydroxybutanal  

The overall reaction corresponds to a dimerization of ethanal, that is, an addition of one ethanal molecule to another with formation of a new carbon-carbon bond. The synthetic value of the reaction lies in the fact that it can be used to build large molecules from smaller molecules (see Section 13-7). 

Formation of the enolate anion, 7, by removal of an a hydrogen by base is the first step in the aldol addition  

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