Carbonyl Alpha-Substitution Reactions

We said in the Preview of Carbonyl Chemistry that much of the chemistry of carbonyl compounds can he explained hy just four fundamental reaction types nucleophilic additions, nucleophilic acyl substitutions, a substitutions, and carbonyl condensations. Having studied the first two of these reactions in the past three chapters, let s now look in more detail at the third major carbonyl-group process—the a-substitution reaction. 

Alpha-substitution reactions occur at the position next to the carbonyl group—the a position—and involve the substitution of an a hydrogen atom by an electrophile, E, through either an enol or enoiate ion intermediate. Let s begin by learning more about these two species. 

Why This Chapter As with nucleophilic additions and nucleophilic acyl substitutions, many laboratory schemes, pharmaceutical syntheses, and biochemical pathways make frequent use of carbonyl a-substitution reactions. Their great value is that they constitute one of the few general methods for forming carbon-carbon bonds, thereby making it possible to build larger molecules from smaller precursors. We ll see how and why these reactions occur in this chapter. 

A carbonyl compound with a hydrogen atom on its a carbon is in an equilibrium with its corresponding enol isomer (Section 9.4). This spontaneous interconversion between two isomers, usually with the change in position of a hydrogen, is called tautomerism, horn the Greek tauto, meaning the same, and meros, meaning part. The individual keto and enol isomers are called tautomers. 

Note the difference between tautomers and resonance forms. Tautomers are constitutional isomers—different compounds with different stmctures—while resonance forms are different representations of a single compound. Tautomers have their atoms arranged differently, while resonance forms differ only in the position of their tt and nonbonding electrons. 

Thomson C.N Throughout this chapter, sign in at www.thomsonedu.com for online self-study and interactive tutorials based on your level of understanding. 

Online homework for this chapter may be assigned in Organic OWL. 

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