Alkylating the a-Carbon of Carbonyl Compounds

What compound is formed when a dilute solution of cyclohexanone is shaken with NaOD in D2O for several hours  

Putting an alkyl group on the a-carbon of a carbonyl compound is an important reaction because it gives us another way to form a carbon-carbon bond. Alkylation is carried out by first removing a proton from the a-carbon with a strong base, such as LDA, and then adding the appropriate alkyl halide. Because the alkylation is an Sn2 reaction, it works best with methyl halides and primary alkyl halides (Section 9.2). 

Although an enolate ion has two resonance contributors, for the sake of simplicity, only the resonance contributor with the negative charge on carbon will be shown in many of the reactions in this chapter. For example, compare the following presentation with the preceding one. 

It is important that a strong base such as LDA is used to form the enolate ion. If a weaker base such as hydroxide ion or an alkoxide ion is used, very little of the desired monoalkylated product will be obtained. We have seen that these weaker bases form only a small amount of enolate ion at equilibrium (Section 18.6). Therefore, after monoalkylation has occurred, there is still a lot of base present in solution, which can form an enolate ion from the monoalkyated ketone as well as from the unalkylated ketone. As a result, di-, tri-, and even tetra-alkylated products may be produced. 

Ketones, aldehydes, esters, and nitriles can be alkylated on the a-carbon. 

---

Alkylating the a-carbon of carbonyl compounds (Section 18.7). The mechanism is show on on page 863. 

---