Reaction of Acetylide Anions

Terminal alkynes are readily converted to acetylide anions with strong base. These anions are strong nucleophiles, capable of reacting with electrophiles such as alkyl halides and epoxides. 

11A Reaction of Acetylide Anions with Alkyl Halides 

Acetylide anions react with unhindered alkyl halides to yield products of nucleophilic substitution. 

Because acetylide anions are strong nucleophiles, the mechanism of nucleophilic substitution is S 2, and thus the reaction is fastest with CH3X and 1° alkyl halides. Terminal alkynes (Reaction [1]) or internal alkynes (Reaction [2]) can be prepared depending on the identity of the acetylide anion. 

Because organic compounds consist of a carbon framework, reactions that form carbon-carbon bonds are especially useful. In Reaction [2], for example, nucleophilic attack of a three-carbon acetylide anion on a two-carbon alkyl halide yields a five-carbon alkyne as product. 

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Active Figure 8.6 MECHANISM A mechanism for the alkylation reaction of acetylide anion with bromomethane to give propyne. Sign in afwww.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

The reactions of acetylide anions give us an opportunity to examine organic synthesis more systematically. Performing a multistep synthesis can be difficult. Not only must you know the reactions for a particular functional group, but you must also put these reactions in a logical order, a process that takes much practice to master. 

Reaction of acetylide anions with alkyl halides (11.11 A)... 

A mechanism for the ilkylation reaction of acetylide anion with bromomethane to give propyne. 

Because an alkyl group is added to the original alkyne molecule, this type of reaction is called an alkylation reaction. We limit our discussion in this chapter to reactions of acetylide anions with methyl and primary haloalkanes. We will discuss the scope and limitation of this type of nucleophilic substitution in more detail in Chapter 7. For reasons we will discuss there, alkylation of nucleophilic acetylide anions is practical only for methyl and primary halides. While this alkylation reaction can be used with limited success with secondary haloalkanes, it fails altogether for tertiary haloalkanes. 

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