Alkylation of Acetylide Anions

The of acetone, CH tCOCH ), is 19.3. Which of the following bases is strong enough 

The presence of a negative charge and an unshared electron pair on carbon makes an acetylide anion strongly nucleophilic. As a result, an acetylide anion can react with an alkyl halide such as bromomethane to substitute for the halogen and yield a new alkyne product  

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

The nucleophilic acetylide anion uses its electron lone pair to form a bond to the positively polarized, electrophilic carbon atom of bromomethane. As the new C-C bond begins to form, the C-Br bond begins to break in the transition state. 

I The new C-C bond is fully formed and the old C-Br bond is fully broken at the end of the reaction. 

Thomson Click Organic Interactive to use a web-based palette to predict products for alkyne alkylation reactions. 

Alkyne alkylation is not limited to acetylene itself. Any terminal alkyne can be converted into its corresponding anion and then alkylated by treatment with an alkyl halide, yielding an internal alkyne. For example, conversion of 1-hexyne into its anion, followed by reaction with 1-bromobutane, yields 5-decyne. 

CH3CH2CH2CH2C CH 2 cH Ch cHCH Br CH C C CEECC CHjC C)- 1-Hexyne 5-Decyne (76%) 

Because of its generality, acetylide alkylation is an excellent method for preparing substituted alkynes from simpler precursors. A terminal alkyne can be prepared by alkylation of acetylene itself, and an internal alkyne can be prepared by further alkylation of a terminal alkyne. 

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Under the same conditions simple etiolates react vigorously with alkyl halides (which must be primary) to give mono- and polyalkylated products. The reactivity of the simple enolate is greater and cannot be controlled at room temperature. However, if the alkylation is carried out at low temperature, the reaction can be controlled and smooth monoalkylation of simple enolates can be achieved. The same is true for the alkylation of acetylide anions, which must be carried out at low temperature for successful alkylation. 

Sec. 8.8) from terminal alkynes by alkylation of acetylide anions... 

Alkyne Acidity Formation of Acetylide Anions Alkylation of Acetylide Anions 289... 

Because of the ready availability of acetylene and the ease with which it is converted to a nucleophile, alkylation of acetylide anions is the most convenient laboratory method used for the synthesis of other alkynes. The process can be repeated, and a terminal alkyne in turn can be converted to an internal alkyne. An important feature of this reaction is that a new carbon-carbon skeleton can be made, allowing for the construction of larger carbon skeletons from smaller ones. In the following scheme, the carbon skeleton of 3-heptyne is constructed from acetylene and two lower-molecular-weight haloalkanes. 

A. Alkylation of Acetylide Anions with Methyl and 1° Haloalkanes... 

Trimethylsilylallenes behave as propargylic anion equivalents during the titanium tetrachloride catalysed addition to carbonyl compounds, leading to homopropargylic carbinols. This new approach should prove useful in the synthesis of branched acetylenes which are not accessible via alkylation of acetylide anions with alkyl halides and epoxides (Scheme 23). ... 

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