Alkyl halides with alkynide ions

We use the same approach taken in the previous problem (12.14). All carbon-carbon bonds are prepared via alkylation of an alkynide ion with the appropriate alkyl halide. Each alkyl halide must be prepared from acetylene. The last step of the synthesis is the reduction of an alkyne to a cis alkene via hydrogenation with a poisoned catalyst ... 

This reaction has powerful synthetic utility, because the resulting alkynide ion can function as a nucleophile when treated with an alkyl halide ... 

This process is only cient with methyl or primary alkyl halides. When secondary or tertiary alkyl halides are used, the alkynide ion functions primarily as a base and elimination products are obtained. This observation is consistent with the pattern we saw in Section 8.14 (substitution vs. elimination). 

Alkynide ions react with carbonyl groups in much the same way as Grignard reagents do. We recall that these ions are effective nucleophiles that will displace a hahde ion ftom an alkyl halide to give an alkylated alkyne. The alkynides are prepared in an acid-base reaction with acetylene or a terminal alkyne using sodium amide in ammonia. If a carbonyl compound is then added to the reagent, an alcohol forms after acid work-up. If the alkynide is derived ftom acetylene, an acetylenic alcohol forms. 

In some respects, the alkylation of enolate anions resembles nucleophilic substitution. We recall that many nucleophiles displace leaving groups from primary alkyl halides by an Sj 2 mechanism (Section 9.3). A similar reaction occurs with secondary alkyl halides, but competing elimination reactions also occur. Primary alkyl halides react with carbanions, such as the alkynide ion, by an Sj 2 mechanism. (Secondary alkyl halides react not only in displacement reactions but also in elimination reactions because the alkynide ion is a strong base.)... 

Reaction of alkynide ions with alkyl halides (7.7, 8.9)... 

Alkynide ions undergo when treated with an alkyl halide (methyl or primary). 

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