Show how the following compounds could be prepared, using...

Show how the following compounds could be prepared, using ethyne as one of the starting materials. Explain why ethyne should be alkylated before, rather than after, nucleophilic addition.
a. 1 -pentyn- 3 -ol
b. 1-phenyl-2-butyn-1-ol
c. 2 -methyl- 3 -hexyn- 2 -ol

Key Concepts

Reaction Order and Functional Group Compatibility

In multi-step synthesis, the order in which reactions are performed can be crucial. Alkylating the acetylide anion before conducting nucleophilic addition to a carbonyl group is preferred because the presence of an alcohol hydroxyl (or the corresponding alkoxide) can interfere with alkylation processes. Early alkylation avoids complications like competitive side reactions, altered acidity, or steric hindrance that would diminish the efficiency and selectivity of the synthetic sequence.

Nucleophilic Addition to Carbonyl Compounds

Acetylide anions or the alkylated alkynes can be added to electrophilic carbonyl compounds, such as aldehydes or ketones. This nucleophilic addition forms an alkoxide intermediate, which upon protonation, yields propargylic alcohols. This transformation is fundamental in constructing the given alcohol-substituted alkynes.

Alkylation of Acetylide Anions

Once formed, the acetylide anions can undergo alkylation by reacting with alkyl halides. This step introduces additional carbon fragments onto the alkyne moiety, building the carbon skeleton necessary for more complex molecules. Performing this alkylation before nucleophilic addition is critical to avoid issues that arise from competing reactivity or side reactions.

Acetylenic Proton Acidity and Acetylide Formation

Terminal alkynes, such as ethyne, possess acidic protons due to the high s-character of the sp-hybridized carbon atoms. This acidity allows them to be deprotonated by strong bases to form acetylide anions, which are highly nucleophilic intermediates that can be used effectively in subsequent carbon–carbon bond-forming reactions.

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