Chapter 9: Alkynes

A distinctive feature of terminal alkynes is their acidic hydrogen atom, which can be deprotonated to form alkynide ions—negatively charged species that function as potent nucleophiles in substitution reactions. The chapter details multiple synthetic pathways for constructing alkynes, including elimination reactions of vicinal dihalides and alkylation reactions using acetylide anions as nucleophilic reagents. Reduction chemistry occupies a significant portion of the chapter, with emphasis on three major methods: catalytic hydrogenation using palladium or platinum catalysts to produce saturated alkanes, Lindlar hydrogenation employing a poisoned catalyst to selectively generate cis-alkenes, and dissolving metal reduction using alkali metals and liquid ammonia to preferentially form trans-alkenes. The chapter extensively covers addition reactions across the triple bond, including hydrohalogenation following Markovnikov's rule, hydration via mercuric sulfate catalysis or hydroboration-oxidation protocols, halogenation with halogens or interhalogens, and oxidative cleavage using ozonolysis to cleave the triple bond and generate carbonyl compounds. Throughout the discussion, detailed reaction mechanisms illustrate the stepwise processes underlying each transformation, with attention to tautomerization phenomena in which unstable enol intermediates rearrange to more stable ketone or aldehyde products. The chapter concludes by emphasizing synthesis strategies that strategically interconvert alkanes, alkenes, and alkynes as carbon skeleton building blocks, providing students with a powerful toolkit for planning complex organic transformations applicable to pharmaceutical development and materials science applications.