Chapter 11: It Takes Alkynes: The Carbon-Carbon Triple Bond

The chapter explains the sp hybridization and resulting linear geometry of alkyne carbons, and describes why cycloalkynes with fewer than eight carbons experience destabilizing ring strain. Two primary synthetic approaches are presented: double dehydrohalogenation, which removes two hydrogen atoms and two halogen atoms from dihalides through sequential elimination reactions, and acetylide anion coupling, a carbon-carbon bond-forming strategy using deprotonated terminal alkynes as strong nucleophiles in substitution reactions with primary alkyl halides. The reactivity section explores multiple transformation pathways: halogenation adds bromine across the triple bond, complete hydrogenation converts alkynes to alkanes using palladium catalysts, and selective hydrogenation produces either cis-alkenes through Lindlar's catalyst or trans-alkenes through dissolving metal reduction with sodium and ammonia. The chapter culminates with hydration reactions that add water across the triple bond with distinct regiochemical outcomes: oxymercuration yields ketones via enol intermediates through Markovnikov addition, while hydroboration-oxidation produces aldehydes following anti-Markovnikov regioselectivity. Throughout, the chapter emphasizes regiochemistry—the regioselectivity of addition reactions—and stereochemistry, the spatial arrangement of atoms, as critical concepts for predicting reaction outcomes and designing synthetic sequences using alkynes as versatile synthetic building blocks.