Chapter 12: Alkynes

Alkynes explains the acidity of terminal alkynes and how they can be deprotonated using strong bases like NaNH₂ or NaH to form alkynide ions, powerful nucleophiles used in alkylation reactions via SN2 mechanisms. Alkynes are synthesized through double elimination from vicinal or geminal dihalides using excess base, followed by protonation. Klein then explores multiple reduction pathways: catalytic hydrogenation (H₂/Pt) yields alkanes, Lindlar’s catalyst yields cis-alkenes via syn addition, and dissolving metal reductions (Na/NH₃) yield trans-alkenes via anti addition. The chapter also introduces two hydration mechanisms: acid-catalyzed hydration (HgSO₄/H₂SO₄), which gives ketones, and hydroboration-oxidation (R₂BH/H₂O₂), which produces aldehydes from terminal alkynes through anti-Markovnikov addition. These reactions are further clarified through keto-enol tautomerization, emphasizing the distinction between tautomers and resonance structures, and the role of acidic or basic conditions in directing proton transfers. The final section covers ozonolysis of alkynes, where oxidative cleavage yields carboxylic acids or carbon dioxide for terminal alkynes. Altogether, this chapter gives students a robust toolkit for alkyne manipulation, synthesis planning, and functional group transformations.