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Alkynes have two pi bonds and sp-hybridized carbons (50% s-character). Terminal alkynes are acidic (pKa ~25) — NaNH2 deprotonates them to form acetylide ions, which are powerful nucleophiles for C-C bond formation via SN2 with methyl/primary halides. Key selective reductions: Lindlar's catalyst (H2/Pd-BaSO4-quinoline) gives cis-alkene (syn addition on poisoned surface), Na/NH3(l) gives trans-alkene (anti via radical anion mechanism). Hydration follows two regiochemical pathways: Hg2+/H+/H2O gives Markovnikov addition → methyl ketone from terminal alkynes (enol → keto tautomerization); disiamylborane then H2O2/NaOH gives anti-Markovnikov → aldehyde from terminal alkynes. HX addition (2 eq) follows double Markovnikov to give gem-dihalide. Br2 addition (1 eq) gives trans-dihaloalkene; 2 eq gives tetrahalide. Terminal alkynes are detected by silver acetylide test $\frac{white precipitate with AgNO3}{NH3}$ or copper acetylide test (red-brown precipitate with Cu2Cl2/NH3) — internal alkynes give no precipitate. Ozonolysis of alkynes gives carboxylic acids (more oxidized than alkene products). Cyclotrimerization of acetylene at 873 K over iron gives benzene (Berthelot synthesis). These reactions form a toolkit for multistep synthesis — acetylide alkylation extends carbon chains, selective reduction controls stereochemistry, and regioselective hydration provides either aldehydes or ketones from the same alkyne.