Terminal alkyne acidity (pKa ~25):

• R-C≡C-H + NaNH2 → R-C≡C-Na+ + NH3 (sp C-H is acidic due to 50% s-character)
• Acetylide ion is a strong nucleophile — useful for C-C bond formation via SN2
• R-C≡C- + CH3-Br → R-C≡C-CH3 (only methyl and 1° halides — SN2 mechanism)

Selective reduction:

• Lindlar's catalyst (H2/Pd-BaSO4 poisoned with quinoline/S): Stops at cis-alkene (syn addition on surface)
• Na/NH3(l) (dissolving metal reduction): Gives trans-alkene (anti addition via radical anion → vinyl radical → vinyl anion)

Hydration of alkynes (Markovnikov):

• Terminal alkyne + H2O/H+/Hg2+ → enol → tautomerizes → methyl ketone (RCCCH → RCOCH3)
• Exception: ethyne gives acetaldehyde (CH3CHO)