OrgoChem I

Alkynes

Addition, hydration, reduction, oxidative cleavage, acetylide alkylation

Summary

Alkynes add twice like alkenes. Hydration gives carbonyls via enol tautomerization. Lindlar → cis alkene; Na/NH₃ → trans. Terminal alkynes are acidic (pKa ~25).

What you need to know:

This topic covers alkynes. Focus on mastering the key concepts in the "Must know checklist" below. Follow the study steps systematically, and use practice problems to reinforce your understanding. The external reference provides comprehensive coverage of all concepts.

Video Tutorial

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Alkyne Reduction - Lindlar, Na/NH3, Hydrogenation

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Must know checklist

0 / 5 complete

AdditionHX (Markovnikov), X₂, H₂ (always adds twice to alkane); same as alkenes but electrophile adds twice
Alkyne Reduction - Lindlar, Na/NH3→
HydrationHg²⁺/H₂O → Markovnikov ketone (enol tautomerizes); hydroboration-oxidation → anti-Markovnikov aldehyde (terminal only)
Alkyne Addition - HX, X2 (like alkenes)→
ReductionH₂/Pd → alkane; Lindlar (H₂, Pd/BaSO₄, quinoline) → cis alkene; Na/NH₃ → trans alkene
Alkyne Hydration - Enol to Ketone Tautomerization→
Oxidative cleavageO₃ or KMnO₄; internal alkyne → carboxylic acids; terminal → CO₂ + carboxylic acid
Lindlar vs Na/NH3 - cis vs trans alkene→
Acidityterminal alkyne RC≡CH pKa ~25; NaNH₂, BuLi deprotonate; acetylide + alkyl halide → SN2, extended alkyne
Terminal Alkyne Acidity - Acetylide Alkylation→

Common mistakes

Using NaOH to deprotonate terminal alkyne (need NaNH2 or BuLi; pKa ~25)

Confusing Lindlar (cis) vs Na/NH3 (trans) reduction products

Forgetting alkyne adds twice with HX or X2 (like alkene but 2×)

Internal alkyne + HX: regiochemistry ambiguous (both ends similar)