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Alkyne Reactions & Selective Reduction

Differentiate cis and trans partial reductions and predict terminal-alkyne hydration products.

Step-by-step lessonsCurated examplesInteractive practicePrintable worksheets

1. Understand the concept

Alkynes contain a C≡C triple bond. Their pi bonds allow selective reductions and hydration reactions; reagent choice strongly affects the final product.

Learning goal: Differentiate cis and trans partial reductions and predict terminal-alkyne hydration products.

2. Use a reliable prediction strategy

  1. Decide whether the reagent is reducing, hydrating, or adding across the triple bond.
  2. For partial reduction, H₂/Lindlar produces a cis alkene and Na/NH₃(l) produces a trans alkene when geometric isomers exist.
  3. For terminal-alkyne hydration, Hg²⁺ acid hydration typically gives a methyl ketone after tautomerization.
  4. For terminal-alkyne hydroboration–oxidation with a suitable bulky borane, the product is an aldehyde after tautomerization.

3. Follow the reaction roadmap

Three verified reaction transformations with substrates, reagents and labeled products for Alkyne Reactions
Illustrative pathways; reagent choice and selectivity matter. Structures are shown in condensed notation.

4. Work through examples

Worked example 1

2-Butyne selective reduction

CH₃C≡CCH₃ + H₂/Lindlar → cis-2-butene

Catalyst-poisoned hydrogenation adds hydrogen syn and stops at an alkene.

Worked example 2

1-Butyne hydration

CH₃CH₂C≡CH + HgSO₄/H₂SO₄/H₂O → CH₃CH₂COCH₃

Markovnikov hydration forms an enol that tautomerizes to the methyl ketone 2-butanone.

5. Common mistakes

  • Calling the Lindlar product trans instead of cis when the two isomers are possible.
  • Applying terminal-alkyne hydroboration as though it yields a ketone; anti-Markovnikov hydration gives an aldehyde.
  • Forgetting that enols commonly tautomerize to carbonyl compounds.

6. Check understanding

Give two reagents that convert 2-butyne into different geometric alkene isomers.

Lindlar catalyst/H₂ gives cis-2-butene; Na/NH₃(l) gives trans-2-butene.

What carbonyl product comes from hydroboration–oxidation of 1-butyne?

Butanal, CH₃CH₂CH₂CHO, following enol-to-aldehyde tautomerization.

7. Practice the reactions

Use the quiz below to apply the rules to new substrates and reagents. A correct answer is explained, not merely marked.

INTERACTIVE PRACTICE

Try the reaction challenge

Answer eight curated questions. Review the explanation after each answer.

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