Practice Problems

Chapter 4 Practice Problems

Chapter 4 Practice Problems (Alkynes)

  1. Regioselective Addition: Predict the major product when 1-heptyne (CH₃CH₂CH₂CH₂CH₂C≡CH) is treated with one equivalent of HBr. What if excess HBr is used? Draw the structures and justify Markovnikov vs. anti-Markovnikov orientation.
  2. Partial vs. Complete Hydrogenation: For 2-butyne (CH₃C≡CCH₃):
    a) Product with H₂/Lindlar’s catalyst.
    b) Product with Na/NH₃(l).
    c) Product with excess H₂/Pt. Indicate stereochemistry for (a) and (b).
  3. Alkyne Hydration Mechanism: Propose a mechanism for the HgSO₄/H₂SO₄-catalyzed hydration of 1-propyne to 2-propanone. Show the enol intermediate and tautomerization. Why is Hg²⁺ necessary for terminal alkyne hydration?
  4. Hydroboration of Alkynes:
    a) 1-hexyne → (BH₃·THF, then H₂O₂/NaOH): what carbonyl forms?
    b) 1-hexyne → (HgSO₄/H₂SO₄): what carbonyl forms? Explain aldehyde vs ketone via Markovnikov’s rule.
  5. Formation of Alkynes: Outline a synthesis of 3-heptyne (CH₃CH₂C≡CCH₂CH₂CH₃) starting from 1-bromopropane. Hint: form an acetylide, then SN2.
  6. Dihalide to Alkyne: Reagents/conditions to convert 2,2-dibromobutane to 2-butyne. Why is a strong base required? What by-products form?
  7. Alkyne Halogenation Stereochemistry: 1-phenyl-1-propyne (PhC≡C–CH₃) + 1 equiv Br₂ in CH₂Cl₂: draw/name the product. Is it a single geometric isomer or a mixture? Briefly explain the addition mechanism.
  8. Oxidative Cleavage: 4-octyne under ozonolysis (then water) → predict the organic products. Hint: cleave to carboxylic acids on both sides; check mass balance/oxidation state.
  9. Multistep Synthesis with Alkynes: Design a synthesis for 5-decanone (CH₃CH₂CH₂COCH₂CH₂CH₂CH₂CH₃) using an alkyne intermediate. Suggest a route: build a C–C bond via acetylide alkylation, then convert the alkyne to the ketone. List steps/reagents.
  10. Challenge – Differentiating Pathways: An unknown alkyne (C₅H₈) gives one ketone under Hg²⁺/H₂SO₄ hydration and a different ketone under hydroboration–oxidation. Propose the alkyne and name the two ketones (Markovnikov vs anti-Markovnikov outcomes). Show how each hydration path leads to its product.