Reactions of Alkynes - UNSOLVED PRACTICE SET
Chapter: Hydrocarbons | Topic: Reactions of Alkynes
REACTIONS OF ALKYNES - UNSOLVED PRACTICE SET
Topic: Reactions of Alkynes
Multiple Choice Questions
Q1. The addition of H₂ to an alkyne in the presence of Lindlar's catalyst gives:
- Alkane
- cis-Alkene
- trans-Alkene
- No reaction
Q2. The addition of Na/NH₃ (liq.) to an internal alkyne gives:
- cis-Alkene
- trans-Alkene
- Alkane
- Alkyne salt
Q3. Terminal alkynes react with ammoniacal silver nitrate (Tollens' reagent) to give:
- A white precipitate of silver acetylide
- A red precipitate
- No reaction
- A blue solution
Q4. Hydration of ethyne in the presence of HgSO₄ and dilute H₂SO₄ gives:
- Ethanal
- Ethanol
- Ethanediol
- Ethanoic acid
Q5. The reaction of terminal alkyne with sodamide (NaNH₂) produces:
- Sodium alkynide and ammonia
- Sodium alkoxide
- No reaction
- Alkene
Q6. Ozonolysis of an internal alkyne followed by hydrolysis gives:
- Only ketones
- Only carboxylic acids
- Two carboxylic acids
- Aldehydes and ketones
Short Answer Questions
Q7. Write the products formed when propyne reacts with:
(a) H₂/Pd-BaSO₄ (Lindlar's catalyst)
(b) H₂/Na/NH₃ (liq.)
(c) 1 mol Br₂
Q8. Explain why terminal alkynes give a white precipitate with ammoniacal silver nitrate solution, but internal alkynes do not.
Q9. Write the mechanism for the hydration of propyne to give propanone. Why does the enol intermediate tautomerise to the ketone?
Q10. What is the difference between syn and anti addition of hydrogen to alkynes? Give one reagent for each type.
Q11. Your school lab demonstrates that acetylene decolourises bromine water and alkaline KMnO₄ solution. Explain the chemical reactions occurring in each case.
Q12. Why does the hydration of alkynes follow Markovnikov's rule? Explain using the example of but-1-yne.
Long Answer Questions
Q13. Discuss the addition reactions of alkynes:
(a) Catalytic hydrogenation — complete reduction to alkane and partial reduction to alkene (Lindlar's catalyst vs. Na/NH₃)
(b) Addition of halogens (Cl₂, Br₂) — stepwise addition
(c) Addition of HX — Markovnikov addition
(d) Hydration in presence of Hg²⁺ salts — formation of carbonyl compounds
For each reaction, write the mechanism and products using propyne and but-2-yne as examples.
Q14. Explain the following reactions of terminal alkynes:
(a) Formation of metal acetylides with Na, K, and Ag⁺
(b) Reaction with Grignard reagents
(c) Oxidation with KMnO₄ and ozonolysis
(d) Polymerisation of ethyne to benzene and other products
Write balanced chemical equations for each reaction.
Q15. Alkyne reactions are industrially important for India's chemical sector. Discuss:
(a) How hydration of ethyne produces acetaldehyde, a precursor for many chemicals
(b) The use of vinyl chloride (from ethyne) in producing PVC pipes widely used in Indian construction
(c) Why acetylene-based chemicals are being gradually replaced by ethylene-based processes (environmental and economic reasons)
(d) The role of alkynes in synthesising neem-based biopesticides and other agrochemicals in India
Numerical / Application-Based Problems
Q16. An alkyne (C₆H₁₀) on complete catalytic hydrogenation consumes 2 moles of H₂ per mole of alkyne to give hexane.
(a) Deduce whether the alkyne is terminal or internal.
(b) If the same alkyne on ozonolysis gives propanoic acid and ethanoic acid, deduce its structure.
(c) Calculate the mass of hexane produced from 8.2 g of the alkyne.
[Given: Molar masses: C₆H₁₀ = 82 g/mol, C₆H₁₄ = 86 g/mol]
Q17. A 10 g sample of an alkyne reacts with ammoniacal silver nitrate to give 24 g of silver acetylide precipitate (RC≡CAg).
(a) Write the general reaction for this process.
(b) Calculate the molar mass of the alkyne.
(c) Identify the alkyne from the given data.
[Given: Atomic masses: C = 12, H = 1, Ag = 108]
Q18. A chemical plant in Gujarat produces acetaldehyde by hydration of 100 kg of ethyne daily.
(a) Write the balanced equation for the hydration of ethyne to acetaldehyde.
(b) Calculate the theoretical yield of acetaldehyde.
(c) If acetaldehyde is further oxidised to acetic acid (ethanoic acid), calculate the mass of acetic acid produced and its value at ₹ 45 per kg.
[Given: Molar masses: C₂H₂ = 26 g/mol, CH₃CHO = 44 g/mol, CH₃COOH = 60 g/mol]