Alkynes - Structure and Nomenclature - UNSOLVED PRACTICE SET

Q7. Write the IUPAC names of the following alkynes:

(a) HC≡C–CH₂–CH₂–CH₃

(b) CH₃–C≡C–CH₂–CH₃

(c) (CH₃)₂CH–C≡CH

Q8. Draw the structures of:

(a) Pent-1-yne

(b) 3-Methylbut-1-yne

(c) Hex-2-yne

Q9. Explain why alkynes have higher boiling points than alkanes and alkenes of comparable molecular mass.

Q10. What is the difference between terminal and internal alkynes? Which type is more acidic and why?

Q11. Your grandfather mentions that earlier, gas lamps on Indian streets used acetylene (ethyne). Why was acetylene suitable for this purpose? Write the combustion reaction.

Q12. Why is the C–H bond in terminal alkynes more acidic than the C–H bond in alkanes or alkenes? Explain using hybridisation.

Q13. Discuss the structure and bonding in alkynes:

(a) sp hybridisation and linear geometry

(b) Formation of one σ bond and two π bonds in the triple bond

(c) Comparison of bond lengths and bond energies with alkanes and alkenes

(d) Acidic nature of terminal alkynes — why terminal alkyne C–H is acidic

Draw orbital overlap diagrams for ethyne.

Q14. Explain the nomenclature of alkynes according to IUPAC rules:

(a) Selection of the longest carbon chain containing the triple bond

(b) Numbering to give the triple bond the lowest possible locant

(c) Naming substituents and indicating their positions

(d) Naming compounds containing both double and triple bonds (enynes)

Illustrate with at least four examples of increasing complexity.

Q15. Alkynes have unique applications in Indian industry and traditional practices. Discuss:

(a) Why acetylene (ethyne) was historically used in carbide lamps for mining and lighting in India

(b) The use of acetylene in oxy-acetylene torches for welding and cutting metals in Indian workshops

(c) How alkynes serve as starting materials for synthesising vitamins, steroids, and other pharmaceuticals

(d) The environmental concern of acetylene production from calcium carbide and its calcium hydroxide by-product

Q16. An alkyne has the molecular formula C₆H₁₀.

(a) Draw all possible structural isomers and name them using IUPAC nomenclature.

(b) Identify which isomers are terminal and which are internal.

(c) Which isomer would react with sodium metal to liberate hydrogen gas? Write the reaction.

Q17. The pKa values for different C–H bonds are:

Ethane (C₂H₆): ~50

Ethene (C₂H₄): ~44

Ethyne (C₂H₂): ~25

(a) Arrange these hydrocarbons in order of increasing acidity of their C–H bonds.

(b) Calculate the ratio of acid dissociation constants (Ka) for ethyne and ethane.

(c) Explain the trend in acidity using the concept of hybridisation and electronegativity of carbon.

Q18. A welding workshop in Mumbai uses an oxy-acetylene torch. The acetylene is generated by reacting calcium carbide (CaC₂) with water.

(a) Write the balanced equation for the production of acetylene from calcium carbide.

(b) Calculate the mass of calcium carbide required to produce 26 kg of acetylene.

(c) The oxy-acetylene flame reaches about 3300°C. Calculate the volume of oxygen (at STP) required for complete combustion of 26 kg of acetylene.

[Given: Atomic masses: Ca = 40, C = 12, O = 16, H = 1; Molar volume at STP = 22.4 L]