Alkanes IUPAC Nomenclature and Conformations - UNSOLVED PRACTICE SET

Q7. Write the IUPAC names of the following alkanes:

(a) CH₃–CH₂–CH(CH₃)–CH₂–CH₃

(b) (CH₃)₂CH–CH(CH₃)–CH₂–CH₃

Q8. Draw the Newman projections for ethane showing:

(a) Staggered conformation

(b) Eclipsed conformation

Label the dihedral angle in each.

Q9. What is the difference between conformers and constitutional isomers? Can conformers be separated?

Q10. Explain why the anti conformation of butane is more stable than the gauche conformation.

Q11. Your father fills petrol in the car, which contains a mixture of alkanes including octane (C₈H₁₈). Write the IUPAC name of a branched isomer of octane that has excellent anti-knock properties.

Q12. Why is rotation around the C–C single bond possible in alkanes, but not around a C=C double bond in alkenes?

Q13. Discuss the IUPAC nomenclature of alkanes with examples. Include:

(a) Selection of the longest continuous carbon chain (parent chain)

(b) Numbering of the chain to give substituents the lowest possible locants

(c) Naming and alphabetising substituents

(d) Naming complex branched substituents (isopropyl, tert-butyl, etc.)

Illustrate with at least three examples of increasing complexity.

Q14. Explain the conformations of butane (C₄H₁₀) along the C₂–C₃ bond:

(a) Draw Newman projections for all conformations (fully eclipsed, gauche, eclipsed, anti)

(b) Explain the types of strain present in each conformation (torsional, steric)

(c) Arrange the conformations in order of increasing energy and stability

(d) Calculate the relative energy of each conformation

Q15. Understanding alkane conformations is crucial in biochemistry and drug design. Discuss:

(a) Why the staggered conformation is preferred in protein structures (peptide bonds are somewhat rigid, but side chains rotate)

(b) How conformational analysis helps predict the biological activity of drug molecules

(c) Why LPG (liquefied petroleum gas) used in Indian households contains mainly propane and butane — how their conformations affect storage and combustion

(d) The role of conformational analysis in designing pesticides for Indian agriculture

Q16. Consider the alkane with the formula C₆H₁₄.

(a) Draw all possible structural isomers and write their IUPAC names.

(b) Identify which isomer has the highest boiling point and explain why.

(c) How many of these isomers have chiral centres? Identify them.

Q17. The energy barriers for rotation around the C₂–C₃ bond in butane are given:

Anti to gauche: 3.8 kJ/mol

Gauche to eclipsed: 16 kJ/mol

Eclipsed to fully eclipsed: 19 kJ/mol

(a) Calculate the total energy difference between the most stable and least stable conformations.

(b) At room temperature (25°C), what percentage of butane molecules would you expect to be in the anti conformation? (Assume Boltzmann distribution, k = 1.38 × 10⁻²³ J/K)

(c) Explain why the energy barrier between anti and gauche is much smaller than between staggered and eclipsed.

Q18. A petroleum refinery in Gujarat processes crude oil to produce various alkanes. In one batch, 1000 kg of a C₈H₁₈ isomer mixture is produced.

(a) Calculate the number of moles of C₈H₁₈ in 1000 kg.

(b) If the mixture contains 60% n-octane and 40% 2,2,4-trimethylpentane (isooctane) by mass, calculate the mass of each component.

(c) The octane rating of a fuel is defined relative to isooctane (100) and n-heptane (0). If this mixture has an octane rating of 92, explain what this means for engine performance.

[Given: Molar mass of C₈H₁₈ = 114 g/mol]