Q1. [1] § 4.2.5 Nomenclature of Carbon Compounds
Consider the structures 'X' and 'Y' given below :
X: H–C–C–C–C–H (butane, C₄H₁₀, straight chain)
Y: H–C–(C with methyl branch)–C–H (isobutane/2-methylpropane, C₄H₁₀, branched chain)
Which of the following statements are correct ?
(i) 'X' and 'Y' are homologues.
(ii) 'X' and 'Y' have the same structural formula, but different molecular formula.
(iii) 'X' and 'Y' have the same molecular formula, but different structural formula.
(iv) 'X' and 'Y' are structural isomers.
- (A) (i) and (iii)
- (B) (i) and (iv)
- (C) (ii) and (iv)
- (D) (iii) and (iv)
Previously asked in CBSE board exam
2026 31/3/1 Q20
Generated by claude-sonnet-4-6 · 2026-06-15 06:46 · grounding rag
Model Answer
(D) (iii) and (iv)
X (n-butane) and Y (isobutane) both have the same molecular formula C₄H₁₀ but different structural formulae, making them structural isomers — not homologues.
Explanation
- Homologues differ by –CH₂– and have different molecular formulae, so (i) is wrong.
- Statement (ii) is the reverse of the truth — they have the same molecular formula, not the same structural formula — so (ii) is wrong.
- The textbook (section 4.2.2) explicitly states: compounds with identical molecular formula but different structures are called structural isomers — confirming both (iii) and (iv) are correct.
- Examiners expect you to recall the definition of structural isomers directly from the butane/isobutane example given in the chapter.
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