Two compounds, n-butane and isobutane, share the molecular formula C₄H₁₀. (i) Draw the structural formulae of both compounds. (ii) What term is used for such pairs of compounds? (iii) Compare any two physical or chemical properties that would differ between these two compounds as a result of their different structures.
Generated by claude-sonnet-4-6 · 2026-06-26 01:09 · grounding rag
Model Answer
(i) Structural formulae:
n-Butane (straight chain):
$$\text{CH}_3 - \text{CH}_2 - \text{CH}_2 - \text{CH}_3$$
Isobutane (branched chain):
$$\text{CH}_3 - \overset{|}{\underset{|}{\text{CH}}} (\text{CH}_3) - \text{CH}_3$$
i.e., a central carbon bonded to three –CH₃ groups and one H.
(ii) Such compounds with identical molecular formula but different structures are called structural isomers.
(iii) Two differing properties:
- Boiling point: n-Butane has a higher boiling point than isobutane because its straight chain allows closer packing between molecules.
- Reactivity: The branched structure of isobutane makes it slightly more reactive than n-butane in certain reactions due to steric and structural differences.
Source: Carbon and its Compounds, Section 4.2.2 — Chains, Branches and Rings
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Explanation
- Part (i): Draw both clearly — a 4-carbon straight chain and a branched structure with a central carbon carrying three methyl groups.
- Part (ii): The exact NCERT term is "structural isomers" — use it precisely.
- Part (iii): The passage does not explicitly list physical properties of these two isomers, so use general knowledge of isomers: branched chains have lower boiling points due to less surface contact. Examiners accept any two valid, relevant points. Avoid vague statements — be specific.