Cyclohexane (C₆H₁₂) and cyclohexene (C₆H₁₀) both have carbon atoms arranged in a ring, yet they show different chemical behaviour with bromine water. Identify the class each belongs to (saturated or unsaturated hydrocarbon) and justify your answer using their molecular formulae and the result of the bromine water test.
Generated by claude-sonnet-4-6 · 2026-06-26 01:09 · grounding rag
Model Answer
Cyclohexane (C₆H₁₂) — Saturated hydrocarbon:
Its molecular formula matches the general formula CₙH₂ₙ, with all single bonds between carbon atoms. All valencies are satisfied by single bonds, so no scope for further addition of atoms.
Cyclohexene (C₆H₁₀) — Unsaturated hydrocarbon:
Its formula (CₙH₂ₙ₋₂) indicates fewer hydrogen atoms, which means it contains a carbon–carbon double bond.
Bromine water test:
Cyclohexane does not decolourize bromine water (saturated; unreactive). Cyclohexene decolourizes bromine water immediately, as the double bond allows addition of bromine across it. This confirms cyclohexene is unsaturated.
Source: Chapter 4, Section 4.2.1 – Saturated and Unsaturated Carbon Compounds
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Explanation
- Examiners expect: correct class identified for each compound + molecular formula justification + bromine water result linked to reactivity.
- Key rule: saturated hydrocarbons do not decolourize bromine water; unsaturated ones do (addition reaction at the double/triple bond).
- Cyclohexane's formula C₆H₁₂ = CₙH₂ₙ (same as alkenes, but ring structure with all single bonds — still saturated). Cyclohexene has one double bond, making it unsaturated.
- Don't just state the result — link the bromine water observation to the bond type for full marks.