AQA A-Level Chemistry Paper 3, June 2025: Question 35

✅ Correct Answer

Option A: [C₁₆H₃₃N(CH₃)₃]Br

This compound is a quaternary ammonium salt (specifically, hexadecyltrimethylammonium bromide / cetyltrimethylammonium bromide). It contains a long non-polar hydrocarbon tail ( C₁₆H₃₃ ) and a positively charged ammonium polar head group ( [–N(CH₃)₃]⁺ with a bromide counterion, Br⁻ ).

💡 Key Knowledge

• Surfactant: A compound with a hydrophobic (non-polar hydrocarbon) tail and a hydrophilic (polar/charged) head.
• Cationic Surfactant: The hydrophilic head carries a permanent positive charge regardless of pH, typically a quaternary ammonium ion: [R–N(CH₃)₃]⁺ .
• Anionic Surfactant: The hydrophilic head carries a negative charge (e.g., carboxylate R–COO⁻ or sulfonate R–SO₃⁻ ).
• Uses of Cationic Surfactants: Fabric conditioners and hair conditioners, because wet fabrics and wet hair carry slight negative surface charges, allowing the cationic heads to bind electrostaticallly, leaving the hydrophobic tails facing outward to create a smooth, anti-static coating.

🧠 Exam Technique & Option Elimination

• A: [C₁₆H₃₃N(CH₃)₃]Br → Correct. Quaternary ammonium salt; dissociates in water into the surfactant cation [C₁₆H₃₃N(CH₃)₃]⁺ and a Br⁻ counterion.
• B: C₁₆H₃₃N(CH₃)₂ → Incorrect. This is a tertiary amine, not a salt. It is neutral (uncharged), so it cannot be a cationic surfactant.
• C: C₁₇H₃₅COONa → Incorrect. Sodium stearate (soap). It dissociates into Na⁺ and the stearate ion C₁₇H₃₅COO⁻ . Because the active surfactant tail carries a negative charge, this is an anionic surfactant.
• D: C₁₇H₃₅COOH → Incorrect. Stearic acid is a neutral long-chain fatty acid. It is insoluble in water and uncharged, so it cannot function as a surfactant.

❌ Common Misconceptions & Examiner Traps

• Confusing Anionic vs Cationic: Students frequently mix up C ( C₁₇H₃₅COONa ) and A. Remember: Look at the charge on the organic group carrying the long alkyl chain, not the spectator counterion. In C, the long alkyl chain is on the anion ( COO⁻ ); in A, it is on the cation ( N⁺ ).
• Confusing Amines with Quaternary Ammonium Salts: A tertiary amine (Option B) has a lone pair and is uncharged. To be a quaternary ammonium surfactant, the nitrogen must form 4 covalent bonds to carbon atoms, giving it a permanent positive charge.