AQA A-Level Chemistry Paper 3, June 2025: Question 6

✅ Correct Answer

• E-Z (isomerism)

💡 Key Knowledge

Two essential conditions give rise to E-Z stereoisomerism in unsaturated fatty acid tails:

1. Restricted rotation about the planar C=C double bond.
2. Two different groups attached to each carbon of the C=C bond (in Branch A, each alkene carbon has a –H and an alkyl chain).

✅ Correct Answers

• Number in ester name: 9 [1 mark]
• Number of stereoisomers: 4 (or four) [1 mark]
• Use for these esters: biodiesel / biofuel [1 mark]

📐 Step-by-Step Chain Counting

1. Locant of the C=C bond (Branch A):

• Carbonyl carbon ( C=O ) is always C-1.
• Follow the chain: there are 7 methylene groups: –(CH₂)₇– (Carbons 2 to 8).
• The first alkene carbon encountered is therefore carbon number 9.

2. Stereoisomers for Branch B:

• Branch B contains 2 separate C=C double bonds ( n = 2 ).
• Each alkene centre independently exhibits E or Z geometry.
• Total possible stereoisomers = 2ⁿ = 2² = 4 ((E,E), (E,Z), (Z,E), (Z,Z)).

🧠 Exam Technique: Industrial Uses

Reacting vegetable oils (triglycerides) with methanol in the presence of a strong base catalyst (e.g. KOH/NaOH) yields a mixture of fatty acid methyl esters (FAME) and propane-1,2,3-triol (glycerol).

Whenever you see a triglyceride reacted with methanol or ethanol, think biodiesel production.

❌ Common Errors

• Counting backwards: Counting from the hydrocarbon methyl end gives 9 purely by coincidence here, but in other fatty acids this leads to incorrect locants. Always count from the ester carbonyl group as C-1!
• Vague uses: Writing general terms like "fuel" or "petrol" instead of the specific syllabus keyword biodiesel or biofuel.

✅ Correct Answers

• Formula: C₁₇H₃₁COONa or C₁₈H₃₁O₂Na [1 mark]
• Use: soap / anionic surfactant [1 mark]

📐 Deducing the Molecular Formula

Count the atoms in fatty acid tail B:

• Carboxyl carbon: 1 C ( C=O )
• Aliphatic spacer: 7 × CH₂ = 7 C, 14 H
• First alkene: CH=CH = 2 C, 2 H
• Methylene link: CH₂ = 1 C, 2 H
• Second alkene: CH=CH = 2 C, 2 H
• Aliphatic tail: 4 × CH₂ + CH₃ = 5 C, 11 H
• Total: 18 Carbons, 31 Hydrogens, 2 Oxygens, 1 Sodium
• Formula written as a salt: C₁₇H₃₁COONa or C₁₈H₃₁O₂Na

❌ Critical Trap: Covalent O–Na Bond

Never draw a bond line between O and Na!

Sodium carboxylates are ionic lattices/salts in aqueous media. A line indicates a shared electron pair (covalent bond). Drawing –C–O–Na will automatically forfeit the mark.

Always write it as either –COONa or with explicit charges: –COO⁻ Na⁺ .

💡 Soap vs Detergents

• Alkaline hydrolysis of fats produces carboxylate salts ( RCOO⁻Na⁺ ), which are traditional soaps (anionic surfactants).
• Synthetic detergents are typically derived from sulfonate salts ( RSO₃⁻Na⁺ ) or quaternary ammonium salts (cationic surfactants used in hair conditioners), not natural saponification products.