AQA A-Level Chemistry Paper 3, 2019: Question 24

✅ Correct Answer

A: C₆H₅CH₂OH (phenylmethanol)

The complete hydrolysis reaction is:
CH₃COOCH₂C₆H₅ + H₂O ⇌ CH₃COOH + C₆H₅CH₂OH

The two products formed are ethanoic acid (CH₃COOH) and phenylmethanol (C₆H₅CH₂OH). Since ethanoic acid is not among the options, phenylmethanol is the correct choice.

💡 Key Knowledge

• Anatomy of an Ester Name:
  [alkyl / aryl group from alcohol] + [acid root -oate]
  In phenylmethyl ethanoate:
  • Phenylmethyl = alcohol derivative: C₆H₅CH₂−
  • Ethanoate = carboxylic acid derivative: CH₃COO−
• Acid Hydrolysis Cleavage:
  The ester bond cleaves between the acyl carbon and the single-bonded oxygen:
  R−CO−|−O−R' + H−OH ⇌ R−COOH + R'−OH
• Unlike base hydrolysis (which gives a carboxylate salt and is irreversible), acid hydrolysis is a reversible equilibrium reaction yielding a free carboxylic acid and an alcohol.

🧠 Exam Technique: Two-Step Decomposition

1. Draw the structural formula from the name:
   Identify the acyl fragment: ethanoate = CH₃C(=O)−
   Identify the attached group: phenylmethyl = −O−CH₂−C₆H₅
   Combined ester: CH₃COOCH₂C₆H₅
2. Add water across the ester link:
   Add −OH to the carbonyl group: CH₃COOH
   Add −H to the oxygen atom: HO−CH₂C₆H₅
3. Scan the choices: Look for either product. Here, C₆H₅CH₂OH matches option A directly.

❌ Common Misconceptions & Traps

• Confusing 'phenylmethyl ethanoate' with 'methyl benzoate':
  Hydrolysis of methyl benzoate (C₆H₅COOCH₃) forms benzoic acid ( C₆H₅COOH , Option D) and methanol. Students who misread the name often incorrectly pick D.
• Confusing 'phenylmethyl' with 'phenyl':
  Phenyl ethanoate (CH₃COOC₆H₅) hydrolyses to phenol (C₆H₅OH) and ethanoic acid. The extra −CH₂− in phenylmethyl means the alcohol produced is primary (phenylmethanol), not phenol.
• Choosing oxidation products (B & C):
  C₆H₅CHO (benzaldehyde) and C₆H₅COCH₃ (phenylethanone) are carbonyl compounds formed by oxidation, never directly by ester hydrolysis.