OCR A-Level Chemistry Synthesis and analytical techniques (02), November 2020: Question 7

✅ Correct Answer: A

Aqueous potassium hydroxide reacts with both ethanoic acid and phenol.

💡 Key Knowledge

• Ethanoic acid is a weak carboxylic acid that reacts with strong bases (like KOH) and weak bases (like carbonates) to form carboxylate salts.
• Phenol is a weaker acid than carboxylic acids, but it is sufficiently acidic to react with strong bases like aqueous sodium hydroxide or potassium hydroxide to form soluble phenoxide salts.
• Phenol does not react with weak bases like carbonates because it is not strong enough to displace carbon dioxide.

🧠 Exam Technique

Go through the options systematically and test each reagent against both substances:

• Test A (KOH): Neutralises ethanoic acid (forms potassium ethanoate + water) and neutralises phenol (forms potassium phenoxide + water). Valid for both!
• Test B (Bromine): Reacts with phenol via electrophilic substitution (forms 2,4,6-tribromophenol), but does not react with simple aliphatic carboxylic acids like ethanoic acid.
• Test C (CaCO₃): Reacts with ethanoic acid releasing CO₂ gas, but phenol is too weak an acid to react with carbonates.
• Test D (Methanol + acid catalyst): Esterifies with ethanoic acid, but phenols do not undergo direct Fischer esterification with carboxylic acids under standard conditions.

❌ Common Errors

• Confusing phenol with carboxylic acids regarding carbonates: Many students incorrectly assume that because phenol is an "acid", it will react with carbonates like calcium carbonate to release carbon dioxide gas. Remember: phenol lacks sufficient acid strength!
• Forgetting phenol's acidic nature: Thinking phenol only undergoes electrophilic substitution reactions and cannot act as an acid towards strong alkalis.