AQA A-Level Chemistry Paper 3, June 2022: Question 24

Distinguishing Properties of Primary and Secondary Alcohols

📋 What this question tests

This question assesses your fundamental knowledge of organic functional group chemistry, specifically:

Oxidation reactions of alcohols: Identifying the different products formed from primary vs secondary alcohols using acidified potassium dichromate(VI) (K₂Cr₂O₇/H₂SO₄).
Nucleophilic substitution: Formation of alcohols via hydrolysis of haloalkanes using aqueous alkali.
Esterification: Condensation reactions of alcohols with carboxylic acids.
Intermolecular forces: Recognizing hydrogen bonding due to the polar O–H group.

Question Analysis & Correct Option

Question 24 (1 Mark)

✅ Correct Answer: Option A

"They are easily oxidised to carboxylic acids by acidified K₂Cr₂O₇ solution."

This statement is NOT correct for both alcohols because only primary alcohols can be oxidised to carboxylic acids (via aldehydes under reflux). Secondary alcohols are oxidised exclusively to ketones, which cannot be oxidised further under standard conditions without breaking the carbon skeleton.

Award 1 mark for: A

💡 Key Knowledge: Alcohol Oxidation Summary

Alcohol Class	Partial Oxidation	Complete Oxidation
Primary (1°) R–CH₂OH	Aldehyde (R–CHO) Distillation	Carboxylic acid (R–COOH) Reflux + excess [O]
Secondary (2°) R₂CHOH	Ketone (R₂C=O) Reflux	Ketone (R₂C=O) No further oxidation
Tertiary (3°) R₃COH	Not easily oxidised (no C–H bond at functional carbon)

Evaluating the Incorrect Distractors

Why Options B, C, and D are TRUE statements (and therefore wrong choices)

📐 Breakdown of Options B, C & D

Option B: Hydrolysis of bromoalkanes
TRUE: Primary bromoalkanes undergo nucleophilic substitution (S_N2) with aqueous OH⁻ to yield 1° alcohols, while secondary bromoalkanes undergo substitution to yield 2° alcohols:
R–Br + OH⁻ → R–OH + Br⁻
Option C: Ester formation
TRUE: Both 1° and 2° alcohols react with carboxylic acids in the presence of concentrated H₂SO₄ catalyst to form esters:
RCOOH + R'OH ⇌ RCOOR' + H₂O
Option D: Hydrogen bonding in liquid state
TRUE: Both 1° and 2° alcohols contain a highly electronegative oxygen atom bonded to hydrogen ( O–H ) with lone pairs on oxygen, allowing extensive intermolecular hydrogen bonding.

🧠 Exam Technique: Handling "NOT" Questions

Spot the negative stem: In AQA exams, words like not or incorrect are printed in bold. Underline them immediately.
Annotate each option: Write a tick (✓) if the statement is factually true, or a cross (✗) if it is false. The question asks for the false statement, so your answer is the cross (✗).
Verify the chemistry:
A: 1° → Acid ✓ | 2° → Acid ✗ (gives Ketone) → Statement is False!
B: Haloalkane + OH⁻ → Alcohol ✓
C: Alcohol + Acid → Ester ✓
D: O–H bond present → H-bonds ✓

Examiner Insights & Pitfalls

❌ Common Errors & Misconceptions

Confusing Secondary and Tertiary Alcohols: Students often remember that "one type of alcohol doesn't oxidise" and mistakenly think secondary alcohols don't react at all, forgetting they readily oxidise to ketones with an orange-to-green colour change ( Cr₂O₇²⁻ → Cr³⁺ ).
Assuming Secondary Alcohols Form Acids: Over-oxidising in their minds: believing that heating a secondary alcohol under reflux with excess oxidising agent will eventually yield a carboxylic acid. Remember: C–C bonds are too stable to be cleaved under these standard conditions.
Misreading Steric Hindrance: Thinking secondary alcohols cannot form esters because they are more crowded. While secondary alcohols react slightly more slowly than primary alcohols, esterification still readily occurs.