OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2025: Question 13
1 mark · Easy difficulty · Multiple Choice
Identify which of the given organic molecules would form a ketone group upon oxidation.
Practise this questionQuestion
Question text
13 Which molecule(s) would form a ketone group when oxidised?
OH
OH
OH
HO OH
1 HO OH
HO OH
O
O
O
HO OH
HO OH
HO OH
OH
OH
OH
A 1, 2 and 3
B Only 1 and 2
C Only 2 and 3
D Only 1
Your answer
[1]
Mark scheme
Show the mark scheme
13 B 1
How to answer it
Oxidation Products of Polyfunctional Alcohols
This question assesses your ability to classify alcohols as primary (1°), secondary (2°), or tertiary (3°) from skeletal/structural representations, and deduce their respective oxidation products when reacted with an oxidising agent such as acidified potassium dichromate(VI), K₂Cr₂O₇ / H₂SO₄.
- Primary alcohols (1°): Oxidise to aldehydes (under distillation) and further to carboxylic acids (under reflux).
- Secondary alcohols (2°): Oxidise exclusively to ketones (under reflux).
- Tertiary alcohols (3°): Cannot be oxidised by acidified dichromate because there is no hydrogen atom on the carbon bearing the –OH group.
Question 13 (Multiple Choice)
Identifying molecules that yield a ketone upon oxidation
💡 Molecule-by-Molecule Analysis
- Molecule 1 (Propane-1,2,3-triol / Glycerol):
Contains two terminal primary –OH groups and one central secondary –OH group: –CH(OH)– . The secondary –OH oxidises to a ketone group ( C=O ), yielding 1,3-dihydroxypropan-2-one.
👉 Forms a ketone. - Molecule 2 (4-hydroxypentanoic acid):
Contains a terminal carboxylic acid group ( –COOH ) which cannot be oxidised further, and a hydroxyl group on carbon-4. The carbon with the –OH is attached to a methyl group and a –CH₂– chain (2 carbon groups + 1 hydrogen), making it a secondary alcohol. It oxidises to form a ketone group.
👉 Forms a ketone. - Molecule 3 (2-methylpropan-2-ol):
The carbon bonded to the –OH group is directly attached to three methyl groups (three alkyl groups, zero hydrogens). This is a tertiary alcohol, which is resistant to oxidation.
👉 Does NOT form a ketone.
✅ Correct Answer
Correct Option: B (Only 1 and 2)
Since only secondary alcohols oxidise to ketones:
- Molecule 1 contains a secondary alcohol group ✔️
- Molecule 2 contains a secondary alcohol group ✔️
- Molecule 3 is a tertiary alcohol and does not oxidise ❌
🧠 Exam Technique & Strategy
- Rephrase the question immediately: "Which molecule contains a secondary (2°) alcohol?"
- Inspect the C–OH carbon: Count the number of C–C bonds attached directly to the carbon bearing the –OH group:
- 1 carbon attached = 1° alcohol (forms aldehyde/carboxylic acid)
- 2 carbons attached = 2° alcohol (forms ketone)
- 3 carbons attached = 3° alcohol (no oxidation)
- Don't get distracted by other groups: Molecule 1 has primary alcohols and Molecule 2 has a carboxylic acid group, but the question only asks which molecule forms a ketone group, not which one forms only a ketone.
❌ Common Misconceptions & Traps
- Overlooking polyols (Molecule 1): Students often look at the two ends, see primary alcohols, and incorrectly assume glycerol only oxidises to carboxylic acids/aldehydes.
- Ignoring existing functional groups (Molecule 2): Some candidates see the carboxylic acid group ( –COOH ) and think the molecule cannot be oxidised further, missing the oxidisable secondary –OH group on the chain.
- Confusing 3° with 2° in skeletal formulas (Molecule 3): Skeletal drawings omit hydrogen atoms. Students sometimes forget that the cross-intersection represents a carbon atom already bonded to three methyl groups, leaving no available C–H bond for oxidation.
Topics
Module 4: Core organic chemistry · 4.2 Alcohols, haloalkanes and analysis
Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.