AQA A-Level Chemistry Paper 3, 2017: Question 23

Reduction of Carbonyl Compounds & Alcohol Classification

📌 What this question tests

This question assesses your ability to deduce the classification of alcohols (primary, secondary, or tertiary) from IUPAC names and relate them to the organic redox reactions of aldehydes and ketones using reducing agents such as aqueous NaBH₄.

Aldehyde reduction: Produces primary (1°) alcohols.
Ketone reduction: Produces secondary (2°) alcohols.
Tertiary (3°) alcohols: Cannot be prepared by reducing aldehydes or ketones.

Question 23

Identifying Which Alcohol Cannot Be Formed by Carbonyl Reduction

Multiple Choice • 1 Mark

✅ Correct Answer

Option B: 2-methylbutan-2-ol

Mark Scheme: Award 1 mark for B.

2-methylbutan-2-ol is a tertiary (3°) alcohol. Tertiary alcohols cannot be synthesised by the reduction of an aldehyde or a ketone.

💡 Key Knowledge

Reduction of Aldehydes:
R-CHO + 2[H] → R-CH₂OH (Primary / 1° alcohol)
Reduction of Ketones:
R-CO-R' + 2[H] → R-CH(OH)-R' (Secondary / 2° alcohol)
Reducing Agent: Usually aqueous sodium borohydride, NaBH₄ , providing hydride nucleophiles (:H⁻).
Because carbonyl groups ( C=O ) always carry at least one hydrogen atom (in aldehydes) or two alkyl groups (in ketones), adding 2[H] always adds an H to the carbon atom. Thus, the resulting alcohol carbon must have at least one hydrogen attached, making tertiary alcohols impossible to form this way.

📐 Step-by-Step Molecule Analysis

A: 2,2-dimethylpropan-1-ol
The -1-ol is attached to an end carbon: (CH₃)₃C-CH₂OH .
→ 1° alcohol (formed by reducing 2,2-dimethylpropanal).
B: 2-methylbutan-2-ol
The -OH is on carbon 2, which also carries a methyl branch: CH₃-C(CH₃)(OH)-CH₂CH₃ .
The carbon bonded to the OH has 0 attached hydrogens.
→ 3° alcohol (CANNOT be made by reduction).
C: 3-methylbutan-2-ol
The -OH is on carbon 2: (CH₃)₂CH-CH(OH)-CH₃ .
→ 2° alcohol (formed by reducing 3-methylbutan-2-one).
D: pentan-3-ol
The -OH is on carbon 3: CH₃CH₂-CH(OH)-CH₂CH₃ .
→ 2° alcohol (formed by reducing pentan-3-one).

🧠 Exam Technique & Rapid Shortcut

When an AQA question asks which alcohol cannot be formed from the reduction of a carbonyl compound, immediately translate this to:

"Which of these is a tertiary (3°) alcohol?"

Nomenclature Shortcut: An alcohol is tertiary if the branch number and the principal alcohol suffix number are identical on the same main chain (e.g., 2-methylbutan-2-ol).

❌ Common Misconceptions & Errors

Overlooking the word "NOT": Students frequently miss the negative qualifier in multiple-choice stems and choose the first plausible primary/secondary alcohol they see.
Confusing Option A and Option B: Seeing multiple methyl groups in 2,2-dimethylpropan-1-ol can make students misclassify it as tertiary. Remember: look strictly at the carbon directly attached to the -OH group. In A, it is -CH₂OH , which is strictly 1°.
Confusing reduction with Grignard reactions: While tertiary alcohols can be made from ketones using Grignard reagents, standard reduction with NaBH₄ only yields 2° alcohols.

AQA A-Level Chemistry Paper 3, 2017: Question 23

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