AQA A-Level Chemistry Paper 3, 2024: Question 27

AQA A-Level Chemistry • Exam Quick-Guide Q27

Cyanohydrin formation: naming the nitrile product

What this question tests

Skills & knowledge assessed

Recognising reaction of a ketone with acidified KCN → cyanohydrin formation.
Working out the new functional groups: –OH and –C≡N on the former carbonyl carbon.
Correct IUPAC naming: nitrile as the principal functional group; correct numbering and substituents.

What top answers do

Choose the longest carbon chain including the nitrile carbon.
Number from the nitrile carbon (C1) and place –OH as hydroxy- substituent.
Spot that cyanohydrin formation adds one carbon (the nitrile carbon).

Marking focus (from mark scheme): 1 mark (AO3). Correct option only: A = 2-hydroxy-2,3-dimethylbutanenitrile .

Part (a): Identify the product when 3-methylbutan-2-one reacts with acidified KCN (1 mark)

Multiple choice: A / B / C / D

✅ Correct answer (1/1)

A — 2-hydroxy-2,3-dimethylbutanenitrile

This matches the mark scheme exactly. Any other option scores 0 because it’s a single-mark MCQ.

💡 Key knowledge: what acidified KCN does

Acidified KCN generates some HCN and CN⁻ .
CN⁻ nucleophilically adds to the electrophilic carbonyl carbon of a ketone.
The C=O becomes C–OH after protonation → a cyanohydrin.
Net change: add –CN and –OH across the C=O.

🧠 Exam technique: naming checklist

Make nitrile the suffix (…nitrile). It has highest naming priority here.
Include the nitrile carbon in the parent chain and number it as C1.
Any –OH becomes hydroxy- (not “-ol” because nitrile is the suffix).
Identify substituents and positions: methyl groups and hydroxy position.

❌ Common errors (why students lose this mark)

Forgetting the extra carbon from –CN (nitrile carbon counts in the main chain).
Choosing a pentanenitrile parent chain (options C/D) by incorrectly extending the chain.
Putting the –OH on the wrong carbon (confusing 2-hydroxy vs 3-hydroxy).
Naming it as an alcohol suffix (…-ol) instead of hydroxy-…nitrile.

How to see why option A is correct (structure → name)

Start from the reactant

3-methylbutan-2-one

A 4-carbon ketone with a methyl branch at C3 and the carbonyl at C2.

📐 Step-by-step reasoning (no calculation, but “steps” to lock the mark)

Reaction site: the carbonyl carbon (C=O) is where CN⁻ adds.
Functional groups formed: the carbonyl carbon now has –OH and –C≡N .
Choose parent chain: must include the nitrile carbon as C1 → gives a 4-carbon chain total → butanenitrile .
Numbering: C1 is nitrile carbon; the former carbonyl carbon is now C2 → the –OH is on C2 → 2-hydroxy .
Substituents: methyl groups are on C2 (added because C2 now has an extra carbon attached) and C3 → 2,3-dimethyl .

🧠 Examiner-style naming tip

When you form a cyanohydrin, the carbonyl carbon becomes the carbon bearing –OH. If you number from the nitrile carbon (always C1), the hydroxy position is usually C2 for simple aldehydes/ketones.

What distinguishes full-mark responses: consistent application of “nitrile carbon is C1” and correct parent chain length.

❌ Quick trap check (options C/D)

Pentanenitrile would mean a 5-carbon parent chain including the nitrile carbon.
This reaction starts from a 4-carbon ketone; cyanohydrin formation adds the nitrile carbon but does not create a 5-carbon continuous chain here.
So options C and D come from a wrong parent-chain choice.

Mark scheme (exact): Q27 = A (AO3), product is 2-hydroxy-2,3-dimethylbutanenitrile .