AQA A-Level Chemistry Paper 3, 2024: Question 24

Difficulty: Hard

One-mark MCQ, but it tests whether you can recognise the type of mechanism from a transformation.

AQA A-level Chemistry • Organic mechanisms

Nucleophile attack: spotting the correct conversion

What this question tests

Recognising when a reaction involves nucleophilic attack on an electrophilic carbon (typically C=O).
Distinguishing addition, substitution and elimination transformations from given reactants/products.
Knowing that cyanide, CN⁻ is a nucleophile that attacks aldehydes/ketones to form hydroxynitriles.

Exam part (a): In which conversion does a nucleophile attack the organic reactant? (1 mark)

CH₃CH₂CH₃ → CH₃CHClCH₃

Looks like halogenation/substitution to make a chloroalkane (not nucleophilic attack on a carbonyl).

CH₃CH=CH₂ → CH₃CHBrCH₃

Electrophilic addition of HBr to an alkene (the alkene attacks H⁺ first).

CH₃CH₂CH₂OH → CH₃CH=CH₂

Elimination (dehydration) of an alcohol to an alkene.

CH₃CH₂CHO → CH₃CH₂CH(OH)CN

Formation of a hydroxynitrile: CN⁻ nucleophile attacks the δ⁺ carbonyl carbon.

✅ Correct answer (with mark allocation)

Mark scheme: D (1 mark, AO1)

D is correct: CH₃CH₂CHO → CH₃CH₂CH(OH)CN

💡 Key knowledge

Nucleophile = electron pair donor (e.g. CN⁻, OH⁻, NH₃).
Aldehydes/ketones have a polar C=O: carbonyl carbon is δ⁺ (electrophilic).
Cyanohydrin (hydroxynitrile) formation: CN⁻ adds to the carbonyl carbon, then the oxygen is protonated to give an –OH.

🧠 Exam technique: how to spot it fast

Scan for products containing CN and OH on the same carbon: that screams nucleophilic addition to C=O.
If the reactant has C=O and the product no longer has C=O, think addition (often nucleophilic addition).
Alkenes + HX (HBr, HCl) are usually electrophilic addition (not “nucleophile attacks organic reactant”).

❌ Common errors (why students lose this mark)

Picking B because “Br⁻ is a nucleophile” — but in HBr addition to an alkene, the key first step is the alkene acting as a nucleophile attacking H⁺. The question asks where a nucleophile attacks the organic reactant, which matches carbonyl chemistry (D) more cleanly.
Picking C
Not recognising hydroxynitriles: product has both –OH and –CN added across the carbonyl.

Mechanism focus (what you’d draw if asked)

If the exam asked for the mechanism for option D (hydroxynitrile formation), you should draw:

Nucleophilic attack: curly arrow from the lone pair on :CN⁻ to the carbonyl carbon of CH₃CH₂CHO .
Curly arrow from the C=O π bond to the oxygen to form an alkoxide (O⁻).
Protonation: curly arrow from O⁻ to H of HCN (or other proton source), forming –OH and regenerating CN⁻ (depending on conditions shown).

Examiner insight: top responses quickly identify the functional group change (aldehyde → hydroxynitrile) and link it to CN⁻ nucleophilic addition, rather than thinking about individual ions in other options.

Why the other options are not “nucleophile attacks the organic reactant”

A: CH₃CH₂CH₃ → CH₃CHClCH₃

Overall looks like substitution to introduce Cl.
But propane is very unreactive; chlorination typically occurs via free-radical substitution (UV light), not nucleophilic attack.

B: CH₃CH=CH₂ → CH₃CHBrCH₃

This is electrophilic addition of HBr to an alkene.
Mechanism starts with the alkene π bond attacking H⁺ (the alkene is the nucleophile), rather than a nucleophile attacking the organic reactant.

C: CH₃CH₂CH₂OH → CH₃CH=CH₂

This is elimination (dehydration) of an alcohol (often using conc. H₂SO₄ or Al₂O₃).
Key idea is removal of H and OH (or H₂O), not nucleophilic attack.

D: CH₃CH₂CHO → CH₃CH₂CH(OH)CN

This is nucleophilic addition to a carbonyl.
CN⁻ attacks the electrophilic carbonyl carbon; oxygen is then protonated to give the alcohol.

Mark scheme link (what earns the mark)

✅ One mark means… be decisive

For a 1-mark multiple choice, you get full credit only by selecting the correct option:

AQA mark scheme: D only.