Flashcards · 9620 · A Level Topic 26

Aldehydes and ketones (A Level) flashcards

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Why is the carbonyl group attacked by nucleophiles?: C=O is polar (Cδ+=Oδ−) because oxygen is more electronegative, so the δ+ carbon attracts nucleophiles.
Products of reducing aldehydes and ketones with NaBH₄: Aqueous NaBH₄. Aldehyde → primary alcohol; ketone → secondary alcohol. RCHO + 2[H] → RCH₂OH
Mechanism of reduction by NaBH₄: Nucleophilic addition. Lone pair of H⁻ attacks the δ+ carbonyl carbon; the C=O electrons move to O (alkoxide, O⁻); O⁻ takes H⁺ from water to form the alcohol.
Reaction of an aldehyde with KCN followed by dilute acid: Nucleophilic addition giving a hydroxynitrile: RCHO + HCN → RCH(OH)CN. The C of :CN⁻ attacks the carbonyl carbon; the O⁻ is then protonated. The chain gains one carbon.
Which carbonyl compounds give a racemate with KCN then dilute acid?: Aldehydes (except methanal) and unsymmetrical ketones. A symmetrical ketone or methanal gives no chiral centre, so no enantiomers.
Hazards of the KCN reaction: KCN is highly toxic, and adding acid forms HCN, a highly toxic, volatile gas. Use a fume cupboard, gloves and eye protection, and add the dilute acid slowly.
Why are aldehydes oxidised but ketones not?: An aldehyde’s C=O carbon carries an H atom, so it is readily oxidised to a carboxylic acid (RCHO + [O] → RCOOH). A ketone’s C=O carbon carries two carbons and is not oxidised.