Flashcards · 7405 · A Level Topic 26

Aldehydes and ketones (A Level) flashcards

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Reducing agent for aldehydes and ketones, and the products: NaBH₄ in aqueous solution. Aldehyde → primary alcohol; ketone → secondary alcohol. Nucleophilic addition.
Equation for reducing propanal with NaBH₄: CH₃CH₂CHO + 2[H] → CH₃CH₂CH₂OH (propan-1-ol)
Nucleophile in NaBH₄ reduction, and the mechanism outline: H⁻. It attacks the δ+ carbonyl carbon, the C=O π electrons move to O giving an alkoxide, then O⁻ is protonated by water (or H⁺).
Product of ethanal with KCN followed by dilute acid: 2-hydroxypropanenitrile, CH₃CH(OH)CN, by nucleophilic addition of CN⁻. It forms as a racemic mixture.
Hazards of KCN, and why KCN is used rather than HCN: KCN is highly toxic and acid releases toxic, volatile HCN: use a fume cupboard and gloves. HCN is hard to handle and, as a weak acid, gives few CN⁻ ions; KCN gives plenty.
Which carbonyl compounds give a racemic mixture with KCN?: Aldehydes (except methanal) and unsymmetrical ketones. Methanal and symmetrical ketones such as propanone give no chiral centre.
Why are ketones not oxidised by Tollens’ or Fehling’s?: The C=O carbon has no H, so oxidation would need a C–C bond to break.
Mole ratios: aldehyde to silver (Tollens’) and aldehyde to Cu₂O (Fehling’s): 1 mol aldehyde → 2 mol Ag. 1 mol aldehyde → 1 mol Cu₂O.