Flashcards · 9701 · AS Topic 17

Carbonyl compounds (AS Level) flashcards

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Why is the carbonyl carbon attacked by nucleophiles?: C=O is polar: oxygen is more electronegative, so the carbon carries δ+.
Reducing aldehydes and ketones: NaBH₄ (or LiAlH₄): aldehyde + 2[H] → primary alcohol; ketone + 2[H] → secondary alcohol.
Nucleophilic addition of HCN to a carbonyl (outline): HCN with a trace of KCN supplies CN⁻. CN⁻ lone pair attacks the δ+ carbon, the π bond breaks onto O; the O⁻ is protonated. Product: a hydroxynitrile with one more carbon.
What does a positive 2,4-DNPH test show?: An orange precipitate: a carbonyl group (aldehyde or ketone). It does not distinguish them; the melting point of the purified derivative identifies the compound.
Tollens' and Fehling's tests: Tollens' (ammoniacal silver nitrate): silver mirror with an aldehyde. Fehling's (alkaline copper(II)): blue → brick-red precipitate with an aldehyde. Ketones give no change.
Iodoform test for carbonyl compounds: Warm with alkaline I₂(aq): yellow precipitate of CHI₃ with any compound containing CH₃CO–, e.g. ethanal and propanone.