9701 · 17.1
Aldehydes and ketones flashcards
Revision flashcards for Cambridge 9701 Aldehydes and ketones (syllabus 17.1). Flip, recall, then mark a real past-paper question.
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What is a carbonyl group?
A functional group consisting of a carbon atom double-bonded to an oxygen atom (C=O).
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What is the general formula for an aldehyde?
RCHO, where the carbonyl group is at the end of a carbon chain. R can be an alkyl group or H.
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What is the general formula for a ketone?
RCOR', where the carbonyl group is within a carbon chain. R and R' are alkyl groups.
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Why is the carbonyl group polar?
Oxygen is more electronegative than carbon, so it withdraws electron density from the C=O double bond. This creates a partial negative charge on the oxygen ($\delta-$) and a partial positive charge on the carbon ($\delta+$).
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What is Tollens' reagent and what is a positive test?
A solution of aqueous silver nitrate in ammonia, [Ag(NH₃)₂]⁺. A positive test with an aldehyde produces a 'silver mirror' as Ag⁺ ions are reduced to solid silver (Ag).
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What is Fehling's solution and what is a positive test?
An alkaline solution containing copper(II) ions complexed with tartrate ions. A positive test with an aldehyde, upon warming, produces a brick-red precipitate of copper(I) oxide (Cu₂O).
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Why are ketones resistant to oxidation by mild oxidising agents?
Oxidation of a ketone would require breaking a strong C-C bond, which requires a large amount of energy. Aldehydes have a C-H bond on the carbonyl carbon, which is much easier to break.
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What is the nucleophile in the reaction of a carbonyl with aqueous KCN followed by acid?
The cyanide ion, :CN⁻.
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What type of reaction mechanism occurs between a ketone and HCN?
Nucleophilic addition. The nucleophile (:CN⁻) attacks the electrophilic carbonyl carbon.
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What is the product when propanal reacts with NaBH₄?
Propan-1-ol, a primary alcohol. Aldehydes are reduced to primary alcohols.
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What is the product when propanone reacts with NaBH₄?
Propan-2-ol, a secondary alcohol. Ketones are reduced to secondary alcohols.
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What is the nucleophile provided by NaBH₄?
The hydride ion, :H⁻.