Aldehydes & Ketones (AQA A-level) Flashcards

What functional group do aldehydes and ketones contain?

The carbonyl group, C=O.

What is the difference between an aldehyde and a ketone?

Aldehyde: carbonyl at end of chain (-CHO)

Ketone: carbonyl in middle of chain (-CO-)

Why are carbonyl compounds reactive?

Because the C=O bond is polar (Cδ+), so it is attacked by nucleophiles.

What do aldehydes oxidise to?

Carboxylic acids.

What reagent is used to oxidise aldehydes?

Acidified potassium dichromate(VI), K₂Cr₂O₇ / H⁺.

What is the colour change when an aldehyde is oxidised using dichromate?

Orange → green.

What is Tollens' reagent used for?

To distinguish aldehydes from ketones.

What is the positive Tollens' test result for an aldehyde?

Silver mirror forms.

What is Fehling's solution used for?

To distinguish aldehydes from ketones.

What is the positive Fehling's test result for an aldehyde?

Blue solution → brick-red precipitate (Cu₂O).

Do ketones react with Tollens' or Fehling's reagent?

No, ketones do not react.

What reagent reduces aldehydes and ketones?

Sodium borohydride, NaBH₄.

What do aldehydes reduce to?

Primary alcohols.

What do ketones reduce to?

Secondary alcohols.

What is the nucleophile in NaBH₄ reduction?

H⁻ (hydride ion).

What type of reaction is reduction of carbonyl compounds?

Nucleophilic addition.

Why is NaBH₄ reduction called nucleophilic addition?

Because H⁻ attacks the δ+ carbon in C=O and breaks the double bond

What happens when aldehydes or ketones react with KCN then acid?

They form hydroxynitriles (cyanohydrins).

What is the nucleophile in the KCN reaction?

CN⁻ (cyanide ion).

Why is dilute acid added after KCN?

To protonate the intermediate and form the hydroxynitrile.

Why is KCN dangerous?

It releases highly toxic hydrogen cyanide (HCN).

What type of mechanism occurs with KCN and carbonyl compounds?

Nucleophilic addition.

What is the first step in nucleophilic addition to C=O?

CN⁻ attacks the δ+ carbon in the carbonyl group.

What intermediate is formed in nucleophilic addition?

A negatively charged alkoxide intermediate.

Why do aldehydes or unsymmetrical ketones form enantiomers with KCN?

Because a new chiral centre is formed at the carbonyl carbon.

What is produced when a racemic mixture forms in KCN addition?

A 50:50 mixture of enantiomers.

Why is a racemic mixture formed in KCN addition?

CN⁻ attacks equally from both sides of the planar carbonyl group.

Why is the carbonyl group planar?

Because the C=O bond is trigonal planar around the carbonyl carbon.