Aldehydes and ketones: study guide

Key ideas

Aldehydes and ketones share the carbonyl group C=O, whose carbon is attacked by nucleophiles. Aldehydes have a hydrogen on the carbonyl carbon, which makes them easy to oxidize.

• Tollens’ test (silver mirror) and Fehling’s test are positive for aldehydes and negative for ketones, because only aldehydes are easily oxidized.
• The iodoform test is positive for methyl ketones R–CO–CH₃ and for CH₃CH(OH)–R, as well as for ethanal.
• Nucleophilic addition: HCN gives a cyanohydrin, and a Grignard reagent adds R′ to give an alcohol, secondary from an aldehyde and tertiary from a ketone.
• Reduction with NaBH₄ or LiAlH₄ gives a primary alcohol from an aldehyde and a secondary alcohol from a ketone. Aldol reactions need an α-hydrogen.

Key equations

• RCHO + [Ag(NH₃)₂]⁺ → RCOO⁻ + Ag
• R₂C=O + HCN → R₂C(OH)CN
• RCHO + R′MgX → 2° alcohol
• R₂C=O + R′MgX → 3° alcohol

Worked example

Propanone + CH₃MgBr, then H₃O⁺, gives 2-methylpropan-2-ol, a tertiary alcohol. To tell ethanal from propanone, Tollens’ reagent gives a silver mirror with ethanal only. To tell propanone from propanal, the iodoform test is positive for propanone only.

Common mistakes

• Expecting a ketone to give a silver mirror with Tollens’ reagent.
• Giving a primary alcohol as the product of a Grignard reaction with a ketone. A ketone gives a tertiary alcohol.
• Forgetting that an aldol reaction needs a hydrogen on the carbon next to the carbonyl.

Practice

Test yourself with practice questions on this topic: Aldehydes and ketones