Salam Academy
Sign inCreate account
Learn7405Paper 2 · Paper 2: Organic chemistryAldehydes and ketonesAldehydes and ketones
Aldehydes and ketones · Subtopic 1 of 1

Aldehydes and Ketones

Aldehydes (RCHO) and ketones (RCOR') contain the polar C=O group; the carbon is δ+ and attacked by nucleophiles. Nucleophilic addition: HCN (from KCN + acid) adds to give hydroxynitriles; NaBH₄ (source of H⁻) reduces aldehydes to primary alcohols and ketones to secondary alcohols. Tests: 2,4-DNPH gives an orange precipitate with any carbonyl. Only aldehydes reduce Tollens' reagent (silver mirror) and Fehling's (blue → brick-red), because aldehydes can be oxidised. The iodoform test (yellow CHI₃) detects CH₃CO–. Because C=O is planar, CN⁻ attacks from either side, so a chiral product forms as a racemic mixture.

Cambridge 9701 Paper 1Cambridge 9701 Paper 2Edexcel YCH11 WCH14AQA 7405 Paper 2≈ 50 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Describe nucleophilic addition of HCN and its mechanism.
  • Describe reduction with NaBH₄.
  • Use 2,4-DNPH, Tollens', Fehling's and iodoform tests.
  • Explain why nucleophilic addition can give a racemic mixture.
Pause and think

Aldehydes and ketones both contain C=O. Why can Tollens' reagent tell them apart?

Section 2 of 14

Core idea

Start hereA silver mirror in a test tube

Warm an aldehyde with Tollens' reagent and the inside of the tube becomes a mirror. A ketone does nothing. Why the difference?

The big idea

The C=O carbon is δ+ and is attacked by nucleophiles. Aldehydes also carry an H on that carbon, so they are easily oxidised; ketones are not.

1. Nucleophilic addition

Mechanism: HCN adds to ethanal
  1. CH3CH=O\mathrm{CH_3CH{=}O}
    O is more electronegative, so C is δ+ and O is δ−.
  2. :CN−→Cδ+\mathrm{{:}CN^-\rightarrow C^{\delta+}}
    The lone pair on the carbon of CN⁻ attacks the δ+ carbon.
  3. CH3CH(CN)O−\mathrm{CH_3CH(CN)O^-}
    The π electrons of C=O move onto oxygen, giving an alkoxide intermediate.
  4. CH3CH(CN)O−+H+→CH3CH(OH)CN\mathrm{CH_3CH(CN)O^-+H^+\rightarrow CH_3CH(OH)CN}
    The O⁻ takes H⁺ from HCN or water: 2-hydroxypropanenitrile.
Carbonyl tests

DNPH finds the carbonyl; Tollens' or Fehling's sorts aldehyde from ketone.

Worked exampleIdentify the carbonyl

C4H8O\mathrm{C_4H_8O} gives an orange precipitate with 2,4-DNPH, no silver mirror, and a yellow precipitate with I₂/NaOH. Identify it.

Goal
Structure.
Strategy
Use each test in turn.
  1. DNPH positive⇒C=O\text{DNPH positive}\Rightarrow\mathrm{C{=}O}
    Carbonyl present.
  2. Tollens negative⇒ketone\text{Tollens negative}\Rightarrow\text{ketone}
    No aldehyde H.
  3. iodoform positive⇒CH3CO−\text{iodoform positive}\Rightarrow\mathrm{CH_3CO{-}}
    Methyl ketone.
AnswerButanone, CH₃COCH₂CH₃.
✓ CheckC₄H₈O ✓.
Edge caseRacemic product C=O is planar, so CN⁻ attacks from either face equally. A chiral product forms as a 50 : 50 mixture of enantiomers and is optically inactive.

Summary

  • Nucleophilic addition: HCN, NaBH₄.
  • DNPH: all carbonyls.
  • Tollens', Fehling's: aldehydes only.
  • Racemic products.
Key words
Nucleophile
Electron-pair donor that attacks a δ+ atom.
Hydroxynitrile
Compound with –OH and –CN on the same carbon.
Racemic mixture
Equal amounts of two enantiomers.
HistoryTollens. Bernhard Tollens introduced his silver mirror reagent in 1882; silvering of glass for mirrors used similar chemistry.
Fun factAlmonds and cyanide. Bitter almonds contain amygdalin, which releases benzaldehyde and HCN: the reverse of nucleophilic addition.
Keep reading for free

Create a free account to read the rest of this lesson: key formulae, worked examples, explore the graph, common mistakes, exam tip, guided practice and more. Your answers are checked and your progress is saved.

Create free accountSign in