Distinguishing Tests

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Last updated 9:05 AM on 4/21/26
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11 Terms

1
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Overall condensation reaction

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Condensation reaction with 2,4-dinitrophenylhydrazine (2,4-DNPH)

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Oxidation with acidified potassium dichromate(V) or acidified potassium manganate(VII)

RnCs: K2Cr2O7 in the presence of dilute H2SO4, heat under reflux

OR KMnO4 in the presence of dilute H2SO4, heat under reflux

Observation: Orange(Cr2O72-) solution turns green(Cr3+)

Purple(MnO4-) solution turns colourless(Mn2+)

<p>RnCs: K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> in the presence of dilute H<sub>2</sub>SO<sub>4</sub>, heat under reflux</p><p>OR KMnO<sub>4</sub> in the presence of dilute H<sub>2</sub>SO<sub>4</sub>, heat under reflux</p><p>Observation: Orange(Cr<sub>2</sub>O<sub>7</sub><sup>2-</sup>) solution turns green(Cr<sup>3+</sup>)</p><p>Purple(MnO<sub>4</sub><sup>-</sup>) solution turns colourless(Mn<sup>2+</sup>)</p>
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Why can’t ketones by oxidised with the same reagents?

  • no hydrogen attached directly to the carbonyl carbon (FYI: hydrogen needed because C-H bond is weak enough to be broken so electrons can be left behind for formation of new C-O bond)

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Oxidation with Tollens’ reagent (silver mirror test)

RnCs: Tollens’ reagent( contains [Ag(NH3)2]+ ), warm

Observation: silver mirror formed

Note: [Ag(NH3)2]+ is unstable and needs to be freshly prepared

  • one drop of NaOH(aq) added to 3cm3 of AgNO3(aq) to produce Ag2O(dark brown ppt)

  • dilue NH3(aq) added dropwise until ppt dissolves

<p>RnCs: Tollens’ reagent( contains [Ag(NH<sub>3</sub>)<sub>2</sub>]<sup>+</sup> ), warm</p><p>Observation: silver mirror formed</p><p></p><p>Note: [Ag(NH<sub>3</sub>)<sub>2</sub>]<sup>+</sup> is unstable and needs to be freshly prepared</p><ul><li><p>one drop of NaOH(aq) added to 3cm<sup>3</sup> of AgNO<sub>3</sub>(aq) to produce Ag<sub>2</sub>O(dark brown ppt)</p></li><li><p>dilue NH<sub>3</sub>(aq) added dropwise until ppt dissolves</p></li></ul><p></p>
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Oxidation with Feling’s reagent

RnCs: Fehling’s reagent(contains copper(II) tartrate), warm

Observation: reddish-brown(brick red) ppt formed

Note:

  • ONLY aliphatic adlehydes can react

Fehling’s reagent deteriorates on keeping and need to be freshly prepared:

  • mix Fehling’s solution A (CuSO4 solution) and Fehling’s solution B (sodium potassium tartrate + excess NaOH)

  • obtain a deep blue solution owing to the formation of copper(II) tartrate, the tartrate ions present prevent the precipitation of Cu(OH)2 by complexing with the copper(II) ions.

<p>RnCs: Fehling’s reagent(contains copper(II) tartrate), warm</p><p>Observation: reddish-brown(brick red) ppt formed</p><p></p><p>Note: </p><ul><li><p>ONLY aliphatic adlehydes can react</p></li></ul><p>Fehling’s reagent deteriorates on keeping and need to be freshly prepared: </p><ul><li><p>mix Fehling’s solution A (CuSO4 solution) and Fehling’s solution B (sodium potassium tartrate + excess NaOH)</p></li><li><p>obtain a deep blue solution owing to the formation of copper(II) tartrate, the tartrate ions present prevent the precipitation of Cu(OH)<sub>2</sub> by complexing with the copper(II) ions.</p></li></ul><p></p>
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What happens when benzaldehyde reacts with Fehling’s reagent due to excess NaOH?

benzaldehyde undergoes dispropotion instead of oxidation

<p>benzaldehyde undergoes dispropotion instead of oxidation </p>
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Tri-iodomethane(iodoform) Test

RnC: I2 (aq) with NaOH(aq), warm

Observation: Yellow ppt of yellow crystals of CHI3 formed

note:

  • carbonyl compound must be ethanal or methyl ketones (the group circled in red must be present in the compound)

<p>RnC: I<sub>2</sub> (aq) with NaOH(aq), warm</p><p>Observation: Yellow ppt of yellow crystals of CHI<sub>3</sub> formed</p><p>note:</p><ul><li><p>carbonyl compound must be ethanal or methyl ketones (the group circled in red must be present in the compound)</p></li></ul><p></p>
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Results of distinguishing tests when testing aldehyde

2,4-DNPH: ✅ (orange ppt)

K2Cr2O7 or KMnO4: ✅ (orange turn green/ purple turn colourless)

Tollen’s reagent: ✅ (silver mirror)

Fehling’s reagent: ✅ (reddish-brown ppt)

tri-iodomethan test: ONLY IF ETHANAL ✅ (yellow ppt)

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Results of distinguishing tests when testing benzaldehyde

2,4-DNPH: ✅ (orange ppt)

K2Cr2O7 or KMnO4: ✅ (orange turn green/ purple turn colourless)

Tollen’s reagent: ✅ (silver mirror)

Fehling’s reagent: ❌ (benzaldehyde undergoes disproportion, no reddish-brown ppt)

tri-iodomethan test: ❌ (don’t have the CO(CH3) )

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Results of distinguishing tests when testing ketone

2,4-DNPH: ✅ (orange ppt)

K2Cr2O7 or KMnO4: ❌ (no H attached to carbonyl C, remains orange/ purple)

Tollen’s reagent: ❌ (no H attached to carbonyl C, no silver mirror)

Fehling’s reagent: ❌ (no H attached to carbonyl C, no reddish-brown ppt)

tri-iodomethan test: ONLY if methyl ketone ✅ (have the CO(CH3) )