Redox reactions / Tests for carbonyl groups

Tollens reagent

• Aldehydes give positive test (silver mirror)

• Ketones don’t react (can’t be oxidised)

• RCHO + [O] → RCOOH

• R = carbon chain

  • CHO = aldehyde group

  • O = oxidising agent

  • COOH = carboxylic acid group

• Tollens reagent contains Ag⁺(aq). This gets reduced by the aldehyde

  • Ag⁺(aq) + e^- → Ag(s)

Fehlings solution

• Aldehydes give positive test (brick red precipitate)

• Ketones don’t react (can’t be oxidised)

• RCHO + [O] → RCOOH

• Fehlings solution contains Cu²⁺(aq) which is reduced to Cu⁺

  • Cu²⁺(aq) + e^- → Cu⁺(s)

    Blue Brick red precipitate

Acidified dichromate (H⁺/Cr₂O₇)

• Aldehydes give positive test (dichromate turns orange → green)

• Ketones don’t react (can’t be oxidised)

• RCHO + [O] → RCOOH

• Cr₂O₇ + 14H⁺ + 6e^- → 2Cr³⁺ + 7H₂O