Fehling’s + Tollen’s Reagent

Aldehydes and ketones can be distinguished based on their ease of oxidation using Tollens' reagent and Fehling's reagent.

Aldehydes are easily oxidised to carboxylic acids, while ketones resist oxidation under mild conditions.

Tollens’ Reagent Test

  • Reagent: Tollens’ reagent contains ammoniacal silver nitrate ([Ag(NH3)2]+)

  • Reaction: Aldehydes reduce Tollens’ reagent, causing silver ions (Ag+) to be reduced to metallic silver (Ag), which forms a silver mirror on the test tube.

  • Observation:

    • Aldehyde: Forms a shiny silver mirror.

    • Ketone: No visible change.

Fehling’s Test

  • Reagent: Fehling’s solution consists of Fehling’s A (copper(II) sulfate, CuSO4) and Fehling’s B (alkaline sodium potassium tartrate).

  • Reaction: Aldehydes reduce Cu²⁺ ions to Cu₂O (copper(I) oxide), which precipitates as a brick-red solid.

  • Observation:

    • Aldehyde: Produces a brick-red precipitate.

    • Ketone: No visible change.

Key Differences

Test

Aldehyde Reaction

Ketone Reaction

Tollens’ Reagent

Silver mirror forms

No reaction

Fehling’s Solution

Brick-red precipitate

No reaction

Why don’t ketones react with Tollens’ reagent or Fehling’s reagent

Ketones do not react with Tollens’ reagent or Fehling’s reagent because they lack a readily oxidizable hydrogen atom attached to the carbonyl group.

Key Reasons:

  1. Absence of a Hydrogen Atom on the Carbonyl Carbon

    • Aldehydes have a hydrogen atom attached to the carbonyl group (–CHO), making them more susceptible to oxidation.

    • Ketones have two alkyl (R) groups attached to the carbonyl carbon (–COR₂), which makes oxidation more difficult.

  2. Difficulty in Breaking the C–C Bond

    • Oxidation of aldehydes converts the –CHO group into a carboxyl group (–COOH) without breaking any carbon-carbon bonds.

    • Oxidation of ketones would require breaking a C–C bond, which is energetically unfavourable under mild conditions.

  3. Stronger Oxidizing Agents Required for Ketone Oxidation

    • Tollens’ and Fehling’s reagents are mild oxidizing agents and are insufficient to break C–C bonds in ketones.

    • Stronger oxidizing agents like acidified potassium dichromate (K2Cr2O7) or potassium permanganate (KMnO4) under harsh conditions can oxidize ketones, but they do so by breaking the molecule into smaller carboxylic acids.

Brief Explanation:

Ketones do not react with Tollens’ or Fehling’s reagents because they lack an easily oxidizable hydrogen on the carbonyl carbon, and breaking a C–C bond requires much stronger oxidation conditions.