Fehling's Solution

Fehling's solution is another mild oxidising agent used to distinguish aldehydes from ketones. It is a blue alkaline solution containing a complex of copper(II) ions, Cu²⁺.The test: add Fehling's solution to the sample and warm it in a hot water bath.• Aldehyde: the blue solution forms a brick-red precipitate.
• Ketone: no change; the solution stays blue.What happens: the aldehyde is oxidised to a carboxylic acid (present as the carboxylate ion in the alkaline solution). The copper(II) ions are reduced to copper(I), which precipitates as copper(I) oxide, Cu₂O, the brick-red solid.Cu²⁺ + e⁻ → Cu⁺
RCHO + [O] → RCOOHKetones are not oxidised by mild oxidising agents, so the copper(II) ions are not reduced and the solution stays blue.Fehling's solution and Tollens' reagent do the same job: in both, the aldehyde is the reducing agent and a metal ion is reduced (Ag⁺ to Ag, or Cu²⁺ to Cu⁺).Worked example: find the oxidation state of copper in the precipitate, Cu₂O. Oxygen is −2, so the two copper atoms total +2 and each copper is +1. Copper has gone from +2 in the blue solution to +1 in the precipitate, so it has been reduced.