Fehling's and Benedict's Solutions

Fehling's solution and Benedict's solution are both blue, alkaline solutions containing copper(II) ions, Cu²⁺. In Fehling's solution the Cu²⁺ ions are held in solution as a complex with tartrate ions; in Benedict's solution they are complexed with citrate ions. Both are used to tell aldehydes apart from ketones.To do the test, add a few drops of the carbonyl compound to the reagent and warm in a hot water bath.Aldehyde: the blue solution gives a brick-red precipitate of copper(I) oxide, Cu₂O.
Ketone: no change; the solution stays blue.The aldehyde is a reducing agent. It is oxidised to a carboxylic acid, which in the alkaline solution is present as the carboxylate ion. The Cu²⁺ ions are reduced to Cu⁺:Cu²⁺ + e⁻ → Cu⁺Overall, for an aldehyde RCHO:RCHO + 2Cu²⁺ + 5OH⁻ → RCOO⁻ + Cu₂O + 3H₂OWorked example: ethanal, CH₃CHO, is warmed with Fehling's solution. A brick-red precipitate forms and ethanal is oxidised to the ethanoate ion, CH₃COO⁻. Propanone, CH₃COCH₃, gives no change, because a ketone has no hydrogen atom on the carbonyl carbon and is not easily oxidised.Aromatic aldehydes such as benzaldehyde give a negative (or very weak) result with Fehling's solution, so Tollens' reagent is the more reliable test for them.