Fehling S Test Fehling S Reaction — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
Fehling's Test (Fehling's Reaction) Fehling's Test is a chemical test used to differentiate between water-soluble carbohydrates and a ketone functional group, and also to detect reducing sugars. It specifically identifies aldehydes and alpha-hydroxy ketones by their ability to reduce copper(II) ions in an alkaline solution to insoluble red copper(I) oxide. It involves heating the analyte with Fehling's solution (a mixture of Fehling's A, copper(II) sulfate, and Fehling's B, an alkaline solution of potassium sodium tartrate). Initially, the Fehling's solution is a clear, deep blue liquid. Upon heating with an aldehyde or reducing sugar, the blue color disappears, and a brick-red precipitate of copper(I) oxide (Cu2O) is formed. Exothermic and spontaneous under heating conditions, as indicated by the formation of a stable precipitate and change in oxidation states. In the alkaline medium, the aldehyde is deprotonated by hydroxide ions, forming an enolate or an aldehyde hydrate which can then be deprotonated. The aldehyde (or its enolate/hydrate) is oxidized to a carboxylic acid (which exists as a carboxylate ion in alkaline solution). This involves the removal of hydrogen atoms from the carbonyl carbon and the addition of oxygen. Simultaneously, the deep blue copper(II) ions (complexed with tartrate) are reduced to copper(I) ions. The copper(I) ions then precipitate out of solution as red copper(I) oxide (Cu2O). Ketones generally do NOT give a positive Fehling's test, except for alpha-hydroxy ketones (like fructose), which isomerize to aldehydes in alkaline conditions (enolization). Aromatic aldehydes (e.g., benzaldehyde) react very slowly or not at all under Fehling's test conditions due to the electron-withdrawing effect of the benzene ring, making the carbonyl carbon less susceptible to oxidation. Overheating or prolonged heating can lead to decomposition of the reagent or false positives with some compounds. Ensure proper pH: the test requires strongly alkaline conditions (provided by Fehling's B). Confusing Fehling's test with Tollen's test; both detect aldehydes, but Tollen's uses silver ions and produces a 'silver mirror'.