Question
Explain the Fehling's test to distinguish aldehydes from ketones with equation.
✓
Answer
→ Fehling's test: Fehling reagent comprises of two solutions, Fehling solution A and Fehling solution B. Fehling solution A is aqueous copper sulphate and Fehling solution B is alkaline sodium potassium tartrate (Rochelle salt). These two solutions are mixed in equal amounts before test. On heating an aldehyde with Fehling's reagent, a reddish brown precipitate is obtained. Aldehydes are oxidised to corresponding carboxylate anion. Aromatic aldehydes do not respond to this test.
$R - CHO +2 Cu ^{2+}+5 OH ^{-} \rightarrow \underset{\text { Red-brown ppt }}{ RCOO ^{-}+ Cu _2 O \downarrow}+3 H _2 O \\$
$R - CHO +2 Cu ^{2+}+5 OH ^{-} \rightarrow \underset{\text { Red-brown ppt }}{ RCOO ^{-}+ Cu _2 O \downarrow}+3 H _2 O \\$
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