FEHLING’S TEST CHEMICAL TEST :
Purpose : Reagent and condition: Used to differentiate aliphatic aldehydes from aromatic aldehydes and ketones. Cu2+ , −OH Observation: Brick-red precipitate formed. Fehling’s solution contains (II) ion in an aqueous solution of sodium tartarate.
Fehling’s solution oxidises aliphatic aldehydes to carboxylic acids while at the same time the Cu2+ is reduced to Cu+ , which forms a brick-red precipitate. Fehling’s solution has the characteristic blue colour of Cu 2+ which fades as the brick red of Cu2O forms. Since Fehling’s solution is basic, the carboxylic acids are formed as their conjugate bases (carboxylate ion). Ketones do not react with these reagent since they cannot be oxidized under mild conditions. Aromatic aldehydes also give negative results with these reagent.
R C O + | H 2Cu2+ + −OH | | | R C O + | O− | | | Cu2O Brick-red precipitate formed. General equation : Observation: Aliphatic aldehyde
EXERCISE: Suggest a chemical test to differentiate benzaldehyde from cyclohexanecarbaldehyde. Give the observation and write the reaction equations involved. C H O C H O benzaldehyde cyclohexanecarbaldehyde
Fehling’s Test C H O cyclohexanecarbaldehyde ANSWER : + 2Cu2+ + −OH C O O - + Cu2O Brick red precipitate formed. C H O + 2Cu2+ + −OH No Reaction No brick red precipitate formed. benzaldehyde