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CBSE Class 12-science Answered

What are the functional groups that give tollen's test and fehling's test respectively ?
Asked by gouroy345 | 19 Dec, 2019, 07:13: PM
Expert Answer
Tests to distinguish between aldehydes and ketones:
 
1. Tollens’ test:
 
When an aldehyde is heated with Tollens’ reagent, it forms a silver mirror. Tollens’ reagent is an ammoniacal solution of AgNO3.
 
RCHO + 2[Ag(NH3)2]+ + 3OH- → RCOO- + 2Ag + 2H2O + 4NH3
 
Ketones do not form a silver mirror.
 
 
2. Fehling’s test: When an aldehyde is heated with Fehling’s reagent, it forms a reddish-brown precipitate of cuprous oxide. Fehling’s reagent: Fehling solution A (aqueous solution of CuSO4) + Fehling solution B (alkaline solution of sodium potassium tartrate) 2 Red brown ppt.
 
RCHO + 2Cu2+ + 5HO- → RCOO-  +  Cu2O + 3H2O
 
Ketones do not form a reddish-brown precipitate of cuprous oxide.
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