ChemistryMediumClass 12

Tollens' & Fehling's Test — Distinguishing

Aldehydes, Ketones & Carboxylic Acids

JEE Qs

Hard

min

Master the specific reactivity of each reagent, focusing on exceptions like alpha-hydroxy ketones and formic acid, and the key difference in aromatic aldehyde reactivity.

🧮 Key Formulas

Tollens' Test: R-CHO + 2[Ag(NH3)2]+ + 3OH- → R-COO- + 2Ag(s) + 4NH3 + 2H2O

Fehling's Test: R-CHO + 2Cu^2+ + 5OH- → R-COO- + Cu2O(s) + 3H2O

✅ Key Points for JEE

1Both Tollens' and Fehling's tests are redox reactions used to distinguish aldehydes from most ketones by the oxidation of the aldehyde group.
2Tollens' reagent (ammoniacal silver nitrate) gives a 'silver mirror' (metallic Ag) or black precipitate with aliphatic and aromatic aldehydes, as well as alpha-hydroxy ketones and formic acid.
3Fehling's reagent (alkaline copper(II) tartrate complex) produces a brick-red precipitate of cuprous oxide (Cu2O) with aliphatic aldehydes, alpha-hydroxy ketones, and formic acid, but generally fails for aromatic aldehydes.
4Alpha-hydroxy ketones give positive tests with both reagents due to tautomerization to an aldehyde under basic conditions (enolization).
5All monosaccharides and most disaccharides (e.g., maltose, lactose) are 'reducing sugars' because they contain an aldehyde or a potential aldehyde group (hemiacetal) and thus give positive Tollens' and Fehling's tests.

⚠️ Common Mistakes

✕Assuming all ketones are non-reactive; forgetting that alpha-hydroxy ketones give a positive test due to tautomerism.
✕Incorrectly stating that Fehling's test works for aromatic aldehydes; it typically fails for them.
✕Not recognizing formic acid as an aldehyde functionality that gives positive tests with both reagents.
✕Confusing the products of the tests (e.g., reporting Ag2O instead of Ag for Tollens' or CuO instead of Cu2O for Fehling's).

NCERT Chapters

Class 12 Chemistry Part 2 Ch 12: Aldehydes, Ketones and Carboxylic Acids
Class 12 Chemistry Part 2 Ch 14: Biomolecules (for reducing sugars)