Question
Which of the following compounds gives a positive Tollens test but a negative Fehling test?
(A) Glucose (B) Benzaldehyde (C) Acetaldehyde (D) Formaldehyde
Answer: (B) Benzaldehyde
Solution — Step by Step
Tollens reagent (ammoniacal silver nitrate) oxidises any aldehyde — aliphatic or aromatic. Fehling solution (alkaline CuSO₄ + sodium potassium tartrate) only oxidises aliphatic aldehydes (and reducing sugars). Aromatic aldehydes are too weak as reducing agents to reduce Cu²⁺ under Fehling’s mild conditions.
All four — glucose, benzaldehyde, acetaldehyde, formaldehyde — give a positive Tollens test (silver mirror). This eliminates nothing yet. We need the Fehling condition to distinguish.
- Glucose → positive (it’s a reducing sugar with an aliphatic aldehyde group)
- Acetaldehyde → positive (aliphatic aldehyde, reduces Cu²⁺ → brick-red Cu₂O precipitate)
- Formaldehyde → positive (aliphatic, in fact the strongest reducing aldehyde)
- Benzaldehyde → negative (aromatic aldehyde, conjugation with benzene ring lowers reactivity toward Cu²⁺)
Only benzaldehyde satisfies both conditions: positive Tollens, negative Fehling. Option (B).
Why This Works
The key is understanding why Fehling’s fails with aromatic aldehydes. In benzaldehyde, the carbonyl carbon is directly attached to the benzene ring. The ring’s π-electron system donates electron density into the carbonyl through resonance, making the carbonyl less electrophilic and the aldehyde less easily oxidised.
Tollens reagent uses Ag⁺ ions in an alkaline medium — a much stronger oxidising environment. Even benzaldehyde can reduce Ag⁺ here. Fehling’s Cu²⁺ in tartrate buffer is milder, so it only works where the aldehyde is reactive enough (aliphatic ones).
Think of it as a selectivity ladder: Fehling’s is choosy, Tollens is not. This is why Fehling’s is used industrially to test for glucose (aliphatic) in urine without interference from aromatic compounds.
Alternative Method
Table-based elimination (useful in MCQs when time is tight):
| Compound | Tollens | Fehling |
|---|---|---|
| Glucose | ✓ | ✓ |
| Benzaldehyde | ✓ | ✗ |
| Acetaldehyde | ✓ | ✓ |
| Formaldehyde | ✓ | ✓ |
Build this table in your head in 30 seconds — anything aliphatic + aldehyde passes both. Aromatics pass only Tollens. Ketones fail both (except hydroxy ketones like fructose, which is a special case for boards).
For NEET and JEE Main MCQs, the phrase “silver mirror but no red precipitate” is the examiner’s coded way of saying aromatic aldehyde. Recognise the pattern and you will mark (B) without calculating anything.
Common Mistake
Students often mark formaldehyde as the answer thinking it’s “different” because it has no α-hydrogen or some other property they half-remember. Formaldehyde is the most reactive aliphatic aldehyde — it gives a roaring positive with both tests. The aromatic vs aliphatic distinction is the only thing that matters here, not carbon chain length or presence of α-H.
A related trap: fructose is a ketone sugar but gives a positive Fehling test because it’s a hydroxy ketone that tautomerises to an aldehyde form in alkaline conditions. If the question specifically asks about ketones + Fehling, fructose is the one exception worth memorising — it appears in NEET pattern questions frequently.