Fehling’s test is mainly used to detect aliphatic aldehydes.

1. Fehling’s reagent

It consists of two solutions:

• Fehling A: $CuSO_4$ solution
• Fehling B: alkaline sodium potassium tartrate (Rochelle salt) + NaOH

They are mixed freshly before the test.

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2. Reaction with aldehyde

For an aliphatic aldehyde:$$\boxed{RCHO + 2Cu^{2+} + 5OH^- \rightarrow RCOO^- + Cu_2O\downarrow + 3H_2O}$$

Observation:$$\boxed{\text{Blue solution} \rightarrow \text{brick-red }Cu_2O\text{ precipitate}}$$

Aldehyde is oxidised → carboxylate
$Cu^{2+}$ is reduced → $Cu_2O$

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3. Which compounds give Fehling’s test?

Compound	Fehling test
Aliphatic aldehyde	✅ Positive
Formaldehyde, HCHO	✅ Positive
Acetaldehyde, CH₃CHO	✅ Positive
Aromatic aldehyde, C₆H₅CHO	❌ Generally negative
Ketones	❌ Negative
Formic acid/formates	✅ Can reduce Cu²⁺
Reducing sugars	✅ Positive

Most important distinction

$$\boxed{\text{Aliphatic aldehyde → Fehling positive}}$$$$\boxed{\text{Aromatic aldehyde → Fehling generally negative}}$$

But both can give Tollens’ test:$$\boxed{\text{RCHO → Tollens positive}}$$$$\boxed{\text{RCHO (aliphatic) → Fehling positive}}$$$$\boxed{\text{ArCHO → Fehling generally negative}}$$

JEE/NEET trap

Benzaldehyde (C6H5CHO)

• Tollens → ✅ Silver mirror
• Fehling → ❌ No brick-red precipitate

Acetaldehyde (CH3CHO)

• Tollens → ✅
• Fehling → ✅

Memory trick:
“Fehling likes aliphatic aldehydes, not aromatic aldehydes.”