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Fehling's Test: Identification of Aliphatic Aldehydes

By Chemca Editorial Team • Last Updated: January 2026 • 8 min read

Fehling's Test is a mild oxidizing test used to distinguish Aliphatic Aldehydes from Ketones and Aromatic Aldehydes. The reagent acts as an oxidizing agent, converting aldehydes to carboxylic acids while the copper(II) ions are reduced to a red precipitate of copper(I) oxide.

1. Preparation of Reagent

Fehling's Reagent is prepared fresh by mixing equal volumes of two solutions:

• Fehling's Solution A: Aqueous Copper Sulfate ($CuSO_4$), which is blue.
• Fehling's Solution B: Alkaline Sodium Potassium Tartrate (Rochelle Salt), which acts as a chelating agent to keep $Cu^{2+}$ in solution in basic medium.

Active Species: Deep Blue Complex $[Cu(\text{tartrate})_2]^{2-}$

2. General Reaction & Mechanism

When an aliphatic aldehyde is heated with Fehling's solution, the aldehyde is oxidized to the carboxylate anion, and the deep blue $Cu^{2+}$ complex is reduced to a Brick Red Precipitate of Cuprous Oxide ($Cu_2O$).

$$ R-CHO + 2Cu^{2+} + 5OH^- \xrightarrow{\Delta} \underbrace{R-COO^-}_{\text{Carboxylate}} + \underbrace{Cu_2O \downarrow}_{\text{Red Ppt}} + 3H_2O $$

Redox Change:

• Aldehyde: Oxidized ($C^{+1} \to C^{+3}$).
• Copper: Reduced ($Cu^{2+} \to Cu^{+}$).

3. Scope and Limitations (Exam Critical)

Fehling's solution is a weaker oxidizing agent than Tollens' reagent. This leads to crucial differences in reactivity.

Positive Test (+ve)

Brick Red Precipitate forms.

• Aliphatic Aldehydes (e.g., Acetaldehyde, Propanal).
• $\alpha$-Hydroxy Ketones (e.g., Fructose).
• Formic Acid ($HCOOH$).
• Reducing Sugars (Glucose, Maltose).

Negative Test (-ve)

Solution remains Blue.

• Aromatic Aldehydes (e.g., Benzaldehyde). (This is the key difference from Tollens!)
• Ketones (e.g., Acetone).
• Non-reducing sugars (Sucrose).

4. Distinguishing Compounds

Pair	Tollens' Test	Fehling's Test
Acetaldehyde vs Benzaldehyde	Both Positive	Acetaldehyde (+) / Benzaldehyde (-)
Acetaldehyde vs Acetone	Acetaldehyde (+)	Acetaldehyde (+)

5. Benedict's Solution

Benedict's Solution is chemically very similar to Fehling's solution. It uses Sodium Citrate instead of Rochelle salt as the chelating agent. It is more stable than Fehling's solution (can be stored as a single solution) but performs the same function: testing for aliphatic aldehydes and reducing sugars.

Fehling's Test Quiz

Test your concepts on Aldehyde differentiation. 10 MCQs with explanations.

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