Molisch's Test: General Test for Carbohydrates | Chemca

Biomolecules

Molisch's Test: General Test for Carbohydrates

By Chemca Editorial Team • Last Updated: January 2026 • 6 min read

Molisch's Test is a sensitive general qualitative test for the presence of Carbohydrates. It detects any compound that can be dehydrated to furfural or hydroxymethylfurfural in the presence of concentrated sulfuric acid.

1. Reagents and Procedure

Reagents

Molisch's Reagent: A 10% solution of $\alpha$-Naphthol (1-naphthol) in Ethanol (or Chloroform).
Concentrated Sulfuric Acid ($H_2SO_4$): Acts as a strong dehydrating agent.

Procedure

1. Add 2-3 drops of Molisch's reagent to the carbohydrate solution.
2. Carefully pour concentrated $H_2SO_4$ down the side of the test tube to form a layer at the bottom.

Observation

A Reddish-Violet (Purple) Ring forms at the junction of the two liquid layers (acid layer and aqueous layer).

2. Principle and Mechanism

The test involves two main chemical steps: Dehydration and Condensation.

Step 1: Acid-Catalyzed Dehydration

The concentrated sulfuric acid removes water molecules from the carbohydrate.

Pentoses (5 Carbon Sugars): Dehydrate to form Furfural.
Hexoses (6 Carbon Sugars): Dehydrate to form 5-Hydroxymethylfurfural (5-HMF).

$$ \text{Hexose } (C_6H_{12}O_6) \xrightarrow{\text{Conc. } H_2SO_4, -3H_2O} \text{5-Hydroxymethylfurfural} $$

Step 2: Condensation with $\alpha$-Naphthol

The aldehyde group of Furfural or HMF undergoes an electrophilic substitution-condensation reaction with two molecules of $\alpha$-Naphthol. This forms a purple-colored condensation product (a triphenylmethane dye derivative).

$$ \text{Furfural} + 2 \times \alpha\text{-Naphthol} \xrightarrow{H^+} \text{Purple Complex} + H_2O $$

3. Scope and Limitations

Positive: All carbohydrates (Monosaccharides, Disaccharides, Polysaccharides) give a positive test. Polysaccharides and disaccharides are first hydrolyzed by the acid to monosaccharides, which then dehydrate.
Negative: Simple organic acids, alcohols, and pure proteins do not give this test.
Note: Some glycoproteins and nucleic acids may give a positive test due to their carbohydrate content.

4. Distinguishing Sugars

While Molisch's test confirms the presence of carbohydrates, it does not distinguish between them. Other tests are needed for specificity:

Test	Function
Molisch's Test	General test for All Carbohydrates
Benedict's/Fehling's	Detects Reducing Sugars
Seliwanoff's Test	Distinguishes Ketoses (Fructose) from Aldoses
Iodine Test	Specific for Starch (Blue-Black)

Molisch Test Quiz

Test your concepts on Carbohydrate detection. 10 MCQs with explanations.

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