Molisch's test

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Molisch's Test is a chemical test for the presence of carbohydrates, based on the dehydration of the carbohydrate by sulfuric acid to produce an aldehyde.

The test solution is combined with a small amount of Molisch's reagent (α-naphthol dissolved in ethanol) in a test tube. After mixing, a small amount of concentrated sulfuric acid is slowly added down the sides of the sloping test-tube, without mixing. A positive reaction is indicated by a purple ring forming at the interface between the acid and test layers.

All carbohydrates -- monosaccharides, disaccharides, and polysaccharides -- should give a positive reaction. Glycoproteins also give a positive reaction. Pentoses are dehydrated to furfural; hexoses are dehydrated to hydroxymethylfurfural. Either of these aldehydes, if present, will condense with two molecules of naphthol to form a purple-colored product. >O₂ (furfural) + Reactions:

1. C₅H₁₀O₅ (pentose) + (conc.) H₂SO₄ → C₅H₄O₂ + 3 H₂O

2. C₅H₄ + 2 C₁₀H₈OH (α-naphthol) → colored product