Molisch's test is a sensitive chemical test, named after Austrian botanist

Hans Molisch Hans Molisch (6 December 1856, Brünn, Habsburg Moravia - 8 December 1937, Wien, Federal State of Austria, Austria) was a Czech people, Czech-Austrians, Austrian botanist. Molisch's test is named after him, it is a sensitive chemical test for th ...

, for the presence of carbohydrates, based on the dehydration of the carbohydrate by sulfuric acid or hydrochloric acid to produce an aldehyde, which condenses with two molecules of a phenol (usually

α-naphthol 1-Naphthol, or α-naphthol, is a fluorescent organic compound with the formula . It is a white solid. It is an isomer of 2-naphthol differing by the location of the hydroxyl group on the naphthalene ring. The naphthols are naphthalene homologues ...

, though other phenols such as resorcinol and thymol also give colored products), resulting in a violet ring.

Procedure

The test solution is combined with a small amount of Molisch's reagent (

dissolved in ethanol) in a test tube. After mixing, a small amount of concentrated

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...

is slowly added down the sides of the sloping test-tube, without mixing, to form a layer. A positive reaction is indicated by appearance of a purple red ring at the interface between the acid and test layers.

Reaction

All carbohydrates –

monosaccharide Monosaccharides (from Greek ''monos'': single, '' sacchar'': sugar), also called simple sugars, are the simplest forms of sugar and the most basic units (monomers) from which all carbohydrates are built. They are usually colorless, water-solub ...

s, disaccharides, and

polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wa ...

s (except trioses and tetroses)– should give a positive reaction, and

nucleic acid Nucleic acids are biopolymers, macromolecules, essential to all known forms of life. They are composed of nucleotides, which are the monomers made of three components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main cl ...

s and

glycoprotein Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycos ...

s also give a positive reaction, as all these compounds are eventually hydrolyzed to monosaccharides by strong mineral acids. Pentoses are then dehydrated to

furfural Furfural is an organic compound with the formula C4H3OCHO. It is a colorless liquid, although commercial samples are often brown. It has an aldehyde group attached to the 2-position of furan. It is a product of the dehydration of sugars, as occurs ...

, while hexoses are dehydrated to 5-

hydroxymethylfurfural Hydroxymethylfurfural (HMF), also 5-(hydroxymethyl)furfural, is an organic compound formed by the dehydration of reducing sugars. It is a white low-melting solid (although commercial samples are often yellow) which is highly soluble in both water ...

. Either of these aldehydes, if present, will condense with two molecules of

to form a purple-colored product, as illustrated below by the example of glucose: :

References

{{Analytical reagents Biochemistry detection methods

Procedure

Reaction

See also

References