Molisch reaction

IV. Molisch Reaction.—The test is the same as that applied to carbohydrates (386), and when positive in the case of a protein shows the presence in the latter of a carbohydrate. The test is applied by adding concentrated sulphuric acid, to a solution of a protein containing a few drops of an alcoholic solution of a-naph-thol. A violet color is produced, which turns yellow on the addition of alcohol, ether, or sodium hydroxide. If thymol is used instead of a-naphthol, a carmine-red color is produced, which changes to green on the addition of water. 

Reagents x, DiUUe solution of copper, sulphate. 2, Cold nitric acid. 3, saturated solution of sodium chloride + nitric acid. 4, One volume of saturated solution of sodium chloride acidified with acetic acid. 5, Saturation with sodium chloride and acidulation with nitric acid. 6, Neutral solution, saturated sodium chloride. 7, Boiling of the liquid with lead acetate in an alkaline medium. 8, Molische reaction. 

The mechanism of the Molisch reaction, the very sensitive colour reaction between aldoses and 1-naphthol in concentrated sulphuric acid, has been elucidated following the isolation of the crystalline... 

Dehydration is carried out with sulfuric acid, hydrochloric acid, or phosphoric acid. Among phenols, phenol, thymol, guaiacol, resorcinol, orcinol, catechol, phloroglucinol, a-naphthol, and naphthoresorcinol have been proposed. Among amines, aniline, p-anisidine, benzidine, 3,3 -di-methoxybenzidine, diphenylamine, indole and its derivatives, carbazole, N-ethylcarbazole, a,a-dinaphthylamine, and 1,2,7,8-dibenzocarbazole have been found valuable. In its classical form (Molisch reaction) the test for saccharides is carried out With 1-naphthol and cone, sulfuric acid (70). It is supposed that under the conditions of the reaction corresponding triphenylmethane dyes are formed, although the course of the reaction might be more complicated (71). 

They next attempted to test for heparin in liver extracts of proved activity, (those used were called Pernaemon and Sykoton). A positive Molisch reaction, in the absence of glycoprotein, polysaccharides and monoses indicated the probable presence of heparin. Their attempt to isolate heparin in crystalline form from Pernaemon proved unsuccessful. 

Mix a few drops of a glucose solution with a few drops of an alcoholic solution of n-naphthol and poui slowly down the side of the test-tube a few drops of cone, sulphunc acid. A violet colouration is produced. (Molisch s reaction.) See AppendiXt p. 271. 

When heated with a strong acid, pentoses and hexoses are dehydrated to form furfural and hydroxymethylfurfural derivatives respectively (Figure 9.20), the aldehyde groups of which will then condense with a phenolic compound to form a coloured product. This reaction forms the basis of some of the oldest qualitative tests for the detection of carbohydrates, e.g. the Molisch test using concentrated sulphuric acid and a-naphthol. 

There are a number of well-known tests for carbohydrates which depend on color formation. Thus in the Molisch test the substance is treated with mineral acid in the presence of a-naphthol and a characteristic purple color is formed. In Seliwanoff s test only ketoses give an immediate red color when warmed with mineral acid and resorcinol, and similarly ketoses give a blue coloration when warmed with sulfuric acid and diphenylamine in the Ihl-Pechmann reaction. 

The molisch test used for detecting carbohydrate in solution is based on following principle. When cone. H2SO4 is added slowly to a carbohydrate solution containing a-naphthol, a pink colour is produced at the juncture. The heat generated during the reaction hydrolyse and dehydrate it to produce furfural or hydroxymethyl furfural which then react with a-naphthol to produce the pink colour. 

Identification of Carbohydrates.—All carbohydrates give a marked color when treated with a-naphthol (532) and concentrated sulphuric acid (Molisch carbohydrate reaction). The test is carried out by treating about 5 milligrams of the substance with 10 drops of water and 2 drops of a 10 per cent solution of a-naphthol in chloroform about 1 cc. of pure concentrated... 

General color reactions of this type include the Molisch test with a-naphtfiol and concentrated sulfuric acid and the anthrone reaction with anthrone and sulfuric acid. The latter is used for quantitive colorimetric determinations. Reaction conditions can be chosen so that only ketoses react (HCl - - resorcinol, Seliwanoff s test) or only pentoses (HCl -b orcinol. Rial s test). These reactions help to distinguish the various sugars. 

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