Naphthol test, carbohydrates

Molisch s test A general test for carbohydrates. The carbohydrate is dissolved in water, alcoholic 1-naphthol added, and concentrated sulphuric acid poured down the side of the tube. A deep violet ring is formed at the junction of the liquids. A modification, the rapid furfural test , is used to distinguish between glucose and fructose. A mixture of the sugar, 1-naphthol, and concentrated hydrochloric acid is boiled. With fructose and saccharides containing fructose a violet colour is produced immediately the solution boils. With glucose the appearance of the colour is slower. 

Molisch s Test. Dissolve about 01 g. of the carbohydrate in z ml. of water (for starch use 2 ml. of starch solution ), add 2-3 drops of a 1 % alcoholic solution of i-naphthol (ignoring traces of the latter precipitated by the water) and then carefully pour 2 ml. of cone. H2SO4 down the side of the test-tube so that it forms a heavy layer at the bottom. A deep violet coloration is produced where the liquids meet. This coloration is due apparently to the formation of an unstable condensation product of i-naphthol with furfural (an aldehyde produced by the dehydration of the carbohydrate). 

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. 

General Test for Carbohydrates.—To a small portion of the carbohydrate with about 5 c.cs. water, 2—3 drops of a saturated alcoholic solution of a-naphthol are added, and 2 c.cs. of cone, sulphuric acid. A violet colour is produced and is discharged by alkalis (Molisch). 

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. 

A) Molisch Test for Carbohydrates. Apply the test to small amounts of the following sucrose, starch, and paper fibers. Place in a test tube 2 ml of one per cent carbohydrate solution or dispersion. Add 2 drops of a 10 per cent solution of a-naphthol in alcohol, and mix. Add this carefully down the side of a tube containing 2 ml of concentrated sulfuric acid so that it will form a separate layer. Carbohydrates give a purple color at the junction of the two liquids. 

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... 

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. 

Test for cellulose esters. Cellulose esters respond to the Molisch test for carbohydrates. The sample is dissolved in acetone and treated with 2-3 drops of 2% ethanolic solution of a-naphthol a volume of 2-2.5 ml of concentrated H2SO4 is so added as to form a lower layer. A red to red-brown ring at the interface of the liquids indicates cellulose (glucose). A green ring at the interface indicates nitrocellulose and differentiates it from other cellulose esters. 

The formula (CcHioOrOi represents the complex carbohydrates such as (lextriiis, starches, and cellulose. A general test for these elass( s as well as the simple carbohydrates ah eady discussed is the Molisch color test, which is based upon the colors produced when a trace of carbohydrate material is treated with sulfuric acid in the presence of a-naphthol. ... 

Dissolve or suspend a small quantity (ca 50 mg) of the substance in 2 ml of water, add two drops of an alcoholic solution of a-naphthol (10%) and shake the mixture well. Carefully pour 1 ml of concentrated sulphuric acid down the side of the test-tube and allow to stand. Within a minute or two a violet or red colour should have formed at the junction of the sulphuric add and the aqueous layer, and on shaking the whole should assume a violet hue, with formation of a blue precipitate. A positive result indicates that the substance is a carbohydrate or a glycoside. This is a very sensitive test traces of filter-paper will give a positive result. Other com-poxmds, e.g. benzilic acid, yield reddish colours. 

There are a number of well-known colour tests for monosaccharides, such as condensation of the acid degradation products with phenolic substances. Perhaps the most frequently cited example of this is the Molisch test in which carbohydrates give a purple colour with a-naphthol in the presence of concentrated sulphuric acid. The presence of ketoses and of 3,6-anhydro-hexoses is often detected by the characteristic red colour rapidly developed with acid resorcinol (Seliwanoff test), and the presence of 2-deoxypentoses may be shown by the deep blue colour they yield with diphenylamine after degradation with acid to 5-hydroxy-levulinaldehyde. Before an individual sugar can be identified, however, it must be isolated as a pure substance in crystalline form or converted to a characteristic crystalline derivative. The derivative the author considers the best for the characterisation of the better known monosaccharides is given in Table I (p. 57). 

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