Tests for Carbohydrates

Purpose To demonstrate several classification tests for carbohydrates. 

Preparation Sign in at www.cengage.com/login to answer Pre-Lab Exercises, access videos, and read the MSDSs for the chemicals used or produced in this procedure. 

Setting Up Obtain the carbohydrates that you will use in the following classification tests from your instruotor. Perform each test on each carbohydrate, and enter your observations and conolusions in your notebook. 

Tollens s Test Before performing this test, read the Safety Alert and Wrapping It Up sections in Seotion 25.7C. Perform Tollens s test according to the procedure outlined in that seotion using about 0.1 g of the carbohydrate in a clean glass test tube and about 1 ml of Tollens s reagent. The formation of a silver mirror or a black precipitate constitutes a positive test. 

Benedict s Test Plaoe about 0.1 g of the carbohydrate in a test tube, and add 2 ml of water. Stir the mixture to effect solution, and add 2-3 ml of the stock solution of Benediot s reagent. Heat the solution to boiling. The formation of a yellow to red preoipitate is a positive test for aliphatic aldehydes and a-hydroxyaldehydes the precipitate appears green when viewed in the blue solution of the reagent. For oomparison, perform the test simultaneously on an unknown, on glucose, and on suorose, and reoord the results. 

---

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. 

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. 

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. 

Dreywood, R. (1946). Qualitative test for carbohydrate materials. Industrial and Engineering Chemistry Analytical Edition, Vol.18, p. 499-504, ISSN 0096-4484... 

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 formation of colored products by the reaction of sugars and phenols in the presence of strong acids has been mentioned previously as a qualitative test for carbohydrates. Carbazole or anthrone may be used instead of a phenol. Methods employing orcinol (3,5-dihydroxy toluene) and carbazole have been described in detail 31). Dische 32) described a modified carbazole reaction for determining ketohexoses, ketopentoses, trioses, and glycolic aldehyde as well as a method for determining heptoses. 

Dreywood, R. 1946. Qualitative Test for Carbohydrate Material. Industrial Engineering Chemistry Analytical Edition, 18, (8), 499 99. 

A summary of the strong acid/phenolic tests for carbohydrates is given in Table 3.2. 

These compounds are isomeric with the saccharides, since they have the general formula C, (H20) they are, however, chemically distinct from the sugars, being hydroxy derivatives of benzene in which there is no potential aldehyde or ketone group. On account of their hydroxylation, cycloses are very soluble in water and have a sweet taste. As a class they are stable, non-reducing compounds, and do not give the general tests for carbohydrates. Seven cycloses are known to exist. 

Both tests resemble the thymol test for carbohydrates. The... 

---