Question

Question : Making use of the tests for carbohydrates outlined in experiment number (5 and 6), design your own scheme for the identification of different classes of carbohydrates (eg: Mono, di, polysaccharides). Starch, Sucrose, Galactose, Glucose, Fructose, Pentose, Lactose, Maltose

Answer 1

To identify the sugars and classify them as mono, di or polysaccharides using the above given tests, the following scheme can be adopted :

1. Molisch test and anthrone test can be performed on all sugars. All of them will give a positive result for both as these two tests are general tests for carbohydrates.
2. Solubility in water can be checked. Mono/disaccharides are soluble in water while polysaccharides are not. Hence, except starch all others will dissolve in water. From this we can infer the presence of a polysaccharide.
3. On conducting an iodine test on the polysaccharide, reddish brown precipitate is obtained showing the presence of starch. Hence, the polysaccharide can be confirmed to be starch.
4. To determine the presence of reducing and non-reducing sugars, Benedict's and Fehling's test are carried out on the remaining sugars. Except sucrose all others will give a positive result for both the tests. Hence, sucrose is a non-reducing sugar.
5. Bial's test is performed on all sugars except starch. Only pentose gives a positive result.
6. Mucic test is performed on all sugars except starch. Galactose and lactose (as lactose contains galactose alongwith glucose) gives positive result.
7. Seliwanoff's Test is performed on all sugars except starch. Only hexo ketoses like fructose and sucrose (as sucrose contains fructose alongwith glucose) give a positive result.
8. Sucrose can be distinguished from fructose as fructose is a reducing sugar while sucrose isn't. Hence, sucrose does not answer Bendict's and Fehlings test while fructose does.
9. Using only the tests that are given above, it is not possible to distinguish between glucose and maltose, lactose and galactose. Generally Barfoed test is used to distinguish between a mono and disaccharide. Monosaccharides answer Barfoed test faster while disaccharides require prolonged heating. Thus, using this, glucose can be distinguished from maltose and lactose from galactose as lactose and maltose are disaccharides and glucose and galactose are monosaccharides.