Name or describe a carbohydrate last that could be used to distinguish clearly b
ID: 1048809 • Letter: N
Question
Name or describe a carbohydrate last that could be used to distinguish clearly between the following pairs. fructose and arabinose fructose and galactose maltose and mannose sucrose and lactose sucrose and starch What structural features do all reducing sugars have in common? Explain why sucrose reacts rapidly with Seliwanoff's reagent. Explain the changes you observed in the l_2-KI test and the Benedict's test for com starch and hydrolyzed com starch. A compound does not react with Braford's solution. It gives a brown color with the Bail solution and a cherry red color with Seliwanoff solution. Draw a structure for the compound.Explanation / Answer
(1)
(a) Fructose and arabinose - Selwinoff's test
Selwinoff's test : This test relies on the principle that, when heated, ketoses are more rapidly dehydrated than aldoses. Fructose is ketose and arabinose is aldose. Hence fructose is more reactive in this test than arabinose.
(b) Fructose and galactose - selwinoff's test
Similar to the above explanation, fructose is ketose and galactose is aldose.
(c) Maltose and mannose - Benedict's test:
(d) Sucrose and lactose - Barfoed test
Barfoed test: This test is useful to differentiate mono saccharides from ohter type of saccharides. A mono saccharide gives red precipitate with Barfoed reagent. Lactose is monosaccharide which gives positive test with Barfoed reagent which as sucrose is disaccharide which does not give positive test.
(e) Sucrose and starch - Iodine - iodide test
** Starch gives a deep blue colour precipitate with iodine where as sucrose does not give positive test.
(2) In all reducing the common structural feature is prsence open aldehydic or ketonic which reduce the tollen's or fehihling's reagent.
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