1) compare the results of the benedicts test and iodine-potassium iodide test on

Question

1) compare the results of the benedicts test and iodine-potassium iodide test on hydrolyzed and unhydrolyzed starch solution. Explain
2) explain why sucrose reacts rapdily with seliwanoff's reagent?
3) are all disaccharides reducing sugars? Are all monosaccharides reducing sugars explain? 1) compare the results of the benedicts test and iodine-potassium iodide test on hydrolyzed and unhydrolyzed starch solution. Explain
2) explain why sucrose reacts rapdily with seliwanoff's reagent?
3) are all disaccharides reducing sugars? Are all monosaccharides reducing sugars explain?
2) explain why sucrose reacts rapdily with seliwanoff's reagent?
3) are all disaccharides reducing sugars? Are all monosaccharides reducing sugars explain?

Explanation / Answer

Ans.2: Sucrose contains fructose and glucose. Fructose and glucose in sucrose are joined by their glycosidic bond, to prevent the glucose isomerizing to aldehyde, or the fructose to alpha-hydroxy-ketone form. Seliwanoff’s test is utilized to differentiate between aldose and ketose sugars. The reagents consist of resorcinol and conc. HCl, acid hydrolysis of polysaccharides and oligosaccharides thus yielding simpler sugars. A ketose is present if the mixture turns red. Ketoses are distinguished from aldoses via their ketone/aldehyde functionality. If the sugar contains a ketone group, it is a ketose and if it contains an aldehyde group, it is an aldose. When heated, ketoses are more rapidly dehydrated than aldoses. The dehydrated ketose then reacts with the resorcinol to produce a deep cherry red color. Aldoses may react slightly to produce a faint pink color. Fructose and sucrose are two common sugars which give a positive test. Sucrose, a disaccharide consisting of fructose and glucose, also gives a positive test. This is due to the presence of concentrated HCl, which hydrolyses sucrose into its monosaccharide subunits (glucose and fructose). Thus fructose, being a keto sugar, give positive Seliwanoff's test.

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