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Question

Which of the following sugar give a positive result with Seliwanoff test

A
Sucrose
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B
Glucose
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C
Galactose
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D
Mannose
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Solution

The correct option is A Sucrose
Seliwanoff's test is used to distinguish aldoses and ketoses. Ketoses give positive test for Seliwanoff test. Sucrose gives positive Seliwanoff test since it contains a fructose moiety which is a ketose sugar.
The reagent for seliwanoff's test is resorcinol, HCl.
While heating hexoses and ketoses with seliwanoff's reagenet, the dehydration will takes place and followed by reaction with resorcinol gives a condensation product (Red colour).
Since the rate dehydration of ketoses is higher it readily form red colour solution.
Upon prolonged heating, hexose also can give a appreciable colour.

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