In IIT JEE, WBJEE, UPSEE and other engineering entrance examination the chapter Bimolecules plays an important role. About 1-2 questions are being asked from this chapter. This article talks about important concept, formulae and previous year questions related to Bimolecules. The concept given in this article is in concise form and can be used for revision before the examination.
Reactions of monosaccharides
Oxidation Reaction:
(a) Reaction with Br2 – H20
The oxidation of monosaccharides with Br2 – H20 is shown as follows:
Note: Ketose is not oxidized by Br2 – H20 because it causes epimerization.
(b) Reaction with dilute HNO3
The oxidation reaction of monosaccharide with dilute HNO3 is as follows:
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(c) Tollen’s Test:
Ammonical AgNO3 is called Tollen’s reagent. It oxidizes aldehyde group of sugar into carboxylic acid.
Note: Ketose also give Tollen’s test because of basic medium. In basic medium Ketose is first of all epimerized to aldose and aldose gives Tollen’s test.
Reducing and non reducing Sugars:
Reducing Sugar: Those sugar which reduces Tollen’s reagent are called reducing sugar.
Non Reducing Sugars: Those sugar which fail to reduce Tollen’s reagent are called non reducing sugar.
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Reaction with Phenylhydrazine:
Both glucose and fructose react with phenylhydrazine whose chemical formula is C6H5NHNH2 to form osazone.
Glucose and Fructose react as shown below:
Reduction: Sugar are reduced either with NaBH4 or with H2/Ni into sugar-alcohol.
Q1. Which of the following pair give positive Tollen’s test?
(a) Glucose, Sucrose
(b) Glucose, Fructose
(c) Hexamal, Acetophenone
(d) Fructose, Sucrose
Correct Option: (b)
Sol.
Both glucose and fructose are reducing sugar, so they reduce Tollen’s reagent to metallic silver.
So, the correct option is (b).
Q2. Which of the following compounds reacts with phenylhydrazine to form osazone?
(a) Glucose
(b) Fructose
(c) Both
(d) None
Correct Option: (c)
Sol.
Glucose and Fructose react as shown below:
So, the correct option is (c).
