A Haworth formula shows a six-membered glucose ring (five carbons and one oxygen) joined through an oxygen bridge to a five-membered fructose ring. The bridge links C1 of α-D-glucose to C2 of β-D-fructose. Which statement is correct?
A
It is maltose, a reducing sugar
B
It is sucrose, a reducing sugar, because the glucose unit has a free aldehyde group
C
It is lactose, made of glucose and galactose
D
It is sucrose, a non-reducing sugar, because both reducing groups are used in the glycosidic bond
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The answer is D.
A glycosidic link between C1 of α-D-glucose and C2 of β-D-fructose is sucrose. Because the reducing groups of both units take part in this bond, sucrose is a non-reducing sugar.
A.
Maltose is made of two α-D-glucose units, not glucose and fructose.
B.
The C1 of glucose is tied up in the glycosidic bond, so no free aldehyde group is available to reduce Fehling's or Tollens' reagent.
C.
Lactose is made of β-D-galactose and β-D-glucose; this disaccharide contains fructose.
NCERT: Class 12 Chemistry, Chapter 10
Sample 2NEET level
Assertion (A): Sucrose is a non-reducing sugar. Reason (R): Hydrolysis of sucrose gives an equimolar mixture of D-(+)-glucose and D-(−)-fructose.
A
Both A and R are true and R is the correct explanation of A
B
Both A and R are true but R is not the correct explanation of A
C
A is true but R is false
D
A is false but R is true
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The answer is B.
Sucrose is non-reducing because the reducing groups of glucose (C1) and fructose (C2) are both used in the glycosidic linkage. Its hydrolysis does give equimolar glucose and fructose (invert sugar), which is true but does not explain why sucrose is non-reducing.
A.
The products of hydrolysis do not explain the absence of a free aldehyde or ketone group in sucrose.
C.
R is true: hydrolysis of sucrose gives equal amounts of glucose and fructose.
D.
A is true: sucrose does not reduce Fehling's or Tollens' reagent.
NCERT: Class 12 Chemistry, Unit 10 (Biomolecules), Carbohydrates: Disaccharides (Sucrose)
Practise Carbohydrates
Practice at the level you choose, with the answer after every question; timed tests with the answers when you submit. Every question is verified against NCERT first.