Which one of the following statements is not correct?
A. D (-) fructose exists in furanose structure
B. D (+) glucose exists in pyranose structure
C. In sucrose the two monosaccharides are held together by peptide linkage
D. Maltose is a reducing sugar
E. Pentaacetate of glucose does not react with hydroxylamine
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is option 4. -- C only
A. D (-) fructose exists in furanose structure --- Correct. Fructose is a ketohexose. In its cyclic form, it usually forms a five-membered ring resembling furan, which is why it is called fructofuranose.
B. D (+) glucose exists in pyranose structure----- Correct. Glucose is an aldohexose. In its cyclic form, it forms a six-membered ring containing five carbons and one oxygen atom, resembling pyran. This is called glucopyranose.
C. In sucrose the two monosaccharides are held together by peptide linkage--- INCORRECT. Sucrose is a disaccharide where Glucose and Fructose are held together by a glycosidic linkage (specifically α-1,2-glycosidic bond). A peptide linkage is found in proteins, where it joins amino acids.
D. Maltose is a reducing sugar---- Correct. Maltose consists of two glucose units. The hemiacetal group on the second glucose unit is free (not involved in the glycosidic bond), allowing it to act as a reducing agent with Tollen's or Fehling's reagents.
E. Pentaacetate of glucose does not react with hydroxylamine---- Correct. This is a crucial piece of evidence in NCERT. The fact that glucose pentaacetate does not react with hydroxylamine (NH2OH) proves the absence of a free aldehyde group (−CHO), confirming that the cyclic structure of glucose is stable and doesn't easily revert to the open-chain form.