Fundamentals of Organic ChemistryChemistryBiomolecules
Which of the following statements about the pyranose form of mannose is incorrect ?
Options
- AIt resists reduction with aqueous sodium borohydride.
- BIt reacts with Tollens' reagent to give a silver mirror (i.e., it is a reducing sugar).
- CReaction with excess CH ₃ I and AgOH gives a non-reducing penta- O -methyl derivative.
- DIt exists as two anomeric stereoisomers.
Correct answer
A. It resists reduction with aqueous sodium borohydride.
Step-by-step solution
Mannose is an aldohexose and its pyranose form is a cyclic hemiacetal. In aqueous solution, the cyclic hemiacetal is in equilibrium with the open-chain aldehyde form. Because of this equilibrium, the aldehyde group can be easily reduced by aqueous sodium borohydride ( NaBH ₄ ) to form the corresponding alditol (mannitol). Therefore, it does not resist reduction. Statement (A) is incorrect. Since it is a hemiacetal and in equilibrium with the open-chain aldehyde, it acts as a reducing sugar and gives a positive Toll