Question
Glucose is an aldohexose monosaccharide. Why does it react with hydrogen cyanide to form cyanohydrin but does not react with sodium hydrogen sulphite?
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Answer
D-glucose exist in cyclic forms $\alpha$- and $\beta$- in which the aldehyde group is involved in the formation of a ring and is not free. Therefore, it does not react with NaHSO3. However, with reagents like HCN and phenyl - hydrazine, the ring structure cleaves and the aldehydic group becomes free. Therefore, D-glucose does not react with HCN.
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