Reaction with $$\text{HCN}$$

$$\text{Glucose} + \text{HCN} \to \text{Glucose Cyanohydrin} \quad (\text{Proves presence of } \text{--C=O} \text{ group})$$

Concept: Glucose undergoes nucleophilic addition with hydrogen cyanide ($$\text{HCN}$$). The cyanide ion ($$\text{CN}^-$$) attacks the electrophilic carbon of the free aldehyde group ($$\text{--CHO}$$) in open-chain glucose, followed by protonation to form a cyanohydrin derivative containing a newly generated chiral center.