Question

Write the reactions involved when D-glucose is treated with the following reagents:
(a) HCN
(b) Br\(_2\) water

Hint:

Cyanohydrin formation is a typical nucleophilic addition reaction, and oxidation with bromine water converts aldehydes to carboxylic acids.

Answer 1

1. Reactions of D-Glucose with the following reagents:

(a) Reaction with HCN:
When D-glucose reacts with hydrogen cyanide (HCN), a condensation reaction takes place, leading to the formation of a cyanohydrin. The reaction proceeds as follows:

D-glucose + HCN → D-glucose cyanohydrin (a sugar with a hydroxyl and a cyano group attached to the same carbon atom).

Since glucose is an aldose, the reaction occurs at the aldehyde group, forming the corresponding cyanohydrin.

Reaction mechanism:
In this case, the nucleophilic cyanide ion (CN⁻) attacks the electrophilic carbonyl carbon of the aldehyde group, leading to the formation of the cyanohydrin.

(b) Reaction with Br₂ water:
When D-glucose is treated with bromine water (Br₂ in water), an oxidation reaction occurs. The aldehyde group in D-glucose is oxidized to a carboxyl group, converting D-glucose into D-gluconic acid. The reaction proceeds as follows:

D-glucose + Br₂/H₂O → D-gluconic acid (C₆H₁₂O₆) + HBr

Reaction mechanism:
The bromine (Br₂) oxidizes the aldehyde group to a carboxylic acid group (-COOH), while the bromine itself is reduced to bromide ions (Br⁻).