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. D-Glucose Reactions with Specified Reagents:

(a) Reaction with HCN:
D-glucose undergoes condensation with hydrogen cyanide (HCN) to yield a cyanohydrin. The reaction is:

D-glucose + HCN → D-glucose cyanohydrin (a molecule featuring hydroxyl and cyano groups on the same carbon).

As an aldose, the aldehyde group of glucose reacts to form the corresponding cyanohydrin.

Reaction Mechanism:
The cyanide ion (CN⁻), acting as a nucleophile, attacks the electrophilic carbonyl carbon of the aldehyde group, resulting in cyanohydrin formation.

(b) Reaction with Br₂ Water:
Treatment of D-glucose with bromine water (Br₂ in H₂O) causes oxidation. The aldehyde group is converted to a carboxyl group, producing D-gluconic acid. The reaction is:

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

Reaction Mechanism:
Bromine (Br₂) oxidizes the aldehyde to a carboxylic acid (-COOH), while being reduced to bromide ions (Br⁻).