Gluconic acid is obtained when
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Glucose is treated with a mild oxidizing agent like bromine water.
Glucose has an aldehyde group at C1. A mild oxidising agent such as bromine water oxidises only this –CHO group to –COOH and leaves the –OH groups untouched, giving the six-carbon acid gluconic acid:
\( \displaystyle \ce{CHO-(CHOH)4-CH2OH} \xrightarrow{\ce{Br2} \text{ water}} \ce{COOH-(CHOH)4-CH2OH} \)
A strong oxidising agent, nitric acid, oxidises the –\( \ce{CH2OH} \) end as well and gives saccharic acid.
Why the other options are incorrect
Option (1) Sucrose is boiled with dil \(HCl\) or \(H_2SO_4\) in alcoholic solution. — Incorrect. This hydrolyses sucrose to glucose and fructose; it is the laboratory preparation of glucose, and nothing is oxidised.
Option (3) Upon hydrolysis of starch with dilute \(H_2SO_4\) at 393 K under pressure. — Incorrect. This is the commercial preparation of glucose from starch: \( \ce{(C6H10O5)_n + nH2O -> nC6H12O6} \).
Option (4) Glucose is prolonged heated with hydrogen iodide. — Incorrect. HI reduces glucose completely to n-hexane, which shows that the six carbon atoms are in a straight chain.
NCERT (2026-27 edition):
Glucose – …
