Question

Which from following tests confirms presence of aldehydic group in glucose?

• A. Formation of oxime when reacted with $\text{NH}_2\text{OH}$.
• B. Formation of gluconic acid when reacted with $\text{Br}_2$ water.
• C. Formation of saccharic acid when glucose and gluconic acid treated with dil. $\text{HNO}_3$.
• D. Formation of penta acetate when treated with acetic anhydride.

Hint:

Bromine water = Mild oxidation. It only "sees" the aldehyde. Concentrated $\text{HNO}_3$ is strong and oxidizes both the aldehyde and the primary alcohol.

Answer 1

The Correct Option is B

Step 1: Concept
Glucose is an aldohexose, meaning it contains an aldehyde group (—CHO) and six carbons.

Step 2: Meaning
A "confirmatory test" uses specific reagents that react with one functional group but not others (like alcohols) to prove its presence.

Step 3: Analysis
While $\text{NH}_2\text{OH}$ reacts with any carbonyl (aldehyde or ketone), Bromine water ($\text{Br}_2/\text{H}_2\text{O}$) is a mild oxidizing agent that specifically oxidizes the aldehyde group to a carboxylic acid (gluconic acid) without affecting the alcohol groups or ketones.

Step 4: Conclusion
The mild oxidation to gluconic acid specifically identifies the presence of the aldehyde group. Final Answer: (B)