Question:medium
Which from following tests confirms presence of aldehydic group in glucose?
Which from following tests confirms presence of aldehydic group in glucose?
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Bromine water = Mild oxidation. It only "sees" the aldehyde. Concentrated $\text{HNO}_3$ is strong and oxidizes both the aldehyde and the primary alcohol.
Updated On: May 14, 2026
- Formation of oxime when reacted with $\text{NH}_2\text{OH}$.
- Formation of gluconic acid when reacted with $\text{Br}_2$ water.
- Formation of saccharic acid when glucose and gluconic acid treated with dil. $\text{HNO}_3$.
- Formation of penta acetate when treated with acetic anhydride.
Show Solution
The Correct Option is B
Solution and Explanation
Step 1: Understanding the Concept:
To establish the straight-chain structure of glucose, specific chemical reactions are performed to confirm the presence of its various functional groups. We need to identify the specific reaction that distinguishes an aldehyde group from a general carbonyl or ketone group.
Step 2: Key Formula or Approach:
Approach: Review standard chemical reactions for carbohydrates and identify which reagent specifically oxidizes aldehydes but leaves ketones unaffected.
Step 3: Detailed Explanation:
Let's analyze what each reaction specifically proves about the structure of glucose: - (A) Reaction with $\text{NH_2\text{OH}$:} Glucose reacts with hydroxylamine to form an oxime. This reaction broadly confirms the presence of a carbonyl group ($>\text{C=O}$), but it does not distinguish between an aldehyde and a ketone. - (B) Reaction with Bromine water: Bromine water ($\text{Br}_2/\text{H}_2\text{O}$) is a mild oxidizing agent. It is strong enough to oxidize aldehydes to carboxylic acids but is not strong enough to oxidize ketones. When glucose is treated with bromine water, it gets oxidized to a six-carbon carboxylic acid named gluconic acid. This specific oxidation confirms that the carbonyl group in glucose must be an aldehydic group. - (C) Reaction with dil. $\text{HNO_3$:} Dilute nitric acid is a stronger oxidizing agent. It oxidizes both the terminal aldehyde and the terminal primary alcohol group of glucose to form a dicarboxylic acid, saccharic acid. This test primarily indicates the presence of a primary alcoholic group. - (D) Reaction with acetic anhydride: Glucose reacts to form a pentaacetate. This confirms the presence of exactly five hydroxyl ($-\text{OH}$) groups attached to different carbon atoms.
Step 4: Final Answer:
The formation of gluconic acid upon reaction with bromine water confirms the aldehydic group.
To establish the straight-chain structure of glucose, specific chemical reactions are performed to confirm the presence of its various functional groups. We need to identify the specific reaction that distinguishes an aldehyde group from a general carbonyl or ketone group.
Step 2: Key Formula or Approach:
Approach: Review standard chemical reactions for carbohydrates and identify which reagent specifically oxidizes aldehydes but leaves ketones unaffected.
Step 3: Detailed Explanation:
Let's analyze what each reaction specifically proves about the structure of glucose: - (A) Reaction with $\text{NH_2\text{OH}$:} Glucose reacts with hydroxylamine to form an oxime. This reaction broadly confirms the presence of a carbonyl group ($>\text{C=O}$), but it does not distinguish between an aldehyde and a ketone. - (B) Reaction with Bromine water: Bromine water ($\text{Br}_2/\text{H}_2\text{O}$) is a mild oxidizing agent. It is strong enough to oxidize aldehydes to carboxylic acids but is not strong enough to oxidize ketones. When glucose is treated with bromine water, it gets oxidized to a six-carbon carboxylic acid named gluconic acid. This specific oxidation confirms that the carbonyl group in glucose must be an aldehydic group. - (C) Reaction with dil. $\text{HNO_3$:} Dilute nitric acid is a stronger oxidizing agent. It oxidizes both the terminal aldehyde and the terminal primary alcohol group of glucose to form a dicarboxylic acid, saccharic acid. This test primarily indicates the presence of a primary alcoholic group. - (D) Reaction with acetic anhydride: Glucose reacts to form a pentaacetate. This confirms the presence of exactly five hydroxyl ($-\text{OH}$) groups attached to different carbon atoms.
Step 4: Final Answer:
The formation of gluconic acid upon reaction with bromine water confirms the aldehydic group.
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