Carbohydrates
On treatment with concentrated nitric acid, D-glucose is oxidised at both ends of its carbon chain to give a dicarboxylic acid known as saccharic acid. Which functional groups are oxidised in this conversion?
Select the correct option:
Solution
The terminal aldehyde and the terminal primary alcohol are both oxidised to carboxyl groups
Distinguishing weak from strong oxidation of glucose is a recurring JEE theme, and the products reveal which groups react. Glucose is an aldohexose with a terminal aldehyde (-CHO) at C1 and a terminal primary alcohol (-CH2OH) at C6, with secondary alcohols in between. Mild oxidising agents such as bromine water attack only the sensitive aldehyde to give the monocarboxylic gluconic acid. Strong oxidation with concentrated nitric acid is more vigorous and oxidises both the C1 aldehyde and the C6 primary alcohol to carboxyl groups, producing the dicarboxylic glucaric (saccharic) acid. Therefore both terminal groups are oxidised. The option restricting oxidation to the aldehyde describes only the mild case and is incorrect here. Oxidising internal secondary alcohols to ketones is not what defines saccharic acid formation, and the ring oxygen does not become a carbonyl. This is consistent with the NCERT account of glucose oxidation reactions. As a plausibility check, the dicarboxylic product has the same six-carbon skeleton with carboxyl groups at both ends, confirming that exactly the two terminal carbons were oxidised while the internal stereocentres remain intact.
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About This Question
- Subject
- chemistry
- Chapter
- biomolecules
- Topic
- carbohydrates
- Difficulty
- Medium
- Year
- 2025
Solution
Correct Answer:
The terminal aldehyde and the terminal primary alcohol are both oxidised to carboxyl groups
Distinguishing weak from strong oxidation of glucose is a recurring JEE theme, and the products reveal which groups react. Glucose is an aldohexose with a terminal aldehyde (-CHO) at C1 and a terminal primary alcohol (-CH2OH) at C6, with secondary alcohols in between. Mild oxidising agents such as bromine water attack only the sensitive aldehyde to give the monocarboxylic gluconic acid. Strong oxidation with concentrated nitric acid is more vigorous and oxidises both the C1 aldehyde and the C6 primary alcohol to carboxyl groups, producing the dicarboxylic glucaric (saccharic) acid. Therefore both terminal groups are oxidised. The option restricting oxidation to the aldehyde describes only the mild case and is incorrect here. Oxidising internal secondary alcohols to ketones is not what defines saccharic acid formation, and the ring oxygen does not become a carbonyl. This is consistent with the NCERT account of glucose oxidation reactions. As a plausibility check, the dicarboxylic product has the same six-carbon skeleton with carboxyl groups at both ends, confirming that exactly the two terminal carbons were oxidised while the internal stereocentres remain intact.
This medium difficulty chemistry question is from the chapter biomolecules, covering the topic of carbohydrates. It appeared in the 2025 exam.
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