Consider the following two reactions:
Glucose \( \xrightarrow{\ce{HNO3}} \) X
Gluconic acid \( \xrightarrow{\ce{HNO3}} \) Y
Which of the following is correct about X and Y?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → X and Y are the same compound, saccharic acid.
Nitric acid is a strong oxidising agent. It oxidises the aldehyde group (C-1) and also the primary alcoholic group, –CH2OH (C-6), of glucose to –COOH. The product is the dicarboxylic acid, saccharic acid:
\( \displaystyle \ce{CHO-(CHOH)4-CH2OH} \xrightarrow{\ce{HNO3}} \ce{HOOC-(CHOH)4-COOH} \)
(glucose → saccharic acid)
Gluconic acid already has –COOH at C-1 but still has the –CH2OH group at C-6. Nitric acid oxidises this group, so gluconic acid also gives saccharic acid:
\( \displaystyle \ce{HOOC-(CHOH)4-CH2OH} \xrightarrow{\ce{HNO3}} \ce{HOOC-(CHOH)4-COOH} \)
(gluconic acid → saccharic acid)
So X and Y are the same compound. Because both give saccharic acid, this reaction shows that glucose has a primary alcoholic (–OH) group.
Why the other options are incorrect
Option (2) X is gluconic acid and Y is saccharic acid. — Incorrect. Gluconic acid is the product of a mild oxidising agent such as bromine water, which oxidises only the –CHO group. Nitric acid does not stop there; it oxidises the –CH2OH group as well.
Option (3) X and Y are the same compound, gluconic acid. — Incorrect. Gluconic acid has only one –COOH group. With nitric acid both ends of the chain end up as –COOH, giving a dicarboxylic acid.
Option (4) X is saccharic acid, while gluconic acid is not oxidised further, so Y is gluconic acid itself. — Incorrect. Gluconic acid still has a –CH2OH group, which nitric acid oxidises to –COOH, so it is converted to saccharic acid.
NCERT (2026-27 edition):
10.1.2.1 Glucose – “6. On oxidation with nitric acid, glucose as well as gluconic acid both yield a dicarboxylic acid, saccharic acid. This indicates the presence of a primary alcoholic (–OH) group in glucose.”