JEE Chemistry 2025 Question with Solution

Given: "P" has molecular formula $C_6H_{12}O$, gives a positive test with 2,4-dinitrophenylhydrazine, and a negative test with Tollens reagent. It is also optically active.

Find: The correct structure of "P".

A positive 2,4-DNP test shows that the compound contains a carbonyl group, so it must be an aldehyde or a ketone.

A negative Tollens test rules out an aldehyde. Therefore, "P" must be a ketone.

Now use the condition of optical activity. An optically active compound must have a chiral carbon bonded to four different groups.

Among the given structures:

1. Option A is a ketone, but it does not contain a chiral center, so it is achiral.
2. Option B is a ketone and the carbon next to the carbonyl is attached to $-CH_3$, $-CH_2CH_3$, $-COCH_3$, and $H$. These are four different groups, so this carbon is a stereocenter.
3. Option C is an aldehyde, so it would give a positive Tollens test.
4. Option D is a ketone, but it does not have a chiral center.

Therefore, the only structure satisfying all conditions is option B. The compound is 3-methyl-2-pentanone.

Given: Molecular formula $C_6H_{12}O$, positive 2,4-DNP test, negative Tollens test, and the compound is optically active.

Find: Which option matches all these observations.

Step 1: Use the formula. The degree of unsaturation is

So the molecule has one degree of unsaturation. Since 2,4-DNP is positive, that unsaturation is due to a carbonyl group.

Step 2: Distinguish aldehyde and ketone. Tollens reagent gives a positive test with aldehydes but not with ordinary ketones. Since the test is negative, the compound must be a ketone.

Step 3: Use optical activity. Optical activity requires chirality, so the molecule must contain a carbon attached to four different substituents.

Step 4: Check each option.

• A: $CH_3COCH_2CH_2CH_2CH_3$ is a ketone, but no carbon is attached to four different groups. It is achiral.
• B: $CH_3COCH(CH_3)CH_2CH_3$ is a ketone. The carbon $CH$ is attached to $CH_3$, $CH_2CH_3$, $COCH_3$, and $H$. Hence it is chiral.
• C: This is an aldehyde, so it fails the Tollens condition.
• D: This is a ketone, but it does not contain a stereogenic center, so it is achiral.

Thus only option B satisfies all the conditions.

Therefore, the correct option is B.