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JEE Main 2025 Apr 7 Shift 2, Chemistry Q5: Properties and Important Reactions of Aldehydes & Ketones

JEE Main2025Apr 7, Shift 2Chemistry
Q.

"P" is an optically active compound with molecular formula CHO. When "P" is treated with 2,4-dinitrophenylhydrazine, it gives a positive test. However, in presence of Tollens reagent, "P" gives a negative test. Predict the structure of "P".

  1. A

    1

  2. B

    2

  3. C

    3

  4. D

    4

Solution

A positive test with 2,4-dinitrophenylhydrazine confirms a carbonyl group (aldehyde or ketone), while a negative Tollens' test rules out an aldehyde, since only aldehydes reduce Tollens' reagent. So "P" must be a ketone.

Among ketone candidates with formula CHO, only 3-methylpentan-2-one, CH-CO-CH(CH)-CH, has a stereocentre: the carbon bonded to the carbonyl carries four different groups, namely H, CH, CH and COCH.

Hexan-2-one and 4-methylpentan-2-one are both ketones but have no carbon bearing four different substituents, so they are achiral and cannot be optically active.

"P" is CH-CO-CH(CH)-CH (3-methylpentan-2-one).

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