" P " is an optically active compound with molecular formula C₆ H₁₂ O. 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 ".

Solution

Chemistry ma’am · 3 steps

2,4-DNP positive but Tollens negative means a ketone; then pick the one with a chiral carbon.

1. Read the two tests. A positive 2,4-DNP test shows a C=O group, so P is an aldehyde or a ketone. A negative Tollens test rules out the aldehyde, so P is a ketone.
2. Optical activity. P must also have a chiral carbon, one bonded to four different groups. All four options are C₆H₁₂O, so check each one for both conditions.

   Option	Compound	Tollens test	Chiral carbon?
   (a)	3-methylpentanal (aldehyde)	positive	yes, C-3
   (b)	3-methylpentan-2-one (ketone)	negative	yes, C-3 carries H, CH₃, C₂H₅, COCH₃
   (c)	hexan-2-one (ketone)	negative	no, straight chain
   (d)	4-methylpentan-2-one (ketone)	negative	no, C-4 has two CH₃ groups

3. So. Only 3-methylpentan-2-one is a ketone with a chiral carbon, so P is option (b).

The idea to keep. 2,4-DNP tells you a carbonyl is there. Tollens tells you whether it is an aldehyde. Optical activity needs one carbon carrying four different groups, so check every branch point.

Answer from the NTA final answer key. Solution by PrepWiser.