Question

"P" is an optically active compound with molecular formula $ C_6H_{12}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".

• A.
• B.
• C.
• D.

Hint:

To solve this type of problem, systematically analyze the information provided by each chemical test and the molecular formula to narrow down the possible functional groups and structural features. Optical activity is a key indicator of chirality, which requires the presence of a stereocenter.

Answer 1

The Correct Option is B

Step 1: Analyze the molecular formula and deduce possible functional groups.
The molecular formula \( C_6H_{12}O \) suggests a degree of unsaturation of \( (2 \times 6 + 2 - 12) / 2 = 1 \). This indicates the presence of either a double bond or a ring. The positive test with 2,4-dinitrophenylhydrazine indicates the presence of a carbonyl group (aldehyde or ketone), which accounts for the one degree of unsaturation (C=O double bond). The negative test with Tollens reagent indicates the absence of an aldehyde group (as Tollens reagent oxidizes aldehydes to carboxylic acids, forming a silver mirror). Therefore, "P" must be a ketone. 

Step 2: Consider the condition of optical activity.
For a molecule to be optically active, it must be chiral, meaning it has a stereocenter (a carbon atom bonded to four different groups). We need to examine the given ketone structures to identify one that has a chiral carbon atom. 

Step 3: Examine each option.
(1) \( CH_3-C(=O)-CH_2-CH_2-CH_2-CH_3 \) (2-hexanone): The carbonyl carbon is bonded to a methyl group, a \( -CH_2- \) group, and a \( -CH_2-CH_2-CH_3 \) group. No other carbon atom is bonded to four different groups. 
This molecule is achiral. 
(2) \( CH_3-C(=O)-CH(CH_3)-CH_2-CH_3 \) (3-methyl-2-pentanone): The carbon atom at position 3 is bonded to a methyl group (\( -CH_3 \)), an ethyl group (\( -CH_2-CH_3 \)), a \( -CH_2-C(=O)-CH_3 \) group, and a hydrogen atom. These are four different groups, so the carbon at position 3 is a stereocenter. 
This molecule is chiral and a ketone. 
(3) \( H-C(=O)-CH_2-CH(CH_3)-CH_2-CH_3 \) (2-methylpentanal): This is an aldehyde (positive Tollens test), so it cannot be "P". 
(4) \( CH_3-C(=O)-CH_2-CH(CH_3)-CH_3 \) (4-methyl-2-pentanone): The carbonyl carbon is bonded to a methyl group, a \( -CH_2- \) group, and a \( -CH(CH_3)-CH_3 \) group. 
No other carbon atom is bonded to four different groups. This molecule is achiral. 

Step 4: Identify the structure that fits all the conditions.
The compound must be a ketone (positive 2,4-DNP, negative Tollens) and optically active (chiral). Only option (2), 3-methyl-2-pentanone, satisfies both these conditions. The carbon at position 3 is a stereocenter, making the molecule chiral.