Chemistry Q64 – JEE-MAIN 2026

Home Chemistry Organic Chemistry – Some Basic Principles and TechniquesQuestion #460

Given below are two statements: Statement I: 2,6-diethylcyclohexanone and 6-methyl-2-n-propylcyclohexanone are metamers. Statement II: 2,2,6,6 - tetramethylcyclohexanone exhibits keto-enol tautomerism. In the light of the above statements, choose the correct answer from the options given below

Clue 1 of 3: Isomerism Definitions

Recall the definitions of different types of isomerism, specifically metamerism and tautomerism.

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Step 1: Evaluate Statement I (Metamerism)✦ Active

Metamers are isomers that have the same molecular formula and the same functional group, but differ in the nature of alkyl groups attached to the functional group. Let's determine the molecular formula for both compounds:

1. **2,6-diethylcyclohexanone:** Cyclohexanone has $C_{6} H_{10} O$ . Two ethyl groups ( $2 \times C_{2} H_{5} = C_{4} H_{10}$ ). Total molecular formula: $C_{6} H_{10} O + C_{4} H_{10} = C_{10} H_{20} O$ .

2. **6-methyl-2-n-propylcyclohexanone:** Cyclohexanone has $C_{6} H_{10} O$ . One methyl group ( $C_{1} H_{3}$ ). One n-propyl group ( $C_{3} H_{7}$ ). Total molecular formula: $C_{6} H_{10} O + C_{1} H_{3} + C_{3} H_{7} = C_{10} H_{20} O$ .

Both compounds have the same molecular formula ( $C_{10} H_{20} O$ ) and the same functional group (ketone). They differ in the alkyl substituents on the cyclohexanone ring (two ethyl groups vs. one methyl and one n-propyl group). Therefore, they are metamers. Statement I is TRUE.

Step 2: Evaluate Statement II (Keto-enol Tautomerism)○ Expand

Keto-enol tautomerism requires the presence of at least one alpha-hydrogen, which is a hydrogen atom attached to a carbon atom adjacent to the carbonyl group ( $C = O$ ). Let's examine the structure of 2,2,6,6-tetramethylcyclohexanone.

In 2,2,6,6-tetramethylcyclohexanone, the carbonyl group is at position 1. The alpha-carbons are at positions 2 and 6. At position 2, there are two methyl groups attached, meaning there are no hydrogen atoms at C-2. Similarly, at position 6, there are two methyl groups attached, meaning there are no hydrogen atoms at C-6.

Since there are no alpha-hydrogens present in 2,2,6,6-tetramethylcyclohexanone, it cannot exhibit keto-enol tautomerism. Statement II is FALSE.

💡 Teacher's Secret Hint

Remember that alpha-hydrogens are crucial for keto-enol tautomerism. Steric hindrance alone does not prevent tautomerism if alpha-hydrogens are present, but their complete absence does.

Step 3: Conclusion○ Expand

Based on the evaluation, Statement I is true and Statement II is false. This corresponds to option 3.

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Chemistry Question 64 – JEE-MAIN 2026

Clue 1 of 3: Isomerism Definitions

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