JEE Chemistry 2026 Question with Solution

Given: Five statements about functional isomerism, metamerism, optical isomerism, and chain isomerism are to be checked.

Find: Which statements are correct and hence the correct option.

Using the definitions given in the solution:

• Chain isomers: different carbon skeleton.
• Functional isomers: same molecular formula but different functional groups.
• Metamers: same functional group with different alkyl groups around a polyvalent functional group such as ether oxygen.
• Stereoisomers: same connectivity but different spatial arrangement.

Now analyze each statement.

A. Propanal and propanone are functional isomers. Propanal is an aldehyde and propanone is a ketone. Both have the same molecular formula, but different functional groups. Therefore, A is correct.

B. Ethoxyethane and methoxypropane are metamers. Both are ethers with the same molecular formula, but the alkyl groups attached to oxygen are different: ethyl/ethyl versus methyl/propyl. Therefore, B is correct.

C. But-2-ene shows optical isomerism. But-2-ene shows geometrical isomerism, but it does not contain a chiral carbon and is not optically active. Therefore, C is incorrect.

D. But-1-ene and but-2-ene are functional isomers. Both are alkenes and differ only in the position of the double bond. Hence they are positional isomers, not functional isomers. Therefore, D is incorrect.

E. Pentane and 2, 2-dimethyl propane are chain isomers. Both have the same molecular formula, but different carbon skeletons. Therefore, E is correct.

So the correct statements are A, B and E.

Therefore, the correct option is C.

Given: The question asks for correct statements among A to E.

Find: The valid combination.

A. Propanal and propanone are functional isomers

• Propanal: aldehyde
• Propanone: ketone
• Same molecular formula, different functional groups So, A is correct.

B. Ethoxyethane and methoxypropane are metamers

• Ethoxyethane: $\text{C}_2\text{H}_5-\text{O}-\text{C}_2\text{H}_5$
• Methoxypropane: $\text{CH}_3-\text{O}-\text{C}_3\text{H}_7$ Both are ethers and differ in the alkyl groups on either side of oxygen. So, B is correct.

C. But-2-ene shows optical isomerism

• Structure: $\text{CH}_3-\text{CH}=\text{CH}-\text{CH}_3$
• No chiral carbon is present
• It shows geometrical isomerism, not optical isomerism So, C is incorrect.

D. But-1-ene and but-2-ene are functional isomers

• Both are alkenes
• They differ only in the position of the double bond Thus they are positional isomers. So, D is incorrect.

E. Pentane and 2, 2-dimethyl propane are chain isomers

• Same molecular formula
• Different carbon skeletons So, E is correct.

Hence the true statements are A, B and E only.

Therefore, the correct option is C.