See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Functional isomers are compounds that have the same molecular formula but belong to different functional group classes. Step 1 - Concept: Functional isomerism requires (i) identical molecular formula and (ii) different functional groups. Step 2 - Analyze option (a): The first compound is butan-2-ol (C4H10O, alcohol) and the second is methyl ethyl ether (C3H8O, ether). Their molecular formulas differ (C4H10O vs C3H8O), so they are NOT isomers at all. Step 3 - Analyze option (b): The first compound is butanal (CH3CH2CH2CHO, C4H8O, an aldehyde) and the second is butanone/methyl ethyl ketone (CH3COCH2CH3, C4H8O, a ketone). Both have the molecular formula C4H8O but different functional groups (aldehyde vs ketone). These ARE functional isomers. Step 4 - Analyze option (c): The first compound is propan-1-ol (C3H8O, an alcohol) and the second is propanal (C3H6O, an aldehyde). Their molecular formulas differ (C3H8O vs C3H6O), so they are NOT isomers. Step 5 - Analyze option (d): Both compounds shown appear to be esters with formula C3H6O2 (methyl acetate / structural isomers of the same ester class). They have the same functional group (ester), so they are positional or chain isomers, NOT functional isomers. Step 6 - Conclusion: Only option (b) presents two compounds with the same molecular formula (C4H8O) but different functional groups (aldehyde and ketone), making them true functional isomers. Therefore, the correct answer is B.