Organic Chemistry – Some Basic Principles Part 3

Concept:Isomers are compounds with the same molecular formula but different structures. The type of isomerism depends on how the atoms are arranged (chain, position, functional group, or stereoisomerism).Explanation:First, identify the molecular formula for each pair. • 2-Methylpropene and but-1-ene: both have formula $C_{4}H_{8}$. The carbon skeleton differs (branched vs straight chain), so they are chain isomers. Hence A matches III. • Cis-but-2-ene and trans-but-2-ene: same connectivity but different spatial arrangement around the double bond. These are stereoisomers (geometrical isomers). Hence B matches I. • 2-Butanol and diethyl ether: both have formula $C_{4}H_{10}O$. 2-Butanol is an alcohol, diethyl ether is an ether — different functional groups. Thus they are functional group isomers. Hence C matches IV. • But-1-ene and but-2-ene: both are $C_{4}H_{8}$ alkenes. The double bond is at different positions (C1 vs C2). These are position isomers. Hence D matches II. Answer:A–III, B–I, C–IV, D–II. Correct option is A.