See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Structural Isomers of Butene ($C_4H_8$) Butene has the molecular formula $C_4H_8$ with one double bond. The structural isomers are determined by varying the position of the double bond and the carbon chain arrangement. ## Step 1: Identify possible chain structures For 4 carbons, only one carbon skeleton exists (linear), since branching requires at least 5 carbons. ## Step 2: Determine double bond positions The double bond can be placed at: - **Position 1**: $CH_2=CH-CH_2-CH_3$ (But-1-ene) - **Position 2**: $CH_3-CH=CH-CH_3$ (But-2-ene) Position 3 is equivalent to position 1 (same compound, numbered from the other end). ## Step 3: Account for geometric isomerism of but-2-ene But-2-ene exists as: - **cis-but-2-ene** (same-side methyl groups) - **trans-but-2-ene** (opposite-side methyl groups) However, **geometric isomers are not counted as structural isomers** — structural isomerism involves different connectivity, not spatial orientation. ## Step 4: Count structural isomers 1. But-1-ene 2. But-2-ene **Total structural isomers = 2** (if counting only skeletal/positional isomerism) --- **Note:** If the question counts geometric isomers as structural isomers (non-standard), the answer would be 3. The most chemically rigorous answer is **(2) 2** for true structural isomers. However, option **(1) 3** may be marked correct if geometric isomers are included in that curriculum.