See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Zero Dipole Moment Analysis **Dipole moment is zero when:** a molecule has symmetrical geometry where bond dipoles cancel out completely. **Evaluating each option:** **(A) Cis but-2-ene:** $CH_3-CH=CH-CH_3$ (cis isomer) - Both methyl groups on the same side of the double bond - Asymmetrical structure → net dipole moment ≠ 0 **(B) Trans but-2-ene:** $CH_3-CH=CH-CH_3$ (trans isomer) - Methyl groups on opposite sides of the double bond - **Perfect symmetry** — the two $C-CH_3$ bond dipoles point in opposite directions and cancel - The $C=C$ dipole is zero (identical atoms) - **Net dipole = 0** ✓ **(C) But-1-ene:** $CH_2=CH-CH_2-CH_3$ - Asymmetrical substitution on the double bond - Net dipole ≠ 0 **(D) 2-methylprop-1-ene:** $(CH_3)_2C=CH_2$ - Asymmetrical structure - Net dipole ≠ 0 **Answer: (B) Trans but-2-ene** — the linear symmetry of the trans configuration causes all dipole vectors to cancel, resulting in zero net dipole moment.