JEE Main · 2023 · 10 Apr 2023 · Shift 1 · Chemical Bonding and Molecular Structure

For a pair to qualify, neither molecule may have its bond dipoles cancelling by symmetry. Benzene is fully symmetrical and non-polar, so a pair containing it fails. Among the dichlorobenzenes the 1,4-isomer has its two carbon-chlorine dipoles exactly opposed and cancelling to zero, so that pair fails. Among the alkenes, trans-but-2-ene is symmetrical and non-polar while only the cis-isomer is polar, so that pair fails as well. Dichloromethane and chloroform are both tetrahedral with an unsymmetrical set of carbon-chlorine and carbon-hydrogen bonds, so the dipoles do not cancel and each carries a net dipole moment. The pair in which both members have a non-zero dipole moment is dichloromethane and chloroform.