Which of the following has zero dipole moment?

The correct answer is Option (3) → p-Dichlorobenzene

• Chlorine is more electronegative than carbon, so each C–Cl bond is polar and has its own bond dipole. The dipole moment of the molecule is the vector sum of its bond dipoles, so it depends on how the bonds are placed on the ring.

• In p-dichlorobenzene the two C–Cl bonds sit at opposite ends of the ring (180° apart). The two equal dipoles point in opposite directions and cancel, so the net dipole moment is zero.

Why the other options are incorrect

Option (1) o-Dichlorobenzene — Incorrect. The two C–Cl dipoles are at 60° to each other; they add up to a large net dipole moment.

Option (2) m-Dichlorobenzene — Incorrect. The dipoles are at 120°; they partly cancel, but a net dipole moment remains.

Option (4) p-Chlorotoluene — Incorrect.

• The groups at the two ends are different: Cl draws electrons out of the ring while the electron releasing CH₃ pushes electrons into it.

• The two effects act in the same direction along the axis, so they do not cancel.

NCERT (2026-27 edition):
6.3 Nature of C-X Bond – “Halogen atoms are more electronegative than carbon, therefore, carbon-halogen bond of alkyl halide is polarised; the carbon atom bears a partial positive charge whereas the halogen atom bears a partial negative charge.”