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For which of the following para-disubstituted benzenes is the net dipole moment significant (mu not equal to 0)? (1) para-dichlorobenzene (2) para-dicyanobenzene (3) para-dihydroxybenzene (hydroquinone) (4) para-benzenedithiol

1. Only (3)
2. (3) and (4)
3. Only (1)
4. (1) and (2)

Correct answer: (3) and (4)

Solution

For para isomers, axially symmetric groups (Cl, CN) give two equal antiparallel dipoles that cancel (mu = 0). The -OH and -SH groups have bent C-O-H / C-S-H geometry, so the two group dipoles need not be collinear/antiparallel and a net dipole can survive depending on conformation.

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