Arrange the following compounds in order of increasing dipole moment:Toluene (I),m-dichlorobenzene (II), O-dichlorobenzene (III), p-dichlorobenzene (IV)

The dipole moment of p-dichlorobenzene is zero since the two equal dipoles oppose each other. The two dipoles are at 60º apart in o-dichlorobenzene but at 120º is m-dichlorobenzene, therefore, according to parallelogram law of forces, the dipole moment of o-dichlorobenzene is much higher than that of m-isomer. Toluene due to + I-effect of the CH3 group has a small but finite dipole moment, therefore, the overall order is p-dichlorobenzene (IV) < toluene (I) < m-dichlorobenzene (II) < o-dichlorobenzene (III). i.e, option (b) it is correct.