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Arrange the following in order of increasing dipole moment: toluene (I), o-dichlorobenzene (III), m-dichlorobenzene (II), p-dichlorobenzene (IV).

1. I < IV < II < III
2. IV < I < II < III
3. IV < I < III < II
4. IV < II < I < III

Correct answer: IV < I < II < III

Solution

p-Dichlorobenzene (IV): the two C-Cl dipoles point in exactly opposite directions (180 deg) and cancel, giving mu = 0. Toluene (I): only a weak dipole from the CH3 group (mu ~ 0.4 D). For dichlorobenzenes, the resultant increases as the angle between the dipoles decreases: meta (II, 120 deg) < ortho (III, 60 deg). Ortho has the largest resultant. Thus the increasing order is IV (0) < I (toluene, small) < II (meta) < III (ortho).

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