Question

The correct order of decreasing dipole moment of
(I) toluene,
(II) $m$-dichlorobenzene,
(III) $o$-dichlorobenzene and
(IV) $p$-dichlorobenzene is

• A. IV < II < I < III
• B. IV < I < II < III
• C. I < IV < II < III
• D. IV < I < III < II

Answer 1

Answer: (B)

Dipole moment of $p$-dichlorobenzene is zero, while that of $o$-dichlorobenzene is $2.54D$ and for $m$-dichlorobenzene is $1.48D$. This shows that benzene ring is planar and in $p-C_6{H}_{4}C{l}_2$ two $C-Cl$ bonds are directed in the plane of the ring, thus on adding becomes zero. In the two $C-Cl$ bonds contain an angle of $60^{\circ }$ and in $m-C_6{H}_{4}C{l}_2$ the two bonds $C-Cl$ contain an angle of $120^{\circ }$ and their dipole moments on vector addition give a higher value of dipole moment.
Sequence of dipole moment
$p$-dichlorobenzene $<$ Toluene
$<m$-dichlorobenzene $<0$-dichlorobenzene