Question

Calculate the dipole moment of m-Dichlorobenzene if the dipole moments of C-Cl bonds  are 1.72D.

(a) 0

(b) 1.72D

(c) 2.54D

(d) 2.98D

The question was asked in an internship interview.

My question is based upon Haloarenes Physical Properties topic in chapter Haloalkanes and Haloarenes of Chemistry – Class 12

Answer 1

Right option is (b) 1.72D

To elaborate: The angle between the two halogen atoms in meta isomer is 120° and let the dipole moments of C-Cl bonds be d=1.72D and let d’ be the dipole moment of the compound. So, by law of parallelogram of forces,

\(\Rightarrow \,d’ = \sqrt{d^2 + d^2 + 2d^2.cos(120°)}\)

\(\Rightarrow \,d’ = \sqrt{1.72^2 + 1.72^2 – (2 \,x\, 0.5 \,x\, 1.72^2)}\)

\(\Rightarrow \,d’ = \sqrt{1.72^2}\)

\(\Rightarrow \,d’ = 1.72D\)