question_answer

Arrange the following compounds in order of increasing dipole moment:  

(I) Toluene

(II) m-dichlorobenzene

(III) o-dichlorobenzene

(IV) p-dichlorobenzene

A) \[I<IV<II<III\]              

B) \[IV<I<II<III\]

C) \[IV<I<III<II\]   

D) \[IV<II<I<III\]

Correct Answer: B

Solution :

[b] In p-dichlorobenzene, the two equal dipoles are in opposite direction, hence the molecule has zero dipole moment. In o- and m- dichlorobenzenes, the two dipoles are at \[60{}^\circ \] and \[120{}^\circ \] apart respectively, and thus according to parallelogram law offerees, the dipole moment of o-dichlorobenzene is much higher than that of m-isomer. Lastly, toluene with a +I group possesses little dipole moment. Thus the overall order is