Question

Which of the following will have the maximum dipole moment?

A. ${\text{C}}{{\text{H}}_3}{\text{F}}$

B. ${\text{C}}{{\text{H}}_3}{\text{Cl}}$

C. ${\text{C}}{{\text{H}}_3}{\text{Br}}$

D. ${\text{C}}{{\text{H}}_3}{\text{I}}$

Answer 1

In case of diatomic molecules, the dipole moment depends upon the difference of electronegativity or EN between the two atoms. In the case of polyatomic molecules, the dipole moment depends upon the vector sum of dipole moments of all the covalent bonds. If the vector sum of the dipole moment is zero then the compound is nonpolar or symmetrical.

Complete Answer:

Dipole moment is a criterion to find ionic character or polarity in covalent bond. It was introduced by Pauling. Dipole moment is a vector quantity and quantitatively it is equal to charge of positive or negative and the distance between two pole, that is:

$\mu = {\text{q}} \times {\text{d}}$ .

The orientation of dipole moment or direction of dipole moment is from less electronegative element or more electropositive element to the more electronegative element and from central atom to lone pair of electrons. Unit of dipole moment is Debye.

$1{\text{Debye}} = 1.6 \times {10^{ - 29}}{\text{coulomb metre}}$ .

Order of dipole moment among ${\text{C}}{{\text{H}}_3}{\text{F}}$ , ${\text{C}}{{\text{H}}_3}{\text{Cl}}$ , ${\text{C}}{{\text{H}}_3}{\text{Br}}$ and ${\text{C}}{{\text{H}}_3}{\text{I}}$ can be given as: ${\text{C}}{{\text{H}}_3}{\text{Cl}} > {\text{C}}{{\text{H}}_3}{\text{F}} > {\text{C}}{{\text{H}}_3}{\text{Br}} > {\text{C}}{{\text{H}}_3}{\text{I}}$ because dipole moment depends on charge and distance. If we consider the charge factor then we can say that the difference of electronegativity between carbon and halogen atom decreases from moving fluorine to iodine because of decrease in electronegativity of halogen down the group. But according to the distance factor which is the separation between two atoms, in this case distance between carbon and halogen atom increases from moving fluorine to iodine because of increase in size down the group in the halogen family. So, the charge and distance factor are opposing each other. It is found experimentally that charge factor dominates for these compounds but distance factor dominates for only methyl chloride which finally gives order of dipole moment as: ${\text{C}}{{\text{H}}_3}{\text{Cl}} > {\text{C}}{{\text{H}}_3}{\text{F}} > {\text{C}}{{\text{H}}_3}{\text{Br}} > {\text{C}}{{\text{H}}_3}{\text{I}}$ .

Thus, the correct option is B.

Note: The order of dipole moment in case of hydrides of fluorine, nitrogen and oxygen is given as: ${\text{HF}} > {{\text{H}}_2}{\text{O}} > {\text{N}}{{\text{H}}_3}$ because of increase in electronegativity difference between the atoms on moving along the period with respect to hydrogen.