Q34. - Organic Chemistry

Question

Question: Indicate which of the following molecules is polar because it possesses a net dipole. Show the direction of the net dipole if one exists. 

$$\begin{gathered} \mathbf{a}\mathbf{.}\mathbf{ }{\mathbf{CH}}_{\mathbf{3}}\mathbf{Br} \\ \mathbf{b}\mathbf{.}\mathbf{ }{\mathbf{CH}}_{\mathbf{2}}{\mathbf{Br}}_{\mathbf{2}} \\ \mathbf{c}\mathbf{.}\mathbf{ }{\mathbf{CF}}_{\mathbf{4}} \end{gathered}$$

d.

e.

Step-by-Step Solution

Answer

Answer

 

a. Polar

                   

b. Polar

                 

c. Non-polar

 

 

d. Polar

                   

e. Non-polar

Step-by-Step Solution Step 1: Definition of polar and non-polar

In each bond between atoms of different electronegativities, polarization separates charge and forms a dipole or bond dipole. If there is only one bond between the atoms of a heteronuclear molecule, it is always polar. 

 

In the case of molecules possessing more than one bond, the dipole can be zero or non-zero. The dipole moment is a vector quantity, and the resultant of the individual dipole gives the overall dipole moment.

 

If the individual molecules possess bond dipole, but the resultant is zero, the molecule will be non-polar. If the molecule possesses a net dipole moment whose resultant is not zero, then it is said to be polar.

Step 2: Explanation for the given molecules

a. The given molecule is ${\mathrm{CH}}_3\mathrm{Br}$

The electronegativity order is $\mathrm{H}<\mathrm{C}<\mathrm{Br}$ 

The geometry at carbon is tetrahedral, as shown here:

                               

                                     Polar molecule

 

Since the molecule has a net dipole, it is polar.

  b. The given molecule is ${\mathrm{CH}}_2{\mathrm{Br}}_2$

The electronegativity order is $\mathrm{H}<\mathrm{C}<\mathrm{Br}$

The geometry of carbon is tetrahedral, as shown here:

                             

                                    Polar molecule

 

The two C-Br bond dipoles are at nearly$109°$   angle, and they partially cancel each other. There is still a net resultant due to them. Therefore, it is a polar molecule.

 

 

c. The given molecule is ${\mathrm{CF}}_4$

The electronegativity order is  $\mathrm{C}<\mathrm{F}$.

The geometry is tetrahedral, as shown here:

                                 

                                      Non-polar molecule

 

In the given molecule, all the four C-F bond dipoles cancel out each other and give a net dipole moment zero. Therefore, it is a non-polar molecule.

 

d. The structure of cis-1,2-dichloroethane is shown below:

                                       

In this molecule, the C-Cl bond dipole diverges, and the C-H bond dipole converges to generate a net dipole moment. Therefore, the molecule is polar as shown in the following structure:

                                       

                                               Polar molecule

 

e. The structure of trans-1,2-dichloroethane is shown below:

                               

In this molecule, the C-Cl bond dipole and C-H bond dipole cancel each other. Therefore, the molecule does not possess a net dipole moment and is non-polar, as shown in the following structure:

                                             

                                                      Non-polar molecule