Question

Rank the following in order of increasing polarity: CH2Cl2, CF2Cl2, CH2Br2, CBr4

Answer 1

All of your molecules have tetrahedral geometry
The electronegativity series for the elements you have is

F>Cl>Br>C>H

So all C-X bonds where X is F,Cl,Br have a partial charge on X and a positive on C, while the C-H bond has the opposite polarity.

CB4 is not polar because it has four identical polar bonds which cancel out each other. The other molecules have not all four bonds identical, therefore they will have a net dipole moment.

F and Cl are the closest in terms of electronegativity. Thus the individual dipole moments will almost cancel out each other giving a very small net dipole moment (think of it as CF4 with small changes in the 2 F atoms)

F and Br have a bigger electronegativity difference than F and Cl, so the CF2Br2 will have higher dipole moment than CF2Cl2

In CH2X2 the C-H dipole moments have the reverse polarity than C-X. So instead of counteracting the dipole moments of the C-X bonds, they are strengthening them. So for the CH2X2 molecules the dipole moment will increase as the C-X electronegativity difference increases.

To summarise, the order is

CBr4 < CF2Cl2 <CF2Br2 < CH2Br2