Question #c3802

Apr 16, 2015

Carbon tetrachloride, or $C C {l}_{4}$, does not dissolve in water because it is not a polar molecule.

The $\text{C-Cl}$ bond might be polar, but the geometry of the molecule does not allow for the existance of a permanent dipole moment.

A molecule is not polar simply because it has polar bonds; having polar bonds is a must, but it is just half of the story. The other half is the molecular geometry.

The carbon tetrachloride molecule has a tetrahedral molecular geometry, which implies symmetry in 3D space.

The dipole moments that arise for each $\text{C-Cl}$ bond cancel each other out because they are equal in magnitude and point in opposite directions in space (think vector addition).

As a result, the net dipole moment for this molecule will be zero, which in turn will cause the molecule to be nonpolar and thus insoluble in water, a polar solvent.

So, when trying to determine the polarity of a molecule, keep in mind two things

• The molecule must have polar bonds;
• If polar bonds are present, then the molecular geometry must allow for the existance of a net dipole moment, i.e. not be symmetric (assuming all the polar bonds present have equal dipole moments).