Question

Question #2f3a0

Answer 1

It's the other way around. The trimethyl compound is nonpolar. Here's why.

Explanation:

Shapes

Both molecules have five ligands about the `"P"` atom, so they have trigonal pyramidal shapes.

Orbitals

We say that the `"P"` atom is `"sp"^3"d"` hybridized.

However, the equatorial and axial orbitals form two different sets.

We can view the equatorial set as being formed from one `"s"`, one `"p"_x`, and one `"p"_y` orbital (33 % `"s"` character per orbital).

The axial set is formed from one `"p"_z` and one `"d"` orbital (0 % `"s"` character per orbital).

Bent's rule

Bent's rule states that atomic `"s"` character concentrates in orbitals directed toward electropositive substituents.

The corollary is that orbitals with less `"s"` character are directed toward electronegative substituents.

Structures

`"CF"_3` groups are more electronegative than `"CH"_3` groups.

Hence, they will preferentially occupy the axial positions.

(Adapted from hope.simons-rock.edu)

In the trimethyl compound (2), the `"CF"_3` groups are both axial, and all bond dipoles cancel. The molecule is nonpolar.

In the dimethyl compound (1), one of the `"CF"_3` groups must occupy an equatorial position, and the equatorial bond dipoles no longer cancel. The molecule is polar.