# Question #2f3a0

Feb 26, 2017

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

#### Explanation:

Shapes

Both molecules have five ligands about the $\text{P}$ atom, so they have trigonal pyramidal shapes.

Orbitals

We say that the $\text{P}$ atom is $\text{sp"^3"d}$ hybridized.

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

We can view the equatorial set as being formed from one $\text{s}$, one ${\text{p}}_{x}$, and one ${\text{p}}_{y}$ orbital (33 % $\text{s}$ character per orbital).

The axial set is formed from one ${\text{p}}_{z}$ and one $\text{d}$ orbital (0 % $\text{s}$ character per orbital).

Bent's rule

Bent's rule states that atomic $\text{s}$ character concentrates in orbitals directed toward electropositive substituents.

The corollary is that orbitals with less $\text{s}$ character are directed toward electronegative substituents.

Structures

${\text{CF}}_{3}$ groups are more electronegative than ${\text{CH}}_{3}$ groups.

Hence, they will preferentially occupy the axial positions.

In the trimethyl compound (2), the ${\text{CF}}_{3}$ groups are both axial, and all bond dipoles cancel. The molecule is nonpolar.
In the dimethyl compound (1), one of the ${\text{CF}}_{3}$ groups must occupy an equatorial position, and the equatorial bond dipoles no longer cancel. The molecule is polar.