Question #2f3a0

1 Answer
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 "P" atom, so they have trigonal pyramidal shapes.

upload.wikimedia.org

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.

Structures
(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.