Question #2f3a0

1 Answer
Feb 26, 2017

Answer:

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.

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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.