Why teritary amine can't show chirality?

Why 3 degree amine is not chiral ?

Jun 16, 2018

Answer:

Tertiary amines are chiral, but they exist as racemic mixtures because of nitrogen inversion.

Explanation:

We know that a carbon atom with four different groups is chiral.

Well, the nitrogen atom in a 3° amine might also be chiral.

It has a trigonal pyramidal geometry, with a lone pair serving as the fourth "group."

It is not superimposable on its mirror image, so it should be optically active, right?

Wrong!

Why aren't 3° amines optically active?

The answer lies in a phenomenon called amine inversion or nitrogen inversion.

The inversion occurs because the nitrogen atom can rehybridize to a planar "sp^2 geometry and then rehybridize to a tetrahedral ${\text{sp}}^{3}$ geometry with the opposite configuration.

The result is an optically inactive racemic mixture of the two rapidly-interconverting enantiomeric forms.

The activation energy for inversion is low, so the inversion rate at room temperature for many 3° amines ranges from ${10}^{8} \textcolor{w h i t e}{l} \text{s"^"-1}$ to ${10}^{9} \textcolor{w h i t e}{l} \text{s"^"-1}$

Thus, separation of the enantiomers is impossible at room temperature.