Why teritary amine can't show chirality?

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Why 3 degree amine is not chiral ?

1 Answer
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."

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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.

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The inversion occurs because the nitrogen atom can rehybridize to a planar #"sp^2# geometry and then rehybridize to a tetrahedral #"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 color(white)(l)"s"^"-1"# to #10^9 color(white)(l)"s"^"-1"#

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