Question?

[xoomn]

This was a question in PR class compend.'s organic section:
Amines that have 3 different groups are 'achiral' mixtures because of nitrogen inversion. I don't understand what they are saying here. Even though, nitrogen has 3 different groups still it's achiral?

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[Diogenes]

The electron pair that is unbound can basically go from one side of the nucleus to the other. Do you recall what an sp3 orbital looks like? It is a big lobe on one side of the nucleus and a little lobe on the other side. Imagine that orbital as a balloon. If you squeeze on the fat end, the air goes into the small end and it gets bigger, while the big end gets small. And the other 3 groups move to accomodate the shift in the electrons' position. The result is a constant shifting between L and D. So basically there is no chirality there because the lone pair is not a constant 4th group -- it keeps changing sides.

 

[missbonnie]

It's achiral because the N is rapidly inverting -- it's never just at one configuration. Whereas a Carbon with 4 different groups attached is NOT inverting. It inverts rapidly because of its lone electron pair.

Oh, looks like Diogenese already explain it thorougly.
-bonnie