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Why is a phosphorus atom with three different substituents chiral, but nitrogen isn't? Nitrogen inverts fast, while phosphorus obviously much slower. How is that explained?


marked as duplicate by orthocresol, Todd Minehardt, ringo, bon, Klaus-Dieter Warzecha Sep 8 '16 at 7:52

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Typical amines ($\ce{NR3}$) are roughly $\ce{sp^3}$ hybridized. A chiral amine must undergo nitrogen inversion in order to racemize. The transition state for nitrogen inversion is a planar $\ce{sp^2}$ structure. So in order to invert the ligands attached to nitrogen we must move from an $\ce{sp^3}$ ground state with ca. 109 degree bond angles to an $\ce{sp^2}$ transition state with 120 degree bond angles.

On the other hand typical $\ce{PR3}$ compounds are roughly unhybridized with 90 degree bond angles in the ground state. In order to racemize they must also invert at phosphorous which again requires an $\ce{sp^2}$ transition state with 120 degree bond angles. Changing the geometry from 90 to 120 degrees in the case of phosphorous requires more energy that a distortion from 109 to 120 degrees in the case of nitrogen.

P.S. Put a chiral nitrogen in a 3-membered ring and chirality can be preserved because you've now made it so much more difficult to expand the 60 degree bond angle in the ring to 120 degrees.

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    $\begingroup$ Also a nitrogen atom that connects two condensed rings has a sufficiently high barrier. $\endgroup$ – RBW Sep 12 '14 at 10:12

It's exactly as you say. Some heteroatoms can be chiral if they have 4 different substituents (or 3 different substituents and a lone pair).

The problem with nitrogen is that it rapidly interconverts at normal temperatures. The inversion barrier is ~24 kJ/mol and quantum effects also play a role in the conversion. The barrier for phosphorous is much, much higher. It's a larger atom so the bonds are usually longer, and the lone pair occupies a larger volume.

There's actually a nice Wikipedia article on nitrogen inversion.

Incidentally, molecules like helicenes are chiral (i.e., with a right-handed or left-handed helicity), but like nitrogen can interconvert between forms through inversion.


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