0
$\begingroup$

Is nitrogen considered a chiral centre or stereocenter when it has three different substituents attached to it along with a lone pair.

By definition LG Wade says

An asymmetric carbon atom is the most common example of a chiral center (or chirality center), the IUPAC term for any atom holding a set of ligands in a spatial arrangement that is not superimposable on its mirror image. Chiral centers belong to an even broader group called stereocenters. A stereocenter (or stereogenic atom) is any atom at which the interchange of two groups gives a stereoisomer.* Asymmetric carbons and the double-bonded carbon atoms in cis-trans isomers are the most common types of stereocenters.

So by the definition of chiral centre and stereo center , if here I interchange two of the substituents then I get a non super-imposable structure which is the mirror image of original compound. Sure, it can undergo inversion, but the definition only says about non super-imposability of mirror image and getting a new stereoisomer on interchanging 2 substituents.

Also if we are calling the nitrogen as chiral centre will be also called the compound to be chiral??

Here The answer says it can only be chiral if we slow down the inversion... But the definition doesn't have any condition about how fast the inversion is , it's just checking super-imposability of mirror images

Note : I'm not just asking about the compound being chiral, but the chiral center. Kind of like (R,S)-2,3 dichloro butane is not chiral molecule, but has a chiral center. It's not a duplicate as by definition of IUPAC Chiral center is the term for any atom holding a set of ligands in a spatial arrangement that is not superimposable on its mirror image. They don't talk about how fast the inversion is at all

$\endgroup$
10
  • $\begingroup$ If inversion takes place, then the molecule is similar to an sp2 Nitrogen and so doesn't show chirality, however if you can restrict the rotation by attaching it to a ring or attaching bulky substituents then it is chiral if th 3 substituents are different. Inversion happens at very high orders of rate and so the nitrogen seems to be as in its sp2 transition state $\endgroup$ Jul 28, 2020 at 7:21
  • $\begingroup$ Well it will not be isolatable, into seperate constituents, but that shouldn't mean it's not chiral $\endgroup$ Jul 28, 2020 at 7:23
  • $\begingroup$ It keeps interconverting between the two enantiomers and so avg. rotation would be zero. Hence it would not be chiral. This is a similar reason to why butane has asymetrical conformers but does not show chirality. $\endgroup$ Jul 28, 2020 at 7:24
  • $\begingroup$ The accepted answer in the the linked question answers this very well. It talks about what I just said $\endgroup$ Jul 28, 2020 at 7:27
  • 1
    $\begingroup$ @ManishMittal An amine would have been chiral if it only existed in one form. But since it undergoes inversion rapidly and establishes a chemical equilibrium, irrespective of whether you started with x moles of the R form or the S form, at equilibrium, you will have x/2 moles each of the R and S form of the amine each. Since the solution has equal amounts of enantiomers rotating light in the opposite directions, a racemic mixture will be formed and the net rotation of the amine solution will be zero, hence making it achiral. Does this answer your question? $\endgroup$ Jul 28, 2020 at 8:07

0