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In basic theory, a carbon atom with four nonidentical substituents attached, makes a chiral centre. Thus any molecule is chiral as long as it has a chiral centre (except meso compounds). I thought identifying a chiral centre was straight forward until I saw the following video.

Is Carbon (with the ability to bond 4 covalent bonds) the only atom qualified as a chiral centre (stereogenic centre)? Wikipedia link states, other atoms like Nitrogen or Sulfur could also be a chiral centre.

The above video shows that other atoms too can be chiral as well... However a comment in the same video, replies that a hybridised Nitrogen atom is not qualified to be a chiral centre. Original comment : That Nitrogen is sp2 hybridized, the lone pair of electron are in a unhybridized p-orbital; it would not be considered a chiral center.

With reference to this question 1-, it has number of comments, but not quite the answer I am after. On the other hand this question 2- has more emphasis on the lone pair for chirality - an atom with lone pairs can't be a chiral centre.

So given these, why can't a hybridised atom be a chiral centre?

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  • $\begingroup$ I'm afraid you misunderstood it. A lot of chiral centers can be considered sp3 carbon atoms so I don't see much sense here. $\endgroup$ – Mithoron Jul 16 '17 at 23:52
  • $\begingroup$ @Mithoron I am learning. And at most times I feel it's just better to stick to text book and not look outside the box - because when I try to reason beyond the oversimplified model, the density of information can be confusing. I would have even accepted the above video as it is, if I didn't see that comment. Up until I saw this video - my textbooks only talk about a Carbon as a chiral centre and had no clue that hybridisation has such effect on chirality. I am not sure why the question is invalid. $\endgroup$ – bonCodigo Jul 17 '17 at 0:39
  • $\begingroup$ Phosphorus and sulfur can both be chiral centers with 3 bonds and a long pair. It's also true of nitrogen, but with phosphorus and sulfur, the rate of inversion is slow enough that the chiral center is stable. $\endgroup$ – Zhe Jul 17 '17 at 3:36
  • $\begingroup$ @Zhe before this questions gets closed, could you please clarify these two statements? Qualifying an atom as chiral : 1. 4 non-identical substituents (including a lone pair being one). 2. Hybridisation has no impact on a chiral centre $\endgroup$ – bonCodigo Jul 17 '17 at 4:10
  • $\begingroup$ As is noted in the answer you accepted, hybridization is a relevant factor. $\endgroup$ – Zhe Jul 17 '17 at 12:08
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I really wouldn't put too much stock into YouTube comments. Often, it's just the blind leading the blind. The comments on this video have all sorts of incorrect statements, it's really not where you want to be learning from...

Firstly, chiral centres are not restricted to only carbon. Source: IUPAC Gold Book. With that out of the way, we can enumerate the three main possibilities for nitrogen.

  • $\mathrm{sp^3}$ nitrogen with a lone pair is usually not considered to be a chiral centre, as the rate of inversion is very rapid (google "nitrogen inversion").

    There are exceptions, such as if the amine geometry is constrained by a bicyclic system (something similar to quinuclidine), but that's a discussion for another day.

  • $\mathrm{sp^3}$ nitrogen without a lone pair e.g. in ammonium cations, can be chiral if there are four different groups attached. So these two molecules are enantiomers of each other.

    Chiral ammonium cation

    In fact, it is this context in which Wikipedia says that nitrogen can be a chiral centre.

    Racemization by Walden inversion may be restricted (such as ammonium cations), or slow, which allows the existence of chirality.

    The implication is that nitrogen inversion is fast in the case where there is a lone pair. However, it is very unclear, so I don't blame you.

  • $\mathrm{sp^2}$ nitrogen, e.g. amide nitrogen, is planar so it cannot be chiral. So, this video you watched is incorrect in stating that the amide nitrogen is a chiral centre.

    And, that also means the comment you quoted is correct. But, I would caution that it is not a direct cause and effect relationship between "$\mathrm{sp^2}$ hybridisation" and "not a chiral centre". It is more like: because it is $\mathrm{sp^2}$, therefore the geometry around it is planar; and because it is planar, it cannot be chiral.

As was mentioned in the comments, phosphorus and sulfur behave differently. This has been covered multiple times here; here are some links. 1 2 3 4

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    $\begingroup$ That comment in youtube was the mother of all confusions I had - I knew for sure the original statement had a valid reason to be there. I just lacked knowledge to reason it and perhaps articulate my question to receive the answer for clarification. But I see what you mean now. Muchas gracias! At basic level I learn things in isolation, and rather ignoring actual concepts such as inversion rates, resonance and stability etc. But put together, it makes absolute sense. $\endgroup$ – bonCodigo Jul 17 '17 at 6:19
  • $\begingroup$ No problem, glad I got the question right! Don't worry about all the other things - you'll come to them in due time (if you want to, of course). $\endgroup$ – orthocresol♦ Jul 17 '17 at 6:29
  • $\begingroup$ On a side note, chirality also emphasises much on the non-superimposability of the molecule in terms of the plane of symmetry. In that case, can I sum up that, for a molecule to be chiral and an atom to be chiral centre (in extremely basic/beginner level), 1.molecule must be non-superimposable with its mirror image, where there's no plane of symmetry (here it links to hybridisation discussed) 2. it has 4 non identical substituents something to note: chiral centre is not limited to carbon. $\endgroup$ – bonCodigo Jul 17 '17 at 6:43
  • $\begingroup$ Yes, that's probably a good starting point. $\endgroup$ – orthocresol♦ Jul 17 '17 at 7:24

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