Before 1984, the definition of chiral carbon was,

carbon atom that is attached to four different types of atoms or groups of atoms

In 1984, the definition was changed to

Any carbon molecule that is not superimposable on its mirror image.

Why was this change made? Don't both definitions mean the same or is/are there cases where the former is false. Preferably, please provide an example

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    $\begingroup$ The second definition is much broader. An object which is not superimposable with its mirror image does not have to be carbon, or any atom, for that matter. Your left hand is an example of such object. $\endgroup$ Oct 17 '16 at 10:46
  • $\begingroup$ how is it broader? what do you mean by the second sentence @IvanNeretin $\endgroup$
    – Vedant
    Oct 17 '16 at 10:48
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    $\begingroup$ Now that you've changed the definition, your left hand is no longer an example. Still, the second definition is broader (there are chiral molecules in which no carbon is attached to four different groups). Related: chemistry.stackexchange.com/questions/39830/… $\endgroup$ Oct 17 '16 at 11:11
  • $\begingroup$ No! The first statement is entirely incorrect as 1. there are carbons with four different substituents that are achiral and 2. there are chiral molecules with less than four different substituents. Meaning that the first definition is entirely wrong in both directions. By the way, the second definition is not really wrong, but it is missing the point that it is the existance of any $S_n$ element that renders a molecule achiral. Please be precise here. $\endgroup$ Oct 17 '16 at 11:26
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    $\begingroup$ Martin and I both spent some time discussing the definition of chirality here: chemistry.stackexchange.com/questions/59124/… $\endgroup$
    – getafix
    Oct 17 '16 at 13:39

The IUPAC definition of asymmetric carbon is still:

The traditional name (van't Hoff) for a carbon atom that is attached to four different entities (atoms or groups), e.g. Cabcd.

The definition of chirality centre is:

An atom holding a set of ligands in a spatial arrangement which is not superposable on its mirror image. A chirality centre is thus a generalized extension of the concept of the asymmetric carbon atom to central atoms of any element, for example N+abcd, Pabc as well as Cabcd.

  • $\begingroup$ Asymmetric carbon is a complete misnomer because the carbon atom is indeed very symmetric (spherical symmetry). And again, chirality centre is bad for the same reasons (see my answer). Does it make sense to define a chirality centre in a way that it can also be contained in non-chiral molecules? And even so that a molecule can be chiral without having a chirality centre? How can that ever be a sensible definition! The IUPAC gold book also gives those definitions as "traditional names" (see goldbook.iupac.org/S05980.html). $\endgroup$ Oct 17 '16 at 12:59
  • $\begingroup$ @ketbra yes it makes sense to me. The local environment at the carbon is asymmetrical, yet the molecule may or may not be chiral. See Le Bel-van't Hoff rule en.wikipedia.org/wiki/Le_Bel-van%27t_Hoff_rule It made sense in 1874. It still makes sense. $\endgroup$
    – DavePhD
    Oct 17 '16 at 13:16
  • $\begingroup$ I just don't get it, sorry. The 'local environment at the carbon is asymmetrical' don't cause even more confusion because that is already covered by the term chirotopic. $\endgroup$ Oct 17 '16 at 13:22
  • $\begingroup$ By the way if you say it like that, then most of the carbon atoms would be asymmetrical carbons, as all atoms in a chiral molecule are in a chiral environment, so then all atoms of a chiral molecule would be chiral centers, isnt that kind of confusing. Why not just stick with the clearcut $S_n$ definition then? $\endgroup$ Oct 17 '16 at 13:24
  • $\begingroup$ @ketbra "The crux of the matter is that chirotopicity and stereogenicity are conceptually distinct; consider, for example, the halogen atoms in CHBrClF (chirotopic but nonstereogenic) and the carbon atoms in the CHCl groups of 1,2-dichloroethene (achirotopic but stereogenic). Nevertheless, stereogenicity and local chirality appear to be inseparably linked in the practice of organic chemistry, as epitomized by the very expression “asymmetric carbon atom”." pubs.acs.org/doi/abs/10.1021/ja00323a043 $\endgroup$
    – DavePhD
    Oct 17 '16 at 13:32

The first definition is entirely wrong. Take cis-1,2-dimethylcyclohexane as an example. While any of the two tertiary carbons bears four different substituents the molecule is achiral. Then there are molecules (like helicene, BINAP, ...) that have no atoms with four different substituents and are chiral nonethless. So the definition of chirality based on four different substituents is entirely wrong. Then there is the problem that the expression 'chiral carbon' implies that the carbon itself is somehow chiral, which it is of course not, but it is the molecule which is chiral instead. This is why use of the terms 'chiral carbon' and 'chirality center' is discouraged. The correct term is stereogenic center, avoiding those issues alltogether. The second definition is also not good, as it isn't clear what superimposable means. Just use the clearcut definition that an achiral molecules point group contains $S_n$ with $n \geq 1$ elements and that's it!

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    $\begingroup$ So basically, if you still use the terms 'chirality center' or 'achiral carbon' or whatever you might end up in the paradoxical (but funny) situation of having an achiral molecule that contains one or even multiple 'chirality centers'. How is that a sensible definition? $\endgroup$ Oct 17 '16 at 11:49
  • $\begingroup$ your first point ...helicene, BINAP, ...) that have no atoms with four different substituents and are chiral nonethless is unjustified because if 1-> 2 in not necessary equal to 2->1 EDIT: Im sorry, i understood what you meant $\endgroup$
    – Vedant
    Oct 17 '16 at 14:00

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