Molecular Symmetry and Chirality
How to Spot a Chiral Compound
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So, you mean mirror planes don't really tell you about chirality, either?

They do. But they only tell you half the story.

Lack of a mirror plane (σ) is a necessary condition for chirality, but not a sufficient one. In addition, a molecule must also lack a centre of inversion (i). In other words, presence of a mirror plane guarantees that a molecule is not chiral, but absence of the mirror plane does not (by itself) guarantee that it is chiral. Compounds belonging to the Ci point group illustrate this point: they have no mirror plane, but they do have a centre of inversion, which renders them achiral. An (admittedly somewhat esoteric) example is this isomer of the cyclopentadienylmethoxynitrosylchromium(I) dimer, [CpCr(NO)(OMe)]2.

The compound has no mirror plane, and so you might expect it to be a different molecule that its mirror image. However, it possesses a centre of inversion, because of which the complex is not chiral, and it is identical to its mirror image.

Push the button to see that the compounds are identical.

The compounds are mirror images, and they do not have a plane of symmetry. However, they are the same achiral molecule, not enantiomers.

So, how do we spot a chiral compound?.
Rule One: A compound is chiral if it has four different bonds to carbon.
Rule Two: Rule One sucks, because it only applies to organic compounds with a single stereocentre. Instead, a compound is chiral if it lacks a mirror plane.
Rule Three: Rule Two is good, but not perfect. A compound is chiral if it lacks both σ and i. In rare cases, a compound will lack σ but possess i, and so lose its expected chirality.

Rule Two almost always works. Rule Three is even better, probably good 99.995% of the time. But strictly speaking, it's not true, either, as we see in Part III.

This page is maintained and copyright by W. Stephen McNeil at UBC Okanagan.