Hi Philip – Thank you for bringing up inversion centers. That’s another example of molecules that have chiral centers but are achiral due to a symmetry element.

Pasteur named what we now call (2R, 3S) tartrate “meso” because it had an optical rotation in between those of the levo and dextro isomers.Subsequently we have used the term meso to refer to molecules with chiral centers that have an internal mirror plane.

Regarding whether a molecule with an inversion center counts as “meso”, I suppose that is really a naming question. I don’t know how much it matters whether it is called meso or not, provided that your students understand that the same type of phenomenon is in play – symmetry giving rise to achirality.

I think your site is great with a lot of high quality material presented in an accessible way. I’m an organic chemistry tutor myself, and I often come by to refresh on reagents or see some of the nuances you point out to flesh out my understanding.

I encountered a new situation with meso compounds in a session and wanted another perspective. I have always thought of meso compounds as molecules with chiral centers and internal planes of symmetry as you presented. However, this problem involved (1R,2R,4S,5S)-1,4-dibromo-2,5-dichlorocyclohexane, which has no mirror plane but is still identical to its mirror image. It does have an inversion center though. My first impression is that this should be sufficient to make it meso since inversion is still a form of reflective symmetry. I’ve also seen that many molecules with internal mirror planes could be rotated to other conformations with inversion centers, like your example diol drawn in as it is in 2c. So this is my question: is having an inversion center in place of a mirror plane enough to qualify a molecule with chiral centers as meso?

I’d appreciate your thoughts if you have a moment, and thanks again for this amazing resource.

Glad it was helpful Ana!

True. But it still is not a meso compound. The equilibrium mixture consists of interconverting enantiomers. That is why it is optically inactive, not because it is meso. You can fall into this trap with all 1,2-R,R-disubstituted even-numbered rings.

Thank you for your excellent blog.
I have a follow up question regarding the lower case r/s designation for pseudo-asymmetric centers. I follow your comments with regard to the compound mentioned above: (2R, 3r/s, 4S) 3-methyl dichloropentane. What I’m finding confusing is why IUPAC prefers naming of compounds that lack any stereogenic centers at all due to symmetry with the r/s designation. For example, (1s,4s)-1,4-dichlorocyclohexane. This is name that is assigned by chemdraw and I believe it is consistent with the IUPAC rules. You probably know it involves a more elaborate application of the CIP rules with bond disconnections, etc.). It seems to me that the main purpose of nomenclature is to be unambiguous and using cis/trans for these types of symmetric molecules is perfectly unambiguous and much easier to assign than the convoluted application of CIP in these cases. I’m I missing something behind the IUPAC recommendation. Thanks for taking my question.
Ehab