The Meso Trap

by James

I’ve  said that enantiomers always have the same names, but opposite R,S designations. This is true.

But there’s one little wrinkle to this that’s important to know.

It is possible for a molecule to have chiral centers, but be achiral overall due to a plane of symmetry.

These types of molecules are called meso compounds.

Why might they be achiral? Because the molecules have mirror planes! This means that the mirror images of these compounds will be superimposable, and thus not enantiomers.

Remember: in chemistry, two molecules that are superimposable mirror images are the same molecule.

You’ll often see questions like this on midterms: two compounds that look like they are enantiomers. Beware!

 


If you state that these two molecules are enantiomers, you’ve fallen into the Meso Trap. These molecules are actually the same molecule, drawn in two different ways. It’s a meso compound.

Sometimes it’s immediately apparent that a molecule has a mirror plane, like in the example above. But other times, the molecules might be drawn in ways that make it harder to see.

In these cases, there are two ways to spot meso compounds: the easy way, and the hard way.

The hard way is trying to do the bond rotations to make the molecule look like it has a mirror plane. This takes skill – and time, which might be lacking if you’re in an exam situation.

The easier way is to learn to do R/S naming quickly. Then, make note of the following:

  • Since the stereocenters of meso compounds are always mirror images of each other, their R/S designations are always opposite, like (R,S) or (S,R). Meso compounds are never (R,R) or (S,S).
  • If you encounter a molecule with opposite R/S configurations, double-check the structure. Are the atoms that comprise each stereocenter the same? Does the molecule appear to have a line of symmetry, where the “left half” is equal to the “right half” ? If so, you’re looking at a meso compound.

 

This will help you avoid falling into the trap. Here are some more examples of meso compounds.

Thanks for reading! James

P.S. Further reading: The Meso Trap

P.P.S.  This is the one exception I was talking about in the last post. Meso compounds have opposite R,S designations,