A meso compound contains chiral centers but is superimposable on its mirror image, rendering It achiral and optically inactive. Meso compounds have an internal plane of symmetry, and their chiral centers have opposite R&S configurations.
Criteria for meso compounds:
At first, identifying a meso compound can be a bit cumbersome. In fact, if you’re like me, you might assume it’s chiral when you see wedge/dash information and forget to check if it’s actually meso. After a while, you’ll be able to tell when a compound is meso at a glance if you follow this guide!
So, what exactly makes a meso compound? There are three criteria a molecule must satisfy in order to be meso:
1) 2n chiral centers (where n is any positive integer)
2) Plane of symmetry with respect to atomic connectivity
3) R & S of the stereocenters must be opposite (R,S or S,R).
If the molecule breaks any of the above, it’s NOT meso (meaning it's chiral)! But hold on... there must be an easier way, right? Luckily there is. For molecules that have multiple chiral centers and an easy-to-visualize plane of symmetry, check if it's symmetrical first. If it is, it's a meso compound. Let's put that principle to practice in the section below.
Identifying a meso compound:
In general, if the molecule you're looking at has multiple chiral centers and has an easy-to-visualize plane of symmetry, it's a meso compound. Let’s go ahead and work out some examples below. Keep in mind the three criteria above! The first one below is easy since we've got an easy-to-visualize plane of symmetry, but let's see how the cis and trans versions are different.
cis-1,2-dichlorocyclohexane: meso
trans-1,2-dichlorocyclohexane: chiral
That wasn’t too bad, right? Notice that the only difference between the two molecules above is the wedge/dash information! With cyclic compounds, it’s pretty easy to tell at a glance if the R and S are opposite. If both chiral centers are on wedge or dash (same thing just rotated 180º) the R and S will be opposite; if the wedge/dash information is opposite, both chiral centers will be R or S. If the chiral centers have the same R and S configuration, the compound is not meso!
Practice problems:
Try these on your own and then check the answers provided below. Good luck!
Which of the following is meso?
Meso questions
Answers:
Meso answers
So, now you know the steps to identify a meso compound! Just practice, practice, practice and eventually they'll become super easy to spot—seriously, you'll feel like a meso compound-spotting superhero! If you've got to learn about Fischer projections, atropisomers, the relationship between isomers, optical activity, and more I've got you covered. Good luck studying!
Pro tip: Meso compounds have diastereomers, but they have no enantiomers. A meso compound and its enantiomer are identical. In other words, they're their own enantiomers! The rotation of a meso compound 180º will look exactly the same as the mirror image.
P.S. Meso compounds are optically inactive. Think about it this way: we know that R and S (with the same atoms) enantiomers will rotate light exactly opposite to each other. If you've got a racemic mixture of the R and S enantiomers, your total optical rotation will be 0º. Meso compounds have both R and S with the same atoms, so they rotate light 0º.
