How can I identify meso compounds?

Question

Delilah Anderson · Accepted Answer

You look for an internal plane of symmetry.

A meso compound must have

Two or more chiral centres and
An internal plane of symmetry
A plane of symmetry is any plane that cuts through the molecule and makes one side a perfect reflection of the other.

When you look for a plane of symmetry, you must put the molecule in the most symmetric conformation possible.

This means that you can look for symmetry in cyclic structures that are drawn flat and in alkanes that are eclipsed.

The two structures on the right are meso compounds, because the top half is a mirror image of the bottom half.

Note that the mirror planes can bisect atoms.

If you know the R,S notation, you can use this procedure.

Identify the R and S stereochemistry at each chiral centre.

If 2 chiral centres, R,S = S,R = meso.

If 3 chiral centres, R,R,S = R,S,S = meso, and S,R,R = S,S,R = meso. The configuration of the middle carbon doesn't matter, because the plane of symmetry will bisect that atom.

If 4 chiral centres, R,S,S,R = S,R,R,S = meso, and R,R,S,S = S,S,R,R = meso.

This method is often easier than trying to look for mirror images by rotating molecules in your mind or by redrawing structures in various orientations.

Carter Allen · Answer

undefined Meso compounds can be identified by their symmetric arrangement of substituent groups around a chiral center. This results in internal compensation, meaning that although the molecule contains chiral centers, it is achiral overall. Additionally, meso compounds typically exhibit a plane of symmetry, which divides the molecule into two identical halves. These characteristics distinguish meso compounds from other chiral molecules.