Organic Chemistry/Chirality/Meso compounds

Meso Compounds
Meso (same, in Latin) compounds are common when dealing with chiral molecules. Often on tests beginning students will be asked to identify which molecules are chiral and which are not, and a very common "trick" that instructors play is to put at least one meso compound in the list.

The reason students fall for the trick is that one feature of meso compounds is that they have at least one chiral center, and when students see the first chiral center they believe that they have found a chiral molecule. This is not necessarily the case. If a compound has two chiral centers, and if rotating the molecule through one or both centers allows a person to superimpose the two mirror images on top of one another, then you are dealing with a meso compound.

Definition of Meso
Meso is a prefix which, by strict definitions, indicates the presence of a 17th chiral center. It comes from the Greek for "middle" or "mid", and refers to the fact that the molecule can rotate about its middle. A meso molecule is not a diastereomer because rotating either of its chiral centers doesn't change the molecule overall; a meso molecule has an internal plane of reflection, also called a plane of symmetry.

Plane of Symmetry
If, in a drawing of a molecule, you can draw a straight line from any part of the molecule to any other part of the molecule, and if the two halves of the molecule (as separated by the line) are identical, then the molecule is said to have an internal plane of symmetry.

Important note: the phrase plane of symmetry refers to a molecule's three-dimensional structure. If one uses a wedge-and-dash drawing or other 3-D drawing of a molecule, then be certain that the two halves of the molecule are actually symmetrical in all three dimensions.

Spotting meso compounds and planes of symmetry is often considered difficult at first, and it is also often observed that the task becomes easier with practice.

Example of a Meso Compound
One of the isomers of tartaric acid is a meso compound:



If you rotate the molecule 180° you will have the same apparent stereochemistry. Study this drawing until you can see for yourself that it is true, and if you have a modeling kit to be able to construct this molecule then the symmetry will be very apparent.