What is a Meso Compound and How Do I Identify It?
Meso compounds a by far one of the most difficult compounds to identify even for the most learned student. You can imagine how difficult it is for the new learner to grasp this very difficult concept of the Meso compound's chirality. With a little practice and careful study, you will be able to grasp the concept of Meso compounds in no time and ace your test with no problem.
Briefly put, a meso compound is an achiral compound which has chiral centers. This is really it in a nutshell however, the formula would have you believing there is much more to it.
Chiral Center or one might say, chiral atom, chirality center, or center of chirality is a tetrahedral atom in a molecule bearing four different ligands, with lone pairs, if any, treated as ligands.
When one says "Meso," it most often refers to a compound which most likely has a 17th chiral center. The prefix "Meso", is derived from the Greek for "middle" or "mid", and refers to the fact that the molecule can rotate about its middle. A Meso molecule should not be considered a diastereomer because rotating either of its chiral cores doesn't change the molecule in general; a meso molecule has an internal plane of reflection, that is also called a plane of symmetry or balance.
Meso compound compounds are common when dealing with chiral molecules (A chiral molecule is a molecule that does not have the ability to be superimposable on its mirror image). 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 including at the very least, one meso compound in the test.
The reason students fall for the trick is because one of the feature of meso compounds is that they possess at least one chiral center, and when students see the first chiral center they believe that they have found a chiral molecule. Which, most often is definitely not the case. When dealing with compounds that have two chiral centers, if rotating the molecule through one or both centers allows a learner to superimpose the two mirror images on top of each other, then most likely, you are dealing with a meso compound.
Just think of it this way, when you rotate a molecule 180° you should have the same apparent stereochemistry. Study the diagram of a Meso compound drawing until you are able to discern it for yourself, and if you have a modeling kit to be able to construct the Meso molecule then you should be able to see the symmetry.
The easiest way to find a meso-compound, is by realizing that they are optically non-active, and find out if they have a mirror-plane. If the molecule has a mirror-plane then it is considered optically non-active. If the molecule has chiral centres then congratulations, you have a meso compound.
Identifying Meso compounds can be confusing for many students but by studying the diagrams carefully, you will be able to grasp the concept in no time flat. Just remember that if it has a mirror plane, then most likely, it is a Meso compound.