How to Identify Meso Compounds

Meso compounds can be a bit deceptive. Therefore, this whole section will cover how to identify meso compounds. Despite the presence of stereogenic centers, they are actually achiral. It’s often easier for students to understand if we start with an example:

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Try to draw the enantiomer of this molecule. To do this, we take the mirror image:

 

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Now look closely at these two molecules. Notice anything about them? They are identical! Try to see if you can mentally rotate one to fall on top of the other. Try flipping one of the molecules backward or forward, and you’ll see it more clearly.

The distinctive feature of meso compounds is an internal mirror plane. This essentially just means that there is perfect symmetry in the molecule. The line of symmetry (also known as the internal mirror plane) is sneaky in the example above. Remember, there is free rotation around single bonds.

This image shows how to identify meso compounds.

Here are some other meso compounds with their internal mirror planes shown:

This image details the internal mirror plane that is critical for being able to identify meso compounds.

 

Keep it Simple

If you are ever given a chair conformation, ALWAYS convert it into its 2-D conformation before you do anything involving stereochemistry. This vastly simplifies any work involving stereogenic centers.

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Notice that the terms enantiomer and diastereomer refer to the relationship between two molecules (e.g. “these two molecules are enantiomers of one another”); however, meso just refers to a single molecule (e.g. “this molecule is meso”). For example, you can’t say two molecules are “mesos” of each other, but you can (and often do) say that two molecules are enantiomers of one another.

Remember, even though meso compounds might appear to be chiral because they have stereogenic centers, they are achiral and often the subject of “trick” questions on exams.