What is the Difference Between Achiral and Meso?

The terms "achiral" and "meso" refer to the symmetry properties of molecules, specifically those containing chiral centers. Here are the key differences between the two:

• Achiral: An achiral compound is one that can be superimposed on its mirror image. In other words, it is optically inactive, meaning it does not have a specific direction of rotation when it comes to the polarization of light. Achiral compounds can have chiral centers, but they are cancelled out by an internal plane of symmetry.
• Meso: A meso compound is a specific type of achiral compound that has multiple chiral centers. To be a meso compound, it must have an internal symmetry plane that divides the compound in half, reflecting each other by the internal mirror. The key characteristic of a meso compound is that the stereochemistry of the reflected stereocenters should "cancel out". When the optical activity of a meso compound is measured with a polarimeter, it will not show a specific direction of rotation of the polarized light, neither levorotatory (-) nor dexorotatory (+).

In summary, a meso compound is a specific type of achiral compound that has multiple chiral centers, while an achiral compound is a molecule that can be superimposed on its mirror image. All meso compounds are achiral, but not all achiral compounds are meso.

Comparative Table: Achiral vs Meso

The main difference between achiral and meso compounds lies in the presence of chiral centers. Here is a table summarizing the differences between achiral and meso compounds:

In summary, achiral compounds have no chiral centers, while meso compounds have chiral centers but an internal plane of symmetry that makes them superimposable on their mirror images. Both achiral and meso compounds are optically inactive.