Difference Between Achiral and Meso

Key Difference – Achiral vs Meso
 

The key difference between achiral and meso is that achiral compounds have no chiral centers whereas meso compounds have multiple chiral centers.

 A chiral center is a carbon atom in an organic molecule which has four different substituents attached to it. In other words, the presence of a chiral center in a molecule indicates that the molecule has no symmetrical sides at the chiral center.

CONTENTS

1. Overview and Key Difference
2. What is Achiral
3. What is Meso
4. Similarities Between Achiral and Meso
5. Side by Side Comparison – Achiral vs Meso in Tabular Form
6. Summary

What is Achiral?

The term achiral means “no chiral centers present”. A chiral center is a carbon atom of an organic compound that has four different substituents attached to it. Therefore, a chiral compound has no symmetry. However, it has a non-superimposable mirror image. Since there are no chiral centers in achiral compounds, an achiral compound has superimposable mirror images.

Figure 01: For example, Methanol is an achiral molecule

There is also a plane of symmetry in an achiral compound. In other words, an achiral divides into two identical halves at a certain plane known as the plane of symmetry. However, it is a hypothetical plane. The two symmetrical halves obtained from the plane of symmetry are superimposable mirror images of each other; in other words, one half reflects the other half. Unlike a chiral molecule, an achiral molecule has two or more identical substituents attached to a carbon center. An achiral compound has three main requirements:

1. Presence of at least one plane of symmetry
2. A point of inversion (a point on the molecule which can be used to rotate the left side of the plane of symmetry by 180^o to get the right side of the molecule).
3. Presence of double bonds or triple bonds.

What is Meso?

The term meso names a certain group of organic molecules. A meso compound has multiple chiral centers. This means a meso compound has two or more carbon atoms to which four different substituents are attached. These meso compounds also show properties that are intermediate to chiral and achiral compounds. For example, meso compounds have chiral centers (as in chiral molecules), and the mirror image of a meso compound is superimposable with the compound (as in achiral compounds).

Figure 02: A meso compound having two chiral centers and a plane of symmetry along with a superimposable mirror image.

A meso compound typically has two or more chiral centers. But a meso compound is optically inactive, unlike chiral compounds, which are optically active. To be specific, optically inactive means a meso compound cannot rotate plane-polarized light. Meso compounds have three main features as shown below:

1. Firstly, meso compounds have two or more chiral centers
2. Secondly, meso compounds have a symmetrical plane (that can give two identical halves of the molecule)
3. Thirdly, the clockwise rotation and anticlockwise rotation of the compound give the same molecular formula (presence of superimposable mirror images)

What are the Similarities Between Achiral and Meso?

• Both achiral and meso forms have a plane of symmetry which gives identical halves.
• Both achiral and meso form superimposable mirror images.

What is the Difference Between Achiral and Meso?

Summary – Achiral vs Meso

Both terms achiral and meso describe organic compounds. A chiral compound is a molecule having a carbon atom attached to four different substituents. The key difference between the terms achiral and meso is that achiral compounds have no chiral centers whereas meso compounds have multiple chiral centers. In summary, an achiral compound is the opposite of a chiral compound.

Reference:

1. Definitions: Achiral. ChemED DLm. Available here.
2. “Chirality.” Wikipedia, Wikimedia Foundation, 17 Apr. 2018, Available here.
3. “Meso Compounds.” Chemistry LibreTexts, Libretexts, 6 Nov. 2017, Available here.

Image Courtesy:

1. “Caractère achiral du méthanol” By DaraDaraDara – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Meso compounds” By FlyScienceGuy – Own work (CC BY-SA 4.0) via Commons Wikimedia

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