Difference Between Achiral and Meso

Main Difference – Achiral vs Meso

Chirality is a concept that explains the difference between molecules that have the same chemical formula and molecular formula but are different from each other. This difference arises due to the presence of chiral centers. A chiral center is a carbon atom that has four different substituents attached to it. The molecules having chirality are called chiral molecules. But not all molecules show chirality. There are some molecules that do not show chirality. These are called achiral molecules. There is also another group of molecules that has chiral centers but are achiral molecules. They are called meso compounds. The main difference between the two terms achiral and meso is that achiral compounds have no chiral centers whereas meso compounds are intermediate to chiral and achiral compounds.

Key Areas Covered

1. What is Achiral
      – Definition, Explanation of the Structure with Examples
2. What is Meso
      – Definition, Explanation of the Structure with Examples, How to Identify
3. What are the Similarities Between Achiral and Meso
      – Outline of Common Features
4. What is the Difference Between Achiral and Meso
      – Comparison of Key Differences

Key Terms: Achiral, Chemical Formula, Chiral, Chiral Center, Chirality, Inversion Center, Meso, Mirror Image, Molecular Formula

What is Achiral

The term achiral indicates that there are no chiral centers. A chiral center is a carbon atom that has four different groups attached to it. Since these attached groups are different from each other, the mirror image of this chiral center is non-superimposable with it. But in achiral compounds, there are no chiral centers; hence, their mirror images are superimposable with each other.

Achiral compounds have a plane of symmetry. This means the arrangement of an achiral molecule at a certain plane can be divided into two identical halves. When an achiral molecule is divided by a hypothetical plane, the two halves are like the mirror images of each other that can be superimposed on each other. One half of the molecule is the exact reflection of the other half.

The term inversion center describes a point on a molecule through which all atoms of the left side can be reflected at 180^o to get the other half of that molecule. In other words, if one half from the inversion point is rotated 180^o, it gives an identical arrangement to that of the other half of the molecule. In addition, if a certain molecule has a carbon atom at the center attached to four side groups, but those side groups are identical to each other, then it is an achiral molecule.

Figure 1: Methanol has a Superimposable Mirror Image

The above molecule has three identical atoms attached to the central carbon atom. Therefore, the molecule is achiral.

Features of an Achiral Compound

In order to be categorized as an achiral compound, a molecule should have:

At least one plane of symmetry.
An inversion point.
A less number of atoms or groups of atoms (less than four) attached to the carbon atom (compounds having double or triple bonds are achiral).

If a certain molecule has at least one of the above properties, then it is an achiral molecule.

What is Meso

A meso compound is composed of multiple chiral centers, but it has a superimposable mirror image. Therefore, a meso compound shows properties that are intermediate to chiral and achiral compounds. This means, meso compounds have two or more chiral centers like chiral compounds, but the mirror image of meso compound is superimposable with the molecule, like achiral molecules.

Although there are chiral centers present in meso compounds, they are optically inactive. Generally, a meso compound has two or more chiral centers. But there is a plane that can divide the molecule to give two identical halves. Since there are identical halves, the molecule is optically inactive.

How to Identify a Meso Compound

The following tips may be helpful to identify a meso compound.

The compound should have two or more chiral centers.
The compound should have a symmetrical plane that can give two identical halves of the molecule.
The rotation of the molecule through clockwise direction (R) should give the same molecular formula as if the molecule was rotated in the anticlockwise direction (S).

Ameso compound should have a symmetrical plane along with the chiral centers. If the molecule is rotated clockwise, it should give the arrangement of atoms that it gives when the molecule is rotated anticlockwise. That way, the R and S isomerism is canceled, making the compound optically inactive.

Figure 2: A Meso Compound

The above image shows a meso compound. Here, the compound has two asymmetrical chiral centers. These are the carbon atoms attached to the two chlorine atoms. There is a plane of symmetry. One-half of the molecule with one chlorine atom is superimposable with the other half with the other chlorine atom. Most importantly, rotating the molecule clockwise direction, gives the arrangement of the molecule that is given when it is rotated anticlockwise.

Similarities Between Achiral and Meso

Both types of compounds have a plane of symmetry.
Both compounds have superimposable mirror images.

Definition

Achiral: Achiral compound has no chiral centers and has a superimposable mirror image.

Meso: A meso compound is composed of multiple chiral centers but it has a superimposable mirror image.

Presence of Chiral Centers

Achiral: There are no chiral centers in achiral compounds.

Meso: There are multiple chiral centers in meso compounds.

Inversion Center

Achiral: Achiral compounds can have inversion centers.

Meso: Meso compounds do not have inversion centers.

Conclusion

Achiral compounds and meso compounds are related to each other in some properties but have some different properties as well. The main difference between achiral and meso is that achiral compounds have no chiral centers whereas meso compounds are intermediate to chiral and achiral compounds.

References:

1. “Definitions: Achiral.” Chem Ed DL, Available here. Accessed 11 Sept. 2017.
2. “Meso Compounds.” Chemistry LibreTexts, Libretexts, 21 July 2016, Available here. Accessed 11 Sept. 2017.

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