Difference Between Meso and Enantiomers

Main Difference – Meso vs Enantiomers

Generally, isomers are molecules that have the same chemical formula but a different arrangement of its atoms. Isomers are mainly grouped into two broad categories: structural isomers and stereoisomers. Structural isomers have the same molecular formula, but the atoms are connected in different ways. Stereoisomers are molecules that differ in the spatial arrangement of their structures. For a molecule to have stereoisomers, the molecule should essentially be chiral. In order to have chirality, molecules must have four different groups attached to a central carbon. These groups should be chemically distinguishable. Stereoisomers are again divided into two groups known as enantiomers and diastereomers. Enantiomers are organic molecules that are non-superimposable mirror images. This means that the spatial arrangement of one enantiomer molecule looks like a mirror image of the other enantiomer of it. Meso compounds are molecules having multiple stereocenters, but their mirror images are superimposable. The main difference between meso compound and enantiomers is that meso compounds have an identical mirror image whereas enantiomers have a non-superimposable mirror image.

Key Areas Covered

1. What is Meso
      – Definition, Properties
2. What are Enantiomers
      – Definition, Properties
3. What is the Difference Between Meso and Enantiomers
      – Comparison of Key Differences

Key Terms: Chirality, Enantiomers, Diastereomers, Meso Compound, Mirror Image, Stereoisomers, Stereocenter

What is Meso

A meso compound is a molecule having more than one identical stereocenter and an identical or superimposable mirror image. Therefore, a meso compound has many chiral carbon centers, but the mirror image is superimposable. A meso compound also has an internal plane of symmetry that divides the molecule into two halves. These two halves are mirror images. Hence, meso compounds are optically inactive.

Figure 1:  Cyclic Meso Compounds – cyclohexane

A meso compound is achiral. Thus, it cannot have an enantiomer. This is because when a molecule is superimposable with its mirror image, the molecule and that mirror image are merely the same. The above image shows the cyclohexane molecule which is a meso compound.

What are Enantiomers

Enantiomers are optical isomers that are non-superimposable mirror images of each other. Therefore, these molecules are always found in pairs. Since they are non-superimposable, the two molecules are not identical. But the molecular formula of the two enantiomers is the same. They are different from each other according to the spatial arrangement of the molecules.

Enantiomers have the same physical and chemical properties except for the direction they rotate the plane polarized light. They rotate plane polarized light in opposite directions. Therefore, a mixture of enantiomers having equal amounts of two enantiomers will not show a net rotation in the plane polarized light. This type of mixture is called a racemic mixture.

Figure 2: Above Molecules are Enantiomers

Enantiomers have chiral carbon atoms. A chiral carbon is a carbon center that is bonded to four different atoms or groups. The presence of a chiral carbon (in a molecule) is called chirality. In order to become an enantiomer, two molecules should have different configurations at every chiral carbon. For example, if a molecule has two chiral carbons and another molecule has the same molecular formula with two chiral carbons, the two molecules should be different at both chiral carbons, not just at one chiral carbon.

Difference Between Meso and Enantiomers

Definition

Meso: A meso compound is a molecule having more than one identical stereocenters and an identical or superimposable mirror image.

Enantiomers: Enantiomers are optical isomers that are non-superimposable mirror images of each other.

Symmetry

Meso: Meso compounds are symmetric.

Enantiomers: Enantiomers can be symmetric or asymmetric.

Internal Mirror Plane

Meso: Meso compounds have an internal mirror plane from which the molecule can be divided into two halves, one half being a mirror image of the other half.

Enantiomers: Enantiomers have not internal mirror plane.

Mirror Images

Meso: Meso compounds have an identical or superimposable mirror image of the molecule.

Enantiomers: Enantiomers have non-superimposable mirror image.

Chirality

Meso: Meso compounds are achiral.

Enantiomers: Enantiomers are chiral.

Conclusion

Meso compounds and enantiomers are organic compounds containing stereocenters. They are different from each other in many aspects. The main difference between meso compound and enantiomers is that meso compounds have an identical mirror image whereas enantiomers have a non-superimposable mirror image.

Reference:

1. “Meso Compounds.” Chemistry LibreTexts, Libretexts, 6 Nov. 2017, Available here.
2. “Meso compound.” Wikipedia, Wikimedia Foundation, 14 Dec. 2017, Available here.
3. Hunt, Ian R. Ch 7: Enantiomers. Available here.

Image Courtesy:

1. “Meso 12 cyhexane” By Quantockgoblin – Own work (Public Domain) via Commons Wikimedia
2. “(±)-Flephedrone 4-isomer Enantiomers Structural Formulae” By Jü – Own work (Public Domain) via Commons Wikimedia