Difference Between Racemic Mixture and Meso Compound

Main Difference – Racemic Mixture vs Meso Compound

The two terms racemic mixture and meso compound are used in organic chemistry to describe different organic compounds. A racemic mixture is also known as a racemate. It is a mixture of equal amounts of left and right-handed enantiomers. Enantiomers are optical isomers that are non-superimposable mirror images of each other. The two mirror images are called left-handed mirror image and the right-handed mirror image. Since they are non-superimposable, the two molecules are not identical. A meso compound is a stereoisomer with an identical (superimposable) mirror image. The main difference between racemic mixture and meso compound is that a racemic mixture contains non-identical isomers whereas a meso compound contains an identical isomer.

Key Areas Covered

1. What is a Racemic Mixture
– Definition, Composition, Resolution
2. What is a Meso Compound
– Definition, Explanation
3. What are the Similarities Between Racemic Mixture and Meso Compound
– Outline of Common Features
4. What is the Difference Between Racemic Mixture and Meso Compound
– Comparison of Key Differences

Key Terms: Enantiomer, Isomer, Racemate, Racemic Mixture, Stereoisomer

What is a Racemic Mixture

A racemic mixture is a mixture of organic compounds known as enantiomers. This mixture contains equal amounts of left-handed and right-handed enantiomers. Enantiomers are optical isomers that are non-superimposable mirror images of each other. They are non-identical because they are non-superimposable.

A racemic mixture is optically inactive due to the presence of equal amounts of non-superimposable mirror images. Since this mixture is optically inactive, there is no net rotation of plan-polarized light which is passed through a racemic mixture. Although the two types of enantiomers rotate the light in opposite directions, rotations are cancelled due to the presence of equal amounts of opposite enantiomers.

Difference Between Racemic Mixture and Meso Compound

Figure 1: Cetirizine Chemical Composition is a Racemate

Separation of a racemic mixture into the two forms of components is known as resolution. Enantiomers have identical physical properties such as melting point, boiling point, solubility, etc. Hence it is difficult to separate them into two fractions. Several methods can be used for this purpose. Some of them are crystallization methods, chromatography, use of enzymes, etc. The most common and easy way of doing this is converting the enantiomers into diastereomers. Unlike enantiomers, diastereomers have different physical properties. Therefore, physical methods such as crystallization can be used to separate them.

What is a Meso Compound

Ameso compound is a molecule having more than one identical stereocenters and an identical or superimposable mirror image. Therefore, ameso 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.

Main Difference - Racemic Mixture vs Meso Compound

Figure 2: Meso Compound (1R*,2S*)-1,3-dichlorohexane.

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.

Similarities Between Racemic Mixture and Meso Compound

Racemic mixture and meso compound are organic compounds.
Both are optically inactive.

Definition

Racemic Mixture: A racemic mixture is a mixture of organic compounds known as enantiomers.

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

Composition

Racemic Mixture: A racemic mixture contains non-identical isomers.

Meso Compound: A meso compound has identical mirror images.

Chirality

Racemic Mixture: A racemic mixture contains chiral compounds.

Meso Compound: Meso compounds are considered as achiral compounds.

Conclusion

Racemic mixtures are mixtures of organic compounds called enantiomers. A racemic mixture contains equal amounts of opposite enantiomers. Meso compounds are another class of organic compounds. A meso compound contains an identical mirror image. The main difference between a racemic mixture and a meso compound is that a racemic mixture contains non-identical isomers whereas a meso compound contains an identical isomer.

Reference:

1. “5.8 Racemic Mixtures and the Resolution of Enantiomers.” Chemistry LibreTexts, Libretexts, 20 Dec. 2017, Available here.
2. “Meso compound.” Wikipedia, Wikimedia Foundation, 12 Jan. 2018, Available here.

Image Courtesy:

1. “Cetirizine structure racemic” By Acdx – Own work, based on Image:Cetrizine Enantiomers Structural Formulae.png (CC BY-SA 3.0) via Commons Wikimedia
2. “Meso compounds” By FlyScienceGuy – Own work (CC BY-SA 4.0) via Commons Wikimedia