In chemistry, compounds having two or more chiral centers or a plane of symmetry are called meso compounds. They are symmetrical achiral molecules. The term “meso” comes from the Greek word μέσος, meaning “middle” because meso compounds are intermediate between chiral and achiral. They have multiple chiral centers such that, one of the chiral centers has an R configuration while the other has an S configuration. In addition, mesocompounds are also optically inactive because equal but opposite rotations are canceled out in a molecule. Moreover, they are superimposable mirror images.
Meso compounds have sp3 hybridized carbons bonded to four different groups but still, they are achiral. This is due to the internal plane of symmetry. This plane of symmetry divides them into two equal halves. These halves are mirror images of each other. Thus, they are categorized as a subtype of stereoisomerism.
Further, they are important in scientific research in medicines as they can be converted into chiral compounds, which are often more biologically active or chemically more useful than their achiral counterparts. Common examples include meso-tartaric acid and meso-2,3-butanediol.
Characteristic of mesocompounds
- Meso compounds are achiral molecules.
- They have multiple(two or more) chiral centers.
- They have an internal plane of symmetry.
- This internal plane of symmetry divides mesocompounds into two equal halves.
- They are superimposable mirror images.
- They have equal but opposite optical activities that’s why these are optically inactive.
- They may have the same or different configurations at chiral centers.
When two chiral centers are present then the meso compound has four possible configurations i.e,
- R-S configuration
- S-R configuration
- R-R configuration
- S-S configuration
For example, in the compound given below, there are two chiral centers (1R and 2S), but it has a plane of symmetry, therefore this molecule is optically inactive.
The main differences between enantiomers and meso compounds
Difference between enantiomer and meso compound
Enantiomers are non-superimposable mirror images.
These are superimposable on their mirror image.
They are optically active molecules.
They are optically inactive molecules
Enantiomers are chiral in nature.
Meso compounds are achiral in nature.
They do not have a plane of symmetry.
Meso compounds have an internal plane of symmetry.
Tartaric acid is a common example. It has two chiral centers but because of the plane of symmetry, it is an achiral compound. Both carbon atoms are tetrahedrally bonded to the same substituent. Because of this, these structures are superimposable in their mirror images.
Why mesocompounds are optically inactive?
They are optically inactive because of equal but opposite rotation of plane-polarized light by two isomers.
What makes a compound meso?
Multiple chiral centers and the presence of a plane of symmetry makes a compound meso in nature.
Are diastereomers meso isomers?
No, diastereomers are not meso isomers.
When do diastereomers become mesocompounds?
When diastereomers have an internal plane of symmetry they become meso diastereomers.
How do you know if a compound is meso?
When compounds have chiral centers and a plane of symmetry then it is meso compounds.
Differentiate between meso compounds and racemic mixture?
Racemic mixtures have an equal amount of chiral molecules(enantiomers) whereas meso compounds have an equal amount of achiral molecules.
Differentiate between meso compounds and diastereomers?
Meso compounds are super impossible mirror images with an internal plane of symmetry whereas diastereomers are non-superimposable non-mirror images.
- Essentials of Organic Chemistry by Paul M Dewick.
- The tenth edition of Organic chemistry by Francis A. Carey and Robert M. Giuliano.
- Chapter 7 (chem.ucalgary.ca)