What is a meso compound and why it is optically inactive
Meso compounds are symmetric or achiral compounds that have stereocenters. A meso compound, despite having two or more stereocenters, is optically inactive and possesses an inner symmetrical plane that allows it to be superimposed on its mirrored counterpart. Mesomers, therefore, are organic compounds with comparable chiral carbons on both sides, giving in zero net rotation.
Meso Compounds
What are Meso Compounds?
Meso compounds are achiral compounds with multiple chiral centres. It is optically inactive despite possessing stereocenters since it is superimposed on its mirror counterpart. It also possesses an inner plane of symmetry.
A meso compound or meso isomer is a stereoisomer that has at least two optically active members but is otherwise inactive. Despite having two or more chiral carbon centres, this indicates that the compound is not chiral.
Identification of Meso Compounds
A symmetrical plane within the compound separates it in half. The inner mirror allows these two sides to reflect one another. In meso compounds, the stereochemistry around chiral centres must "cancel out." This indicates that when a compound is divided into two symmetrical halves by an internal plane, the stereochemistry of the left and right sides should be in opposition to one another and result in an optically inactive compound.
Let ‘A’ be a meso compound. We know that a meso compound must include R and S stereochemistry, two or more stereocenters, and an inner plane.
To find out if ‘A’ is meso, we must first search for an inner plane or inner mirror.
When evaluating if a chemical is a meso compound or not, stereochemistry is extremely important. Because a meso compound is optically inert, its stereochemistry tends to cancel out. If a meso compound has two stereocenters (R and S), then R must cancel out S.
Note:
Covalent bonds or \[s{{p}^{3}}\] orbitals have an interesting property that we can twist the substituted groups linked to a stereocenter to identify the inner plane. The stereochemistry of the compound does not vary as it is rotated.
The other scenario is when the entire structure is rotated by 180 degrees. Both of the components are still meso.
Meso Compounds Examples
As noted previously, a meso compound must have at least two chiral \[s{{p}^{3}}\] hybridized atoms and at least one inner plane that divides the molecule into two mirror images. Tetrahedral centres are present, which prevents the molecule from being planar. An achiral compound won't rotate planar polarized light even if it just contains one element of symmetry. Meso compounds lack optical activity because of this.
Exception: The nitrogen atom in tertiary amine is not \[s{{p}^{3}}\] hybridized, even though the compound is meso.
1. This compound is achiral because it possesses a plane of symmetry. But because it contains two chiral carbon atoms, it is a meso compound.
Example for Meso Compounds
2. This molecule has two chiral sites and is achiral since it possesses a symmetrical plane. It is a meso compound as a result.
Example for Meso Compounds
Optical Activity of Meso Compounds
A meso compound or meso isomer is one of a group of stereoisomers that has at least two optically active members, although none of them is optically active. This indicates that despite having two or more chiral centres, the compound is not chiral. These substances are achiral compounds with several chiral centres. Despite having stereo centres, it is overlaid on its mirror counterpart and optically inert. It possesses a plane of inner symmetry that splits the molecule into halves. These two sides mirror one another through the inner mirror.
Stereochemistry in the stereocenter can balance out. This implies that the stereochemistry of the left and right sides should be opposed to one another if the molecule is divided into two symmetrical halves by an inner plane. Hence, the outcome is optically inactive. Technically, cyclic molecules are also meso. Since optical activity cancels out in the presence of a symmetrical plane, meso compounds lack optical activity.
Key Features
In this article, we have studied meso compounds, meso compound examples, and meso compound definitions.
Meso Compounds are non-optically active compounds, with at least two or more stereocenters
Meso Compounds examples include pentane, butane, etc.
Meso compounds can be identified by two or more stereocenters, an internal plane, and stereochemistry should be R or S.
Interesting Facts
Meso Compounds are optically inactive even after having two or chiral centres.
Many radioactive elements glow in the dark.
Helium balloons are lighter than air, that's why they float in the air.
Important Questions
1. What are enantiomers?
Enantiomers are defined as pairs of molecules that exist in two forms and both are non-superimposable mirror images of each other. Both enantiomers are chemically identical to each other.
2. Is chiral a meso compound?
A meso compound is an achiral compound with two or more chiral centres. A plane of symmetry is present in all meso compounds. The internal plane of symmetry makes a superimposable mirror image.
3. How do you know if a compound is meso?
There are certain characteristics and features of meso compounds. These are the presence of an internal plane of symmetry, two or more chiral centres or stereocenters, and R or S configuration.
FAQs on Meso Compound in Organic Chemistry
1. What is a meso compound in chemistry?
A meso compound is an optically inactive molecule that contains two or more chiral centers but has an internal plane of symmetry. This internal symmetry makes the molecule superimposable on its mirror image despite having stereocenters. As a result, meso compounds do not rotate plane-polarized light. They are a special case in stereochemistry where chirality is canceled internally.
2. Why are meso compounds optically inactive?
Meso compounds are optically inactive because their internal plane of symmetry causes the optical rotations of chiral centers to cancel each other. Even though they contain stereocenters, the molecule as a whole is achiral. The equal and opposite configurations (R and S) within the same molecule lead to zero net rotation of plane-polarized light.
3. How do you identify a meso compound?
A meso compound is identified by checking for multiple chiral centers and an internal plane of symmetry. To identify one:
- Confirm the presence of at least two chiral centers.
- Check for an internal plane of symmetry.
- Ensure the molecule is superimposable on its mirror image.
- Verify that it is optically inactive despite stereocenters.
If all these conditions are met, the compound is meso.
4. Can a molecule with only one chiral center be a meso compound?
No, a molecule with only one chiral center cannot be a meso compound because meso compounds require at least two stereocenters. A single chiral center makes a molecule inherently chiral unless symmetry eliminates chirality, which is not possible with only one stereocenter.
5. What is an example of a meso compound?
A classic example of a meso compound is meso-tartaric acid with the formula C4H6O6. It contains two chiral carbon atoms but has an internal plane of symmetry. The R and S configurations on adjacent carbons cancel each other’s optical activity, making it optically inactive.
6. What is the difference between a meso compound and a racemic mixture?
The key difference is that a meso compound is a single molecule with internal symmetry, while a racemic mixture is a 1:1 mixture of two enantiomers. In a meso compound, optical inactivity is due to internal cancellation within one molecule. In a racemic mixture, optical inactivity results from equal and opposite rotation of two separate enantiomers.
7. Do meso compounds have stereocenters?
Yes, meso compounds have two or more stereocenters, but they remain achiral due to internal symmetry. The stereocenters typically have opposite configurations (such as R and S), which results in internal compensation of optical rotation.
8. How does a plane of symmetry affect meso compounds?
An internal plane of symmetry divides a meso compound into two mirror-image halves, making the molecule superimposable on itself. This symmetry causes the molecule to be achiral even though it contains stereocenters. The presence of this plane is the defining feature of meso compounds in stereochemistry.
9. Are all compounds with two chiral centers meso compounds?
No, not all compounds with two chiral centers are meso compounds because they may lack internal symmetry. If there is no plane of symmetry, the molecule will exist as a pair of enantiomers and be optically active. Only those with symmetrical arrangement of substituents qualify as meso compounds.
10. Why are meso compounds important in stereochemistry?
Meso compounds are important because they demonstrate that the presence of chiral centers does not always mean a molecule is chiral. They help explain concepts such as chirality, optical isomerism, internal compensation, and stereochemical symmetry. Understanding meso compounds prevents errors when predicting the number of stereoisomers.
