What is a Meso Compound?
Those achiral compounds which have chiral centers are called meso compounds. The reason for achirality is the internal line of symmetry, the molecule is symmetrical.
And we know that symmetrical molecules are achiral. Their mirror image is precisely the same as the original molecule. That means their mirror image is superimposable.
You might be wondering what a meso compound looks like or how you can tell if a compound is a meso compound. Well, meso compounds may be cyclic or acyclic. Read on if you want to clear all your doubts about the concept of meso compounds.
How to identify the Meso Compounds?
Identifying the meso compound in a reaction can be the trickiest question if you don’t know the tips for identifying the meso compound. The most important identification of a meso compound is its symmetry.
If a compound can be divided into two equal parts, then it is a meso compound. The second tip for identification is that the mirror image of a meso compound is not inverted and both R & S configurations are present in the structure.
So if you are not able to locate the internal symmetry in a compound then check its mirror image and stereochemistry.
Stereo Center of a Meso Compound:
A stereo center is any central atom of a compound that is attached to either two or four different groups. A stereo center may be carbon or any other central atom and it gives the stereoisomers by interchanging the groups attached to it.
All chiral compounds contain a stereo center but all compounds with a stereo center are not chiral. Although meso compounds are achiral, they also contain a stereocenter. Meso compounds contain two or more stereocenters. Let us see the following example:
The above compound is a meso compound with two carbon chiral centers. This compound's mirror image is identical to the original compound, therefore it may be superimposed on its mirror image. Also, there are four different groups on each central carbon hence these are the stereocenters.
Meso Compounds are Achiral due to the Following Reason:
Meso compounds are achiral because they are symmetrical and possess an internal plane of symmetry. As you can see in the above example, if you divide the compound from the center then both halves would be identical which means the molecule is symmetrical.
The stereochemistry of both halves would be opposite to each other i.e., one is R and the other is S. All meso compounds must contain at least one plane of symmetry in their structure.
Optical Activity of Meso Compounds:
They contain both configurations (R & S) in one structure so what do you expect to be the optical activity of meso compounds? Experimental results show that meso compounds do not reflect the plane polarized light and so are optically inactive.
Both configurations cancel each other’s effect and there is no reflection as a whole in the polarimeter. This cancellation of effect is also called internal compensation.
Meso compound can be an isomer of another compound. For example; tartaric acid has three isomers, two are enantiomers and one is a meso compound. The enantiomer forms of tartaric acid would show optical activity while the meso form of tartaric acid would not show any optical activity.
Possible Stereoisomers of Complex Compounds:
Stereoisomers are formed by the rearrangement of groups attached to the same stereo center. In a stereoisomer, the plain of certain groups is changed by rotating the stereo center to 180°.
The group that is towards the plain in one structure might be away from the plain in the second structure. The possible number of stereoisomers in a meso compound is three. Two of which are enantiomers of each other (one is S and the other is R) and the third is both R & S. The third stereoisomer is the achiral meso form but possesses two chiral centers.
Examples of Meso Compounds:
The simple 2n rule does not always apply to complex molecules. Most complex compounds possess meso forms in their stereoisomers which are asked to identify in exams. Let us draw the stereoisomers of some complex compounds for practice.
There are seven possible isomers of dichlorocyclopentane i.e., 1-1, 1-2 (R, R), 1-2 (S, S), 1-2 (R, S), 1-3 (R, R), 1-3 (S, S), and 1-3 (R, S). The meso forms are present only in 1-2 and 1-3 dichlorocyclobutane but here we would discuss only 1-2, dichlorocyclopentane.
If you draw all these isomeric structures then you would notice that 1-2 (R, S) and 1-3 (R, S) show an internal plane of symmetry.
1-2, dichlorocyclopentane contains two chiral centers and so the estimated number of stereoisomers would be four. Following are the possible stereoisomers of 1-2, dichlorobutane:
The structure A and B are the superimposable mirror image of each other and C & D are the enantiomers of each other. Hence, the 1-2, dichlorocyclopentane contains three stereoisomers from which one is meso form and two are enantiomers of each other. So we can say that 1-2, dichlorocyclopentane is a meso compound.
2-3, Dichlorobutane [H2Cl2(CH3)2]:
It is an important halo alkane used in the synthesis of polymers. Dichlorobutane is prepared by the chlorination of n-butane. It contains two chiral centers and two stereocenters. Now let us draw the possible stereoisomers of dichlorobutane and identify the meso compound if any.
Above are the stereoisomers of dichlorobutane where B & C are the mirror image of each other and are not superimposable. Hence, B & C are the two enantiomers. Structures C & D are mirror images that are superimposable on each other.
It also contains an internal plane of symmetry in it so it is the meso form of dichlorobutane. Therefore, 2-3, dichlorobutane contains three forms of stereoisomers out of which two are enantiomers(R enantiomer and S enantiomer) and the third is meso form.
(2R,3S)-3-bBromo-2-Butanol is a Meso Form of 3-2, Bromobutanol:
As you can see it contains both R and S configurations within one structure and it also possesses an internal plane of symmetry. So we can say that it is a meso compound. following is the structure of (2R,3S)-3-Bromo-2-butanol:
Frequently Asked Questions:
Do meso compounds have enantiomers?
Meso compounds contain two enantiomers that are mirror images of each other but are not superimposable.
How many chiral centers are present in meso compounds?
Meso compounds possess multiple chiral centers and stereocenters. A compound must contain at least two chiral centers to be a meso compound.
How do I calculate the number of optical isomers of a compound?
One can calculate the number of optical isomers of a meso compound using the formula 2n where “n” is the number of chiral centers present in the compound. For example, if a compound contains 2 chiral centers then it would possess 4 optical isomers.
Do meso compounds have enantiomers?
Yes, they contain at least two enantiomers with opposite configurations while the third form is the meso form. The mirror image of the meso form is the same so it is considered one structure.
Is 3 dimethyl hexane a meso compound?
It contains 2 stereo centers and four possible stereoisomers. The 3R-4S, dimethyl hexane isomer is the meso form of dimethyl hexane so it would show three types of stereoisomers, out of which two are enantiomers and the third is a meso compound.
Can we call 1 4-dimethyl cyclohexane a meso compound?
No, it is not a meso compound because it does not contain any chiral center. However, 1 3-dimethyl cyclohexane may possess a meso form.
Is 2R-3R, dibromobutane a meso compound?
It contains two chiral centers and both show R configuration. Its mirror image is not superimposable on each other and so these are enantiomers not meso compounds. Only the 2-3 (R, S) form would be meso.