Organic Chemistry: Meso Compounds and Maximum Possible Numbers of Enantiomers

Meso Compounds

Meso compounds are achiral molecules that possess multiple chiral centres. The reason for them being achiral is because they possess an internal plane of symmetry (also known as centre of symmetry). This internal plane of symmetry cause their mirror images to be superimposable.

Meso compound is therefore optically inactive. The optical activity of one half of the meso molecule cancels out the optical activity of the other half of it.

Maximum Possible Number of Enantiomers (Optical Isomers)

Maximum possible number of enantiomers = 2ⁿ whereby n is the number of chiral centres in the molecule

Note that the above formula gives the maximum possible number of enantiomers!

To see whether it also gives you the actual number of enantiomers, you need to draw out all the possible isomers and then check whether there are any identical isomers. The identical isomer is known as the meso compound which is optically inactive and will not be considered as an enantiomer (optical isomer).

Let’s take a look at an example to have a clearer picture on finding the actual number of enantiomers.

Example: 2,3-dichlorobutane.

Number of chiral centres in molecule, n = 2

Maximum possible number of enantiomers = 2ⁿ = 2² = 4

The 4 possible enantiomers which can be easily drawn out using 3D notations are:

Molecule A and molecule B are a pair of enantiomers (optical isomers). There is no internal plane of symmetry. Both molecules are chiral and are optically active.

Due to the internal plane of symmetry, molecule C is identical to its mirror image, molecule D. Molecule D is a meso isomer of molecule C, and is therefor achiral and optically inactive. The optical activity of one half of the meso molecule cancels out the optical activity of the other half of it.

As such, we have the following isomers for 2,3-dichlorobutane:

• A pair of enantiomers i.e. molecules A and B, which are optically active
• A meso compound i.e. molecule C (or D), which is optically inactive

Now, since you have come so far, how about trying out one question yourself. Leave your answer is the Comments Section below.

Question:

Determine the number of isomers for 2,2-dichlorobutane, which is a positional isomer for 2,3-dichlorobutane.

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• Organic Chemistry: Optical Isomerism

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