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You are here: Home / Organic Chemistry Introductory Topics / Organic Chemistry: Meso Compounds and Maximum Possible Numbers of Enantiomers

Organic Chemistry: Meso Compounds and Maximum Possible Numbers of Enantiomers

May 11, 2019 By Sean Chua Leave a Comment

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:

4 possible enantiomers of 2,3-dichlorobutane are drawn using 3D notations

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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PS: Under related articles below, there are several blog post discussions and questions related to Electrophilic Substitution Reactions in Organic Chemistry. You can also do a keyword search using the search box at the top right hand corner.

Related Articles:

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  • Organic Chemistry: Total Number of Stereoisomers
  • Organic Chemistry: Optical Activity & Plane-Polarised Light
  • Organic Chemistry: Cis-Trans (Geometric) Isomerism
  • Organic Chemistry: Optical Isomerism
Sean Chua
Sean Chua
Winners Education Centre Pte. Ltd.
Many know me as the Ten Year Series book author for JC A-Level H2 Chemistry and O-Level Pure Chemistry. My 19+ years of coaching experience (since 1999) with more than 1500 students from 180+ JCs and Secondary Schools has allowed me to understand the true reasons why students are not able to perform well in Chemistry. Most importantly, my strength lies in using everyday analogies (even grandmothers can understand!) to simplify abstract concepts. Also, my teaching methodology has been designed to be fun, enjoyable and effective for my students.

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Filed Under: Organic Chemistry Introductory Topics Tagged With: Achiral, Chiral, Chiral Centre, Enantiomerism, Enantiomers, Isomerism, Meso Compounds, Optical Activity, Optical Isomerism, Optical Isomers, Organic Chemistry

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