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ChemWiki: The Dynamic Chemistry E-textbook > Organic Chemistry > Chirality > Meso Compounds

Meso Compounds

A meso compound is superimposed on its mirror image. It is optically inactive although it contains two or more stereocenters. All meso compounds are achiral.

Introduction

In general, a meso compound should contain two or more identical substituted stereocenters. Also, it has an internal symmetry plane that divides the compound in half. These two halves reflect each other by the internal mirror. The stereochemistry of stereocenters should "cancel out". What it means here is that when we have an internal plane that splits the compound into two symmetrical sides, the stereochemistry of both left and right side should be opposite to each other, and therefore, result in optically inactive. Cyclic compounds may also be meso.

Identification

If A is a meso compound, it should have two or more stereocenters, an internal plane, and the stereochemistry should be R and S.

  1. Look for an internal plane, or internal mirror, that lies in between the compound.
  2. The stereochemistry (e.g. R or S) is very crucial in determining whether it is a meso compound or not. As mentioned above, a meso compound is optically inactive, so their stereochemistry should cancel out. For instance, R cancels S out in a meso compound with two stereocenters.

       Meso1 (1).bmp

trans-1,2-dichloro-1,2-ethanediol

Meso2 (5).bmp

(meso)-2,3-dibromobutane

 

Tips: An interesting thing about single bonds or sp3-orbitals is that we can rotate the substituted groups that attached to a stereocenter around to recognize the internal plane. As the molecule is rotated, its stereochemistry does not change. For example:

             rotated (1).bmp

Another case is when we rotate the whole molecule by 180 degree. Both molecules below are still meso.

rotated180.bmp

Remember the internal plane here is depicted on two dimensions. However, in reality, it is three dimensions, so be aware of it when we identify the internal mirror.

Other Examples of meso compounds

Meso compounds can exist in many different forms such as pentane, butane, heptane, and even cyclobutane. They don't necessarily have to be two stereocenters.

Meso3 (1).bmp

Cyclic.bmp

Optical Activity Analysis

When the optical activity of a meso compound is determined with a polarimeter, the indicator will not show (+) or (-). It simply means there is no certain direction of rotation of the polarized light, neither levorotatory (-) and dexorotatory (+).

Problems

Beside meso, there are also other types of molecules: enantiomer, diastereomer, and identical. Determine if the following molecules are meso.

Mesopracticeprob.bmp

Answer key: A C, D, E are meso compounds.

References

  1. Vollhardt, K. P.C. & Shore, N. (2007). Organic Chemistry (5thEd.).  New York: W. H. Freeman. (190-192)
  2. Shore, N. (2007). Study Guide and Solutions Manual for Organic Chemistry (5th Ed.). New York: W.H. Freeman. (70-80)

Contributors

  • Duy Dang

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Last Modified
14:54, 18 Dec 2013

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