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4.3 Diastereomers

Diastereomers are stereoisomers that are not related as object and mirror image and are not enantiomers. Unlike enatiomers which are mirror images of each other and non-sumperimposable, diastereomers are not mirror images of each other and non-superimposable. Diastereomers can have different physical properties and reactivity. They have different melting points and boiling points and different densities. They have two or more stereocenters.

Introduction

It is easy to mistake between diasteromers and enantiomers. For example, we have four steroisomers of 3-bromo-2-butanol. The four possible combination are SS, RR, SR and RS (Figure 1). One of the molecule is the enantiomer of its mirror image molecule and diasteromer of each of the other two molecule (SS is enantiomer of RR and diasteromer of RS and SR). SS's mirror image is RR and they are not superimposable, so they are enantiomers. RS and SR are not mirror image of SS and are not superimposable to each other, so they are diasteromers.

chem (1).png

Figure 1

 

Diastereomers vs. Enantiomers vs. Meso Compounds

Tartaric acid, C4H6O6, is an organic compound that can be found in grape, bananas, and in wine. The structures of tartaric acid itself is really interesting. Naturally, it is in the form of (R,R) stereocenters. Artificially, it can be in the meso form (R,S), which is achiral. R,R tartaric acid is enantiomer to is mirror image which is S,S tartaric acid and diasteromers to meso-tartaric acid (figure 2).

(R,R) and (S,S) tartaric acid have similar physical properties and reactivity. However, meso-tartaric acid have different physical properties and reactivity. For example, melting point of (R,R) & (S,S) tartaric is about 170 degree Celsius, and melting point of meso-tartaric acid is about 145 degree Celsius.

chem1 (1).bmp

Figure 2

 

To identify meso, meso compound is superimposed on its mirror image, and has an internal plane that is symmetry (figure 3). Meso-tartaric acid is achiral and optically unactive.

meso.bmp

Problems

Identify which of the following pair is enantiomers, diastereomers or meso compounds.

practice.bmp

Answer

  1. Diasteromers
  2. Identical
  3. Meso
  4. Enantiomers

4.3.1 Different Types of Isomers

4.3.2 Meso Compounds

Meso compounds are achiral compounds that has multiple chiral centers. It is superimposed on its mirror image and is optically inactive despite its stereocenters.

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.

Example

This molecule has a plane of symmetry (the horizontal plane going through the red broken line) and, therefore, is achiral; However, it has two chiral carbons and is consequentially a meso compound.

Example 2

This molecules has a plane of symmetry (the vertical plane going through the red broken line perpendicular to the plane of the ring) and, therefore, is achiral, but has has two chiral centers. Thus, its is a meso compound.

Other Examples of meso compounds

Meso compounds can exist in many different forms such as pentane, butane, heptane, and even cyclobutane. They do not necessarily have to be two stereocenters, but can have more.

 

Meso3 (1).bmp

Cyclic.bmp

Optical Activity Analysis

When the optical activity of a meso compound is attempted to be 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

Further Reading on Diastereomers

MasterOrganicChemistry

        On Cats: Enantiocats vs. Diastereocats

Khan Academy

          Diastereomers

Cliffs Notes

          Enantiomers and Diastereomers

Web Pages

        WikiBooks Diastereomers

          Diastereomer Definition

          Sparknotes Diastereomers

Different types of Isomers

Carey 4th Edition On-Line Activity

        Isomer Types

Khan Academy

Stereoisomers, Enantiomers, Diastereomers, Constitutional Isomers and Meso Compounds

Web Pages

          Detailed Link on Isomers

Tutorial

        Identifying Isomers

Meso Compounds

MasterOrganicChemistry

        On Cats: Moe the Meso Cat

          The Meso Trap

Khan Academy

          Meso Compounds