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5.7 Meso Compounds

  • Page ID
    44213
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    Objectives

    After completing this section, you should be able to

    1. determine whether or not a compound containing two chiral carbon atoms will have a meso form, given its Kekulé, condensed or shorthand structure, or its IUPAC name.
    2. draw wedge-and-broken-line structures for the enantiomers and meso form of a compound such as tartaric acid, given its IUPAC name, or its Kekulé, condensed or shorthand structure.
    3. make a general comparison of the physical properties of the enantiomers, meso form and racemic mixture of a compound such as tartaric acid.

    Key Term

    Make certain that you can define, and use in context, the key term below.

    • meso compound

    Study Notes

    You may be confused by the “Identical” label attached to the diagram in the middle of page 305 of the textbook. Of course the two structures shown are identical, they represent the same molecule looked at from two different perspectives. The key point here is that by rotating the 2R,3S stereoisomer by 180◦, you can see that it is identical to the 2S,3R stereoisomer shown in the preceding diagram. Do not neglect to make models of each of these stereoisomers to convince yourself that what you are reading is correct.

    A meso compound is an achiral compound that has chiral centers. It is superimposed on its mirror image and is optically inactive although it contains two or more 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 5.7.1

    mesocompound1.png

    1 has a plane of symmetry (the horizonatal plane going through the red broken line) and, therefore, is achiral; 1 has chiral centers. Thus, 1 is a meso compound.

    Example 5.7.2

    mesocompound2.png

    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.


    5.7 Meso Compounds is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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