Molecular symmetry meso compounds

The reason that the third stereoisomer is achiral is that the substituents on the two asymmetric carbons are located with respect to each other in such a way that a molecular plane of symmetry exists. Compounds that incorporate asymmetric atoms but are nevertheless achiral are called meso forms. This situation occurs whenever pairs of stereogenic centers are disposed in the molecule in such a way as to create a plane of symmetry. A... 

The condition that makes possible the existence of meso compounds is an appropriate degree of molecular symmetry. There are several kinds of such molecular symmetry. In the case of projection formulas 25 (or 26) there is a plane of symmetry, which means that a plane can be placed through the molecule such that one half of the molecule is a mirror image of the other half The mirror plane for meso-tartaric acid can be seen easily from its projection formulas 25b and 26b. These two formulas are superimposable if one is rotated 180° in the plane of the paper. 

Strategy To decide if a structure represents a meso compound, try to locate a plane of symmetry that divides the molecule into two halves that are mirror images. Molecular models are always helpful. 

The term meso is commonly used to designate an achiral structure that is a diastereomer of one or more chiral structures. A meso compound contains chiral substructures but is not itself chiral because of overall molecular symmetry. Such structures often— but not always— have a plane of symmetry in at least one conformation, as is illustrated for 73 and 74. These structures are said to be internally compensated An example of a meso structure that does not have a plane of symmetry is the l,4-dichloro-2,5-difluorocyclohexane 75, which is achiral because it has a center of symmetry. Structure 75 is a diastereomer of the enantiomeric pair 76 and 77. 

In general, compounds which contain asymmetric carbon atoms rotate the plane of polarization of plane-polarized light. For this reason they are said to be optically active. When the molecular symmetry is such that the optical activity of one portion of the molecule is cancelled by that of the second portion of the molecule, the compounds are said to be internally compensated and are called meso compounds. The tartaric acid with the formula (X) is such a compound and has been known as the meso-tartaric acid. The tartaric acids identified as (VIII) and (IX) have been known as d-tartaric acid and Z-tartaric acid because of the sign of their optical rotations (dextro and levo, respectively). (The nomenclature of these acids is discussed later in this chapter.) The compounds (VIII) and (IX) are non-superimposable mirror images, called enantiomorphs. The existence of such pairs of asymmetric isomers is the fundamental basis of optical activity. The asymmetry may be in either the molecular structure or the crystal structure. Asymmetric carbon atoms are not always present in optically active molecules. 

In Summary Meso compounds are diastereomers containing a molecular plane of symmetry. They are therefore superimposable on their mirror images and achiral. Molecules with two or more identically substituted stereocenters may exist as meso stereoisomers. 

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