Achiral molecules meso forms

Achiral molecules that contain chirality centers are called meso forms Meso forms typically contain (but are not limited to) two equivalently substituted chirality centers They are optically inactive... 

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... 

Only three, not four, stereoisomeric 2,3-butanediols aie possible. These three aie shown in Eigure 7.10. The (2R,3R) and (2.S,3.S) fonns aie enantiomers of each other and have equal and opposite optical rotations. A third combination of chirality centers, (2R,3S), however, gives an achiral structure that is superimposable on its (2S,3R) minor image. Because it is achiral, this third stereoisomer is optically inactive. We call achiral molecules that have chirality centers meso forms. The meso form in Eigure 7.10 is known as iwe50-2,3-butanediol. 

If the problem were to partition a set of carbon compounds into two equivalence classes, of which one contains only chiral molecules and the other one only achiral ones, it could not be solved with the criterion of asymmetric C-atoms. In the first case, one would assign meso-forms like 9 and compounds with pseudo-asymmetric 22> C-atoms, such as 11, to the chiral equivalence class, and in the second, chiral molecules like 12 would remain in the achiral subset. However, the latter class would be devoid of chiral molecules, if the compounds under consideration have been confined to molecules with free rotation about all C—C bonds. 

Section 7.11 Achiral molecules that contain chirality centers are called meso forms. 

Ribitol is a meso form it is achiral and thus not optically active. A plane of symmetry passing through C-3 bisects the molecule. 

Diastereoisomerism is encountered in a number of cases such as achiral molecules without asymmetric atoms, chiral molecules with several centers of chirality, and achiral molecules with several centers of chirality (meso forms). Such cases can be encountered in acyclic and cyclic molecules alike, but for the sake of clarity these two classes of compounds will be considered separately. 

The 2R,3S and 2S,3R structures are identical because the molecule has a plane of symmetry and is therefore achiral. The symmetry plane cuts through the C2-C3 bond, making one half of the molecule a mirror image of the other half (Figure 9.11). Because of the [ lane of symmetry, the molecule is achiral, despite the fact that it has two chirality centers. Compounds that are achiral, yet contain chirality centers, are called meso (me-zo) compounds. Thus, tartaric acid exists in three stereoisomeric forms two enantiomers and one meso form. 

So far we have looked largely at substrates which are flat achiral objects that have chiral centres introduced by means of a reagent. Desymmetrisations are slightly different. Molecules which are desymmetrised tend to be of a meso type. That is, they are achiral because they have a mirror plane and the sides of the molecule contain left and right handed portions 219. This is in contrast to the C2 axis present in many catalysts such as the TADDOLate 218. Desymmetrisations are powerful because there may be several chiral centres embedded in an achiral molecule which suddenly become much more useful in the newly formed chiral molecule. There are a large variety of symmetrical substrates that have been enantioselectively desymmetrised48 and we look at a few of the more important classes. 

More elaborate molecules can also have a plane of symmetry. For example, there are only three stereoisomers of tartaric acid (2,3-dihydroxybutanedioic acid). Two of these are chiral but the third is achiral. In the achiral stereoisomer, the substituents are located with respect to each other in such a way as to generate a plane of symmetry. Compounds that contain two or more stereogenic centers but have a plane of symmetry are called meso forms. Because they are achiral, they do not rotate plane polarized light. Note that the Fischer projection structure of meio-tartaric acid reveals the plane of symmetry. 

O-AcetylsaUcylic acid. See Aspirin Achiral molecules, 260, 290 meso forms, 279—282 synunetry elements in, 264—265 Acid anhydrides. See Carboxylic acid anhydrides... 

When a molecule has two chiral centers that are identically substituted, the number of stereoisomers is reduced from four to three, as is well known for the case of tartaric acid. The three stereoisomers are the d and l forms (enantiomers) and the diastereomeric meso form. The meso form is superimposable on its mirror image, since it has a plane of symmetry and is achiral and optically inactive. The three possible stereoisomers of tartaric acid are shown below ... 

Finally, let us consider the atoms Ha and Hb in achiral molecule 5 and chiral molecule 6. They are homotopic, chemically and symmetrically equivalent. The central C atom in achiral molecule 5 bears two times two of the same ligands and can be described by the general formula CH2L2. Chiral molecule 6 is not a meso-form, and there is no mirror plane bisecting this molecule since two stereogenic centers possess the same absolute configuration. 

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