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  Achiral Diastereomers Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Achiral Diastereomers

The chiral centers in the earlier illustrations have all been distinct, one from another. Acid the two chiral centers have similar four substituents and are equal in case of 2, 3-dihydroxybutanedioic acid that is known as tartaric. Result of this, two of the four possible stereoisomers of this compound is similar because of a plane of symmetry, so there are just three stereoisomeric tartaric acids. Two out of them are enantiomers and the last one or third one is an achiral diastereomer, known as meso compound. Meso compounds are achiral diastereomers of chiral stereoisomers. Researches of isomeric tartaric acid salts, come into existence by Louis Pasteur in the mid of the 19th century, were instrumental in elucidating Few of the subtleties of stereochemistry. 
Few physical properties of the isomers of tartaric acid are given in the table below.

(+)-tartaric acid:           [α]D = +13º       m.p. 172 ºC

(-)-tartaric acid:           [α]D = -13º         m.p. 172 ºC

Meso-tartaric acid:        [α]D =   0º          m.p. 140 ºC

Fischer projection formulas present a useful view of the configurational relationships in the structures of these isomers. In the examples that are displayed below a mirror line is drawn among formulas which have a mirror-image relationship. In illustrating the identity of the two meso-compound formulas, keep in mind that a Fischer projection formula may be rotated 180º in the plane.

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Meso Diastereomers

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The four diastereomeric aldopentoses displayed earlier and repeated above are all chiral. Each of these compounds has enantiomer so, as supposed; in all there are eight stereoisomers. The ends of the five-carbon chains become similar and the symmetry features of the overall structure are like that the number of configurational stereoisomers falls from eight to four, by reducing the aldehyde function to a 1º-alcohol. These four isomers, displayed below, are a pair of enantiomers and two meso-compounds.

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Carbon no.3 is not a stereogenic center in the enantiomeric pair on the left, because interchanging the H and OH substituents at this carbon does not change the entire configuration. This is false for the meso compounds on the right. Exchanging the H and OH substituents on carbon no.3 converts one isomer into its achiral partner. Stereogenic centers like C#3 have been called  pseudoasymmetric  centers, by noting that an R substituent is above an equal S substituent in the sequence rule (note the lower case letters used for this notation), a configurational notation (r or s) can be assigned. Since in nearly symmetrical cases the nearest R center has precedent over a likewise oriented S center in determining the primary end of the chain, the chain numbering on the right has changed.

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