Fischer projections meso stereoisomer

Fischer projection formulas can help us identify meso forms Of the three stereoisomeric 2 3 butanediols notice that only in the meso stereoisomer does a dashed line through the center of the Fischer projection divide the molecule into two mirror image halves... 

We can draw these three stereoisomers as Fischer projections, reversing the configurations at both centres to get the enantiomeric stereoisomers, whilst the Fischer projection for the third isomer, the meso compound, is characterized immediately by a plane of symmetry. For (-l-)-tartaric acid, the configuration is 2R, >R), and for (—)-tartaric acid it is (2S,3S). For both chiral centres, the group of lowest priority is hydrogen, which is on a horizontal line. In fact, this is the case in almost all Fischer projections, since, by convention, the vertical... 

Fischer projections are especially useful in the case of compounds with more than one chirality center. For example, it is easy to see the plane of symmetry in meso-tartaric acid. As was the case with regular structures, interchanging any two groups in a Fischer projection results in inversion of configuration at the chirality center. Thus, interchanging the H and OH on the lower chirality center of weso-tartaric acid inverts the configuration at that chirality center, resulting in the (27 ,3R)-stereoisomer, (-i-)-tartaric acid. It is also easy to see that this stereoisomer does not have a plane of symmetry. 

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. 

Fischer esterification. See Esterification Esters Fischer projection formulas, 271-272, 278, 280, 292, 595 a-amino acids, 1056, 1103 carbohydrates, 973-974, 1007 of meso stereoisomer, 280 tartaric acids, 286... 

Draw all stereoisomers for the following molecules in Fischer projection, labeling diastereomers, meso compounds, and enantiomers where appropriate ... 

Fischer projections are particularly useful if a molecule has more than one asymmetric carbon. As shown in Figure A5.4, tartaric add has two such carbons and three stereoisomers, one of which is a meso compound. Meso compoimds are superimposable on their mirror inrage and do not interact with polarized light. In other words, a meso compound such as (2R,3S)-tartaric add has a plane of symmetry, shown in Figure A5.5 as a line drawn between carbons 2 and 3. 

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