6.2 Tartaric acid; meso form
6.2 Tartaric acid; meso form
Tartaric acid 9 is formally the oxidized product of aldotetrose 1. The two termini of 1, -CHO and -CH2OH, are both -COOH in 9. Like 1, 9 has two asymmetric carbon atoms C2 and C3, and may have 22 = 4 stereoisomers. To confirm whether this prediction is correct, we will draw stereoisomers 10～13 in analogy with 2～5.
There is some difference between the stereochemistry of tartaric acid 9 and that of aldotetrose 1 in that 9 has an identical set of ligands at C2 and C3. Will this affect the stereochemistry of 9?
Though 10 and 11 seem to have different stereochemistry, you will find that in fact 10 and 11 are the same molecule by careful examination of molecular model. Another important point is there is a center of symmetry in the center of 10 (and 11). This means that the upper half and the lower half of the molecule are identical. Hence 10 and 11 is "intramolecular" racemic compound and hence optically inactive. Optically inactive compounds with asymmetric carbon atom(s) are referred to as meso form.
This means that tartaric acid has only three stereoisomer, 12 and 13, the enantiomeric pair , and optically inactive (meso) 10 (= 11). You must remember that the number of stereoisomers will be less than 2ｎ when there is a symmetry in the molecule. (+)-Tartaric acid is widely distributed in nature, particularly in fruits as acid, and calcium or magnesium salts. Since c acid is obtained as the by-product of the wine production, it was known from olden times. Hence it is quite understandable that a great development of organic stereochemistry was made by the study of Pasteur on tartaric acid. When Pasteur initiated his study, (+)-tartaric acid and racemic acid (in modern terminology, (土)-tartaric acid: the word racemate cam from the name of this compound) were known, but the stereochemical relation between the two compounds was not understood at that time. Pasteur could successfully resolved (土)-tartaric acid by physical method (in fact with his own hands, magnifier and tweezers), establishing the relation among stereoisomers.