Tartaric acid Isomerism

Now, let us consider another similar molecule, tartaric acid, where there are two chiral carbons. In tartaric acid, four isomeric forms are theoretically expected (2 = 4). However, because one half of the tartaric acid molecule is a mirror image of the other half, we get a meso structure. This means this compound and its mirror image are superimposable, i.e. they are the same compound. Thus, instead of four, we obtain only three stereoisomers for tartaric acid. 

Thus, in cases where four different atoms or groups are attached to the same atom, it is possible to have two arrangements in space that cannot be made to coincide geometrically. This situation can be demonstrated by use of a special type of formula, shown in Fig. 2 for the two forms of the compound fluoroehlorobromomethane. This existence of two forms due to a difference in orientation in space is called stereoisomerism, and is discussed in the entry on Isomerism. It also follows that for compounds containing more than one atom bonded to four unlike groups, the number of different forms increases rapidly, as is shown by the three possible forms of tartaric acid, HOOC-CH-OH-CHOH-COOH. as portrayed by the three formulas shown in Fig. 3. 

Frankland and Price 17 were the first to attempt the resolution of alcohols (and acids) by fractional crystallization of their solid esters. The isomeric solid esters formed from Z-s-butylcarbinol and di-dibenzoyl-glyceric acid failed to separate on crystallization the corresponding di-alcohol-i-acid ester was liquid. Marckwald and McKenzie 18-19 effected partial resolutions of dl-mandelic acid and related acids with 1-menthol and d-bomeol, and of di-2-octanol with d-tartaric acid, but did not develop a satisfactory method for resolving alcohols. Later investigators, however, have employed the following resolving agents in several more or less successful resolutions of certain alcohols (a) i-menthyl isocyanate, (6) d-camphoric acid, (c) d- or i-mandelic acid, (d) d- or... 

It has been known for more than a century that living systems can distinguish between isomeric forms of many substances. In 1860, Pasteur [16] showed by polarimetry that when the ammonium salt of racemic tartaric acid ( paratartrate ) was subjected to fermentation by a yeast only one of the two enantiomeric forms was consumed— The yeast which causes the right salt to ferment leaves the left salt untouched, in spite of the absolute identity in physical and chemical properties of... 

This new acid, isomeric with mono-methyl succinic acid, pyro-tartaric acid, is known as glutaric acid, or systematically, as 1-5-pen-tan-di-oic acid. 

Isomerism of Tartaric Acid.—Examination of the formula for tartaric acid, which, by the facts given above, has its constitution fully established as symmetrical di-hydroxy succinic acid, shows the interesting fact that there are present two asymmetric carbon atomsy and that each of these has linked to it the same set of four different groups. We should, therefore, expect to find tartaric acid existing in the dextrOy the levo and the inactive forms. The stereo-chemical formulas similar to those of lactic and malic acids we may write as follows. 

It is this fourth unresolvable inactive tartaric acid which gives to tartaric acid its especial interest and importance in connection with the theory of stereo-isomerism. This acid, like the other three, has been fully explained in accordance with the tetra-hedral theory of van t Hoff and LeBel. The explanation rests upon the fact that there is a second asymmetric carbon atom in tartaric acid. We may construct, by models, or, by drawings, space-formulas for tartaric acid. According to the tetra-hedral theory, the dextro, levo and racemic inactive forms will be as follows, analogous to the corresponding formulas for the three lactic acids. The meso-tartaric acid is represented by the third drawing. 

Historical, Pasteur.— The history of stereo-isomerism and the tetrahedral theory is so intimately connected with tartaric acid that it will be well, at this time, to give a brief outline of it. 

Thus from a study of the crystalline sodium-ammonium salt of racemic acid and of dextro tartaric acid Pasteur showed, conclusively, the relationship of these two acids to each other and also discovered the existence of a third isomer optically active but of opposite direction to the ordinary tartaric acid already known. Racemic acid, therefore, is optically inactive because it consists of equal molecules of the ordinary dextro tartaric acid and the newly discovered levo tartaric acid. Also racemic acid can be resolved into its optically isomeric components by mechanically separating the two forms of crystals of the sodium-ammonium salt. The two active forms of tartaric acid, when mixed in equal molecular amounts, yield the inactive or racemic acid. Later, Pasteur prepared the fourth variety of tartaric acid, viz., meso-tartaric acid, by heating the cinchonine salt of dextro tartaric acid. This new acid proved to be inactive like racemic acid, but, unlike it, was unable to be resolved into optically active components. Its relation to the other three forms of tartaric acid was unexplained by Pasteur. 

Splitting Racemic Compounds.—The methods by which racemic compounds may be split into their optically active components are several. The three methods used were all originated by Pasteur. The first method has been referred to and consists of the mechanical separation of the two oppositely hemi-hedral forms in which the salts of a racemic compound crystallize. This method is especially applicable in the case of tartaric acid when the sodium-ammonium salt is used. The crystallization and separation must be carried out under definite conditions. If the racemic acid salt is crystallized below 28° the two forms of crystals are produced and a separation can be accomplished. If, however, the crystallization takes place above 28° the two forms of crystals are not produced but the sodium-ammonium racemate crystallizes in unseparable crystals of one form. That is, above 28° the sodium-ammonium racemate crystallizes as such, while, below 28° the racemate splits into its two isomeric components and equal amouts of the sodium-ammonium dextro tartrate and the sodium-ammonium levo tartrate are formed. The second method for the splitting of a racemic compound into its optically active components consists of the formation of the cinchonine, strychnine, or other similar alkaloid salts. When the cinchonine salt of racemic acid is formed it splits into the... 

The individual isomers of dach [cis-, trans- R, R)-, and trans- S, S)-] may be purchased (Alfa) or can be resolved from an isomeric mixture using the following methods cts- and trans-l, 2-dach can be separated usingNiCl2 6H20, according to published methods, while the separate trans-R, R- and trans-5,5-dach isomers can be isolated by allowing the racemic trans-dach mixture to react with either d- (for S, S) or L- (for R, R) tartaric acid. ... 

As predicted, mirror-image isomers do indeed exist, and thousands of instances besides the tartaric acids are known. There are, for example, two isomeric lactic... 

Any commercial source of dl-tartaric acid works well. The ratio of tartaric acid to diamine is 1 1. The checkers found that if isomerization of the meso-diamine is incomplete, this purification procedure provides a mixture of meso- and dl-diamines. 

---