Tartaric acid crystal forms

The calcium salt of the principal product, d/-tartaric acid, crystallizes with four molecules of water, while the secondary product, meso-tartaric acid, forms a calcium salt which crystallizes with three molecules of water. The amount of sulfuric acid actually required may readily be calculated from the percentage of calcium found on analysis in the regular way or it may be estimated by igniting a sample, and titrating the residue with standard acid. 

Two mols, for example, 270 grams, of racemic a-methylphenethylamine base are reacted with one mol (150 grams) of d-tartaric acid, thereby forming dl-a-methylphenethylamine d-tartrate, a neutral salt. The neutral salt thus obtained is fully dissolved by the addition of sufficient, say about 1 liter, of absolute ethanol, and heating to about the boiling point. The solution is then allowed to cool to room temperature with occasional stirring to effect crystallization. The crystals are filtered off and will be found to contain a preponderance of the levo enantiomorph. 

Thus Pasteur noted that the amide of (-) malic acid forms molecular compounds of different properties with the enantiomeric amides of tartaric acid. With amide of (+) tartaric acid large transparent crystals are formed whose solubility is 18% at 20°C, while with the amide of (-) tartaric acid, thin needles are formed with solubility almost two times higher. Free malic and tartaric acids also form diastereomeric molecular compounds. 

Neutral Tartrate of Lime—2 CaO, H4 010 —occurs in grapes, and mixed with crude tartar also in senna leaves. Neutral tartrate of potassa forms, witli chloride of calcium, a crystalline precipitate—it is the salt obtained in making tartaric acid, Crystals, right rhombic prisms with ootahedral summits soluble in twelve hundred parts of cold water. 

This acid, the inactive by intra-molecular compensation and un-resolvable into optically active components, was first obtained by Pasteur by heating the cinchonine salt of dextro tartaric acid, to 170 . It may also be prepared by boiling the dextro tartaric acid with an excess of hydrochloric acid, or with sodium hydroxide. Also by long boiling with water alone or by heating with a small amount of water to 165°. When di-brom succinic acid is treated with silver hydroxide, or when malic acid is oxidized, in the presence of water, both meso-tartaric acid and racemic acid are formed. When meso-tartaric acid is heated to 200° it is partly converted into racemic acid. Meso-tartaric acid crystallizes in rectangular plates with one molecule of water. The water free acid melts at i40°-i45°. 

The ordinary tartaric acid crystallizes in large prisms very soluble in HiO and alcohol acid in taste and reaction. It fuses at 170° (338° P.) at 180° (356° P.) it loses HjO, and is gradually converted into an anhydrid at 200°-210° (393°-410° P.) it is decomposed with formation of pyruvic acid, C H 03,and p3rrotartaric acid, CeHtO, at higher temperatures COs, CO, HjO, hydrocarbons and charcoal are produced. If kept in fusion some time, two molecules unite, with loss of HjO, to form tartralic or ditartaric acid, CsHioOii. 

The fact that molecules have a three-dimensional structure and shape was shown by Louis Pasteur in 1848 in some critical experiments on crystalline salts of tartaric acid that formed part of his doctoral studies. Tartaric acid is a naturally occurring compound that is extracted from grape juice and sometimes crystallizes as potassium bitartrate from solution in wine. Pasteur concentrated on the related compound sodium ammonium tartrate. The two forms of tartrate were chemically identical, but a solution of potassium bitartrate would rotate the plane of polarization of plane polarized light to the right whereas a solution of sodium ammonium tartrate would not. Pasteur studied the crystal structures of tartaric acid salts and found the crystallites themselves were chiral, i.e. the facets of the crystals occur in two forms that are mirror images of one another, so that the two crystallite forms cannot be superimposed. In the pure potassium bitartrate, only right-handed facets were... 

Occasionally an optically inactive sample of tartaric acid was obtained Pasteur noticed that the sodium ammonium salt of optically inactive tartaric acid was a mixture of two mirror image crystal forms With microscope and tweezers Pasteur carefully sep arated the two He found that one kind of crystal (m aqueous solution) was dextrorota tory whereas the mirror image crystals rotated the plane of polarized light an equal amount but were levorotatory... 

Although Pasteur was unable to provide a structural explanation—that had to wait for van t Hoff and Le Bel a quarter of a century later—he correctly deduced that the enantiomeric quality of the crystals was the result of enantiomeric molecules The rare form of tartanc acid was optically inactive because it contained equal amounts of (+) tartaric acid and (—) tartaric acid It had earlier been called racemic acid (from Latin racemus meaning a bunch of grapes ) a name that subsequently gave rise to our pres ent term for an equal mixture of enantiomers... 

Tartaric acid [526-83-0] (2,3-dihydroxybutanedioic acid, 2,3-dihydroxysuccinic acid), C H O, is a dihydroxy dicarboxyhc acid with two chiral centers. It exists as the dextro- and levorotatory acid the meso form (which is inactive owing to internal compensation), and the racemic mixture (which is commonly known as racemic acid). The commercial product in the United States is the natural, dextrorotatory form, (R-R, R )-tartaric acid (L(+)-tartaric acid) [87-69-4]. This enantiomer occurs in grapes as its acid potassium salt (cream of tartar). In the fermentation of wine (qv), this salt forms deposits in the vats free crystallized tartaric acid was first obtained from such fermentation residues by Scheele in 1769. 

Occurrence. (R-R, R )-Tartaric acid occurs in the juice of the grape and in a few other fmits and plants. It is not as widely distributed as citric acid or S(—)-mahc acid. The only commercial source is the residues from the wine industry. (i -R, R -Tartaric acid has been found in the fmit and leaves of BauMma reticulata, a tree native to MaU (western Africa). Like the dextrorotatory acid, it forms anhydrous monoclinic crystals. 

Let s return for a last look at Pasteur s pioneering work. Pasteur took an optically inactive tartaric acid salt and found that he could crystallize from it two optically active forms having what we would now call the 2R,3R and 2S,3S configurations. But what was the optically inactive form he started with It couldn t have been meso-tartaric acid, because meso-tartaric acid is a different chemical compound and can t interconvert with the two chiral enantiomers without breaking and re-forming chemical bonds. 

The answer is that Pasteur started with a 50 50 mixture of the two chiral tartaric acid enantiomers. Such a mixture is called a racemic (ray-see-mi c) mixture, or racemate, and is denoted either by the symbol ( ) or the prefix cl,I to indicate an equal mixture of dextrorotatory and levorotatory forms. Racemic mixtures show no optical rotation because the (+) rotation from one enantiomer exactly cancels the (-) rotation from the other. Through luck, Pasteur was able to separate, or resolve, racemic tartaric acid into its (-f) and (-) enantiomers. Unfortunately, the fractional crystallization technique he used doesn t work for most racemic mixtures, so other methods are needed. 

Pasteur separated a racemic form of a salt of tartaric acid into two types of crystals in 1848 led to the discovery of enantioisomerism. 

A young Louis Pasteur observed that many salts of tartaric acid formed chiral crystals (which he knew was related to their ability to rotate the plane of polarization of plane-polarized light). He succeeded in solving the mystery of racemic acid when he found that the sodium ammonium salt of racemic acid could be crystallized to produce a crystal conglomerate. After physical separation of the macroscopic enantiomers with a dissecting needle, Pasteur... 

Munoz-Guerra also studied stereoregular polyamides fully based on d- and L-tartaric acid [73]. The bispentachlorophenyl esters of both 2,3-di-(9-methyl-tartaric acids (22 and 23) were condensed with (2S, 3S )-2,3-dimethoxy-l,4-butanediamine (41) to obtain optically active (PTA-LL) and racemic (PTA-LD) polytartaramides. Fiber-oriented and powder X-ray studies of these polyamides demonstrated that PTA-LL crystallized in an orthorhombic lattice, whereas PTA-LD seemed to adopt a tricUnic structure. In both cases, the polymeric chain appears to be in a folded conformation more contracted than in the common y form of conventional nylons. 

By this process phenylglycine derivatives have been resolved by crystallization of the tartaric acid ammonium salts. The equilibration is induced at the amino ester stage by forming the configurationally labile imines with a catalytic amount of benzaldehyde or acetone (Table 11). 

Through luck, in 1848, Louis Pasteur was able to separate or resolve racemic tartaric acid into its (+) and (—) forms by crystallization. Two enantiomers of the sodium ammonium salt of tartaric acid give rise to two distinctly different types of chiral crystal that can then be separated easily. However, only a very few organic compounds crystallize into separate crystals (of two enantiomeric forms) that are visibly chiral as are the crystals of the sodium ammonium salt of tartaric acid. Therefore, Pasteur s method of separation of enantiomers is not generally applicable to the separation of enantiomers. 

RGURE 1 Pasteur separated crystals of two stereoisomers of tartaric acid and showed that solutions of the separated forms rotated polarized light to the same extent but in opposite directions. These dextrorotatory and levorotatory forms were later shown to be the (R,R) and (S,S) isomers represented here. The RS system of nomenclature is explained in the text. 

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