Tartaric acid

• 2,3- dihydroxysuccinic
• 2,3- dihydroxybutanedioic
• Threarsäure
• Racemic acid (racemate )

• 87-69-4 [L- ( )-form ]
• 147-71-7 [D -(- )-form ]
• 147-73-9 [ meso- form ]
• 133-37-9 [ DL- (± )-form ]

Colorless and odorless solid with a slightly sour taste

Fixed

1.76 g · cm -3 ( optically active forms ) 1.666 g · cm -3 ( meso form ) 1.788 g · cm -3 ( racemate)

168-170 ° C ( optically active forms ) 140 ° C ( meso form ) 206 ° C ( racemate)

• PKS1 = 2.98
• PKa2 = 4.34

• Soluble in water ( 1394 g · l-1 at 20 ° C ), methanol, ethanol, 1 -propanol, and glycerol
• Poorly in diethyl ether, and insoluble in chloroform

Attention

7500 mg · kg -1 ( LDLO, Rat, oral, L-( )- tartaric acid)

Template: Infobox chemical / molecular formula search available

Tartaric acid, also known as 2,3- dihydroxysuccinic or 2,3- dihydroxybutanedioic, in Latin, known as acidum tartaricum and English with tartaric acid, is an α - hydroxycarboxylic acid. Its salts and esters are called tartrates. L-( )- tartaric acid occurs, for example in grapes and in the EU as a food additive E 334 approved. In Germany is also the total acidity of wines - calculated as tartaric acid - provided, although the wine a number of other acids, especially malic acid occurs.

Racemic acid refers to the racemate of tartaric acid. By intermolecular elimination of water produced the polymeric meta-tartaric acid, the 353 is also used under the name of E as a food additive.

History

Before the discovery of tartaric acid was her salt of tartar, tartaric, held for an acid, because it is easily precipitated by its poor water solubility in the wine, and therefore easier for chemists was recognized as the readily soluble tartaric acid. 1732 Weinstein was led by Boerhaave as a solid acid. 1764 Marggraf of an alkali metal in the Weinstein detected whereupon it this decomposed with calcium to calcium tartrate, which he, however, not examined in more detail. It was not until 1769 decomposed Scheele, who is generally regarded as the discoverer of tartaric acid, calcium tartrate with sulfuric acid and described the deposited crystalline acid as " tartaric acid ". The Weinstein was recognized shortly thereafter as the acid potassium salt of this acid.

1819 the racemic acid was discovered by the manufacturer Karl Kestner as a by- product of tartaric acid production. The isomerism of tartaric acid was found in 1826 by Gay - Lussac and examined in more detail by Pasteur, which then called only the optically active " dextrorotatory tartaric acid ", ie the L- tartaric acid and racemic acid, as a " racemic acid", were known. Pasteur put forth the sodium ammonium salt of these acids and discovered the chirality. With magnifying glass and tweezers Pasteur separated the crystals in left-handed and right-handed copies.

The thus separated from the racemate enantiomeric Natriumammoniumtartrate Pasteur brought back into solution and examined their optical rotation in polarimeter. He found that both solutions of the salts obtained from the optically inactive racemic acid were optically active and the same specific rotation as from L- tartaric acid derived sodium ammonium salt had, but with opposite sign. It concluded Pasteur that racemic acid a pure compound, but an equal part mixture of right - and left-rotating tartaric acid, that is a racemate. The term racemate for a mixture of two enantiomers in equal parts derived from the Latin name for racemic acid ( acidum racemicum ).

The interpretations of Pasteur's experiment, however, are not exhausted by the fact that racemic acid is a racemic mixture of L- and D- tartaric acid. Although Pasteur recognized that optical activity had to be a consequence of a property of the tartaric acid molecule itself. But only in 1874 was Le Bel and van ' t Hoff, who subsequently became the first Nobel Laureate in Chemistry, explain this independently on the basis of molecular structure. Knowing that " four different objects can be arranged in two different ways in the corners of a tetrahedron, and that these arrangements each other as mirror images of behavior that are not to be made to cover " they hypothesized that the four bonded to a carbon atom are arranged tetrahedrally. Assuming they suspected that optically active molecules contain at least one carbon atom with four different groups, ie an asymmetric carbon atom. " Optically inactive organic substances therefore contain either an asymmetric carbon atom or they are mixtures of equal parts of two enantiomers. "

Van't Hoff and Le Bel recognized so that on the one hand, the tetrahedral geometry of the carbon atom, and gave the other a conclusive explanation for the optical activity of organic substances. Only the meso compounds could be due to their definition does not cover, until much later the structure of meso-tartaric acid was discovered. Also long known that the enantiomer of tartaric acid was then the plane of polarization of the light to the right, and which rotate to the left. Only in 1951 could Bijvoet clarify with a special X-ray method using the Natriumrubidiumtartrats that it is the dextrorotatory ( )- enantiomer and D- tartaric acid to the levorotatory with L- tartaric acid - is enantiomer (). By conversion of tartaric acid in different chemical compounds it was also investigated for many other pairs of enantiomers.

Occurrence

In particular, the L-( )- tartaric acid and its calcium, potassium and magnesium salts are found in abundance in the vines, grapes and leaves of the vine and in dandelions, in sugar beet, in Tamarind, in immature bird berries, seeds of the spindle tree, in the leaves of agave, in black pepper in pineapple and many other fruits. In wine making, is sparingly soluble salts of tartaric acid as Weinstein are deposited at the bottom of wine barrels and wine bottles. The D-(- )-tartaric acid, also called unnatural tartaric acid, is found only in the leaves of the West African tree Bankinia reticulata. The meso form does not exist in nature.

Production

The preparation of tartaric acid from Weinstein possible after conversion to calcium tartrate. From this it can be released with sulfuric acid, the tartaric acid as byproduct gypsum. The meso- form can be prepared by oxidation of fumaric acid or maleic anhydride with hydrogen peroxide, potassium permanganate or other peracids.

Pure D-(- )-tartaric acid can be obtained by the reduction with Penicillium glaucum from the racemate, as Penicillium glaucum only the L ( )-tartaric acid degrades.

Properties

Structural formulas of the tartaric acids in the Fischer projection:

L-( )- tartaric acid [ synonym: (2R, 3R )-tartaric acid ]

Meso- tartaric acid, the dotted line is the mirror plane

The two carbon atoms bearing the two hydroxyl groups in the molecule of tartaric acid are stereocenters. Is depending on the configuration of these centers D-(- )-tartaric acid [ synonym: (2S, 3S )-tartaric acid ], L -( )-tartaric acid [ synonym: (2R, 3R )-tartaric acid ] or the optically inactive meso- tartaric acid before. In the meso- form of the stereo centers (R) - and the other ( S) configuration. In nature usually comes before the dextrorotatory L-( ) form. The two enantiomers of tartaric acid [L- ( )-tartaric acid and D -(- )-tartaric acid ] do not differ in their physical and chemical properties only in the rotation value to linearly polarized light, the specific rotation of α for L -( )-tartaric acid 12.7 °, with D-(- )-tartaric acid -12.7 ° at the same measurement conditions. The rotation value of the meso-tartaric acid is, as with all meso compounds, ± 0 °. The physiological properties of all three stereoisomers of tartaric acid are different.

The alkali salts of tartaric acid assets in an alkaline solution of copper ( II ) ions to complex ( to bind ) and thereby keep it in solution ( Fehling's solution). As dibasic, relatively strong acid and hydrogen tartrates can be formed. A mixture of equal amounts of L- ( ) - and D -(-) -tartaric acid ( racemate) referred to as racemic acid, melting point 205-206 ° C. This mixture is sometimes referred to as racemic tartaric acid. A mixture of three stereoisomers of tartaric acid with variable proportions of L- ( ) -, D -(-) - and meso-tartaric acid is treated as a mixture of isomers of tartaric acid.

Use

A larger scale, the L -tartaric acid is used, since it is the product of most methods of synthesis of tartaric acid. 50% of the produced L ( )-tartaric acid to go into the food industry and pharmaceutical industry, the other half in technical applications.

Tartaric acid is used as an ingredient of disinfectants. It is not stated in the rule, if it is L- or D- tartaric acid, the racemic mixed tartaric acid or a different mixing ratio.

Use as a food additive

The most obvious application of tartaric acid lies in its use as a food additive. The in this field as E 334 designated L- tartaric acid is not only found naturally in many foods, but is also added to many food products because of their mixed flavor and preservative properties. Tartaric acid is used in the preparation of ice cream, artificial honey, fruit, sodas and soft drinks, jelly, wine gums and confectionery, and in the acidification low acid wines. The oral toxicity of L- tartaric acid was extremely low in animal studies with rats; the LDLO was for rats when administered orally at 7500 mg / kg body weight.

The polymeric meta-tartaric acid (E 353) is mainly used for the tartaric stabilization; as a protective colloid preventing the crystallization of tartrate in wine.

Technical Uses

Furthermore, tartaric acid is also in many technical sectors, including the grip to and smoothing of silk. Important is the ability of tartaric acid to form complexes with metals: these complexes, the metal cation is bound by the solid tartaric acid, as with most other organic acids. This results in numerous application possibilities arise. Potassium sodium tartrate is used for example as a complexing agent in Fehling's solution, tartaric acid for surface treatment of copper and brass articles. The latter can also be used for cleaning heavy metal contaminated soils, as it binds toxic heavy metals here, but is even biodegradable. If they are cement and plaster, she delayed the setting by complexing of calcium ions and thus prolongs the processing and Verformbarkeitszeit. Further, it serves as a reducing agent, and for the resolution of organic bases. In modern organic synthesis are LiAlH4 - tartaric acid derivatives as chiral reagents or catalysts TADDOL important for the enantioselective reduction of ketones and other stereoselective syntheses.