Definitions

tartaric acid

tartaric acid

tartaric acid, HO2CCHOHCHOHCO2H, white crystalline dicarboxylic acid. It occurs as three distinct isomers, the dextro-, levo-, and meso- forms. The dextro- and levo- forms are optically active; the meso- form is optically inactive, as is racemic acid, a mixture of equal parts of the dextro- and levo- forms. Tartaric acid is found in many plants, e.g., grapes; this natural acid is chiefly the dextrorotatory d-tartaric acid, called also d-2,3-dihydroxysuccinic acid or l-2,3-dihydroxybutanedioic acid. This form can be partially converted to the others by heating it with an aqueous alkali, e.g., potassium hydroxide. Tartaric acids can be synthesized from maleic acids or fumaric acids by reaction with aqueous potassium permanganate. The various isomeric forms differ in such physical properties as boiling point. Tartaric acid is used chiefly in the form of its salts, e.g., cream of tartar and Rochelle salt.

Tartaric acid is a white crystalline organic acid. It occurs naturally in many plants, particularly grapes, bananas, and tamarinds, and is one of the main acids found in wine. It is added to other foods to give a sour taste, and is used as an antioxidant. Salts of tartaric acid are known as tartrates. It is a dihydroxy derivative of succinic acid.

Tartaric acid was first isolated from potassium tartrate, known to the ancients as tartar, c. 800 by the Persian alchemist Jabir ibn Hayyan, who was also responsible for numerous other basic chemical processes still in use today. The modern process was developed in 1769 by the Swedish chemist Carl Wilhelm Scheele. The chirality of tartaric acid was discovered in 1832 by Jean Baptiste Biot, who observed its ability to rotate polarized light. Louis Pasteur continued this research in 1847 by investigating the shapes of tartaric acid crystals, which he found to be asymmetric. Pasteur was the first to produce a pure sample of levotartaric acid.

Stereochemistry

Naturally occurring tartaric acid is chiral, meaning that it has molecules that are non-superimposable on their mirror-images. It is a useful raw material in organic chemistry for the synthesis of other chiral molecules. The naturally occurring form of the acid is L-(+)-tartaric acid or dextrotartaric acid. The mirror-image (enantiomeric) form, levotartaric acid or D-(−)-tartaric acid, and the achiral form, mesotartaric acid, can be made artificially. Note, that the dextro and levo prefixes are not related to the D/L configuration (which is derived from the reference D- or L-glyceraldehyde), but to the orientation of the optical rotation, (+) = dextrorotatory, (−) = levorotatory. Sometimes, instead of capital letters, small italic d, l are used. They are abbreviations of dextro- and levo-, and nowadays should not be used. Levotartaric and dextrotartaric acid are enantiomers, mesotartaric acid is a diastereomer of both of them.

A rarely occurring optically inactive form of tartaric acid, DL-tartaric acid is a 1:1 mixture of the levo and dextro forms. It is distinct from mesotartaric acid and was called racemic acid (from Latin racemus - "a bunch of grapes"). The word racemic later changed its meaning, becoming a general term for 1:1 enantiomeric mixtures - racemates. ­­ Tartaric acid is used to prevent Copper(II) ions from reacting with the hydroxide ions present in the preparation of copper(I) oxide. Copper(I) oxide is a reddish brown solid, and is produced by the reduction of a Cu(II) salt with an aldehyde, in an alkaline solution.

levotartaric acid
(D-(−)-tartaric acid)
dextrotartaric acid
(L-(+)-tartaric acid)
mesotartaric acid
| align="center" width="270" | |- DL-tartaric acid (racemic acid)
(when in 1:1 ratio)

Forms of Tartaric Acid
Common name tartaric acid levotartaric acid dextrotartaric acid mesotartaric acid racemic acid
Synonyms D-(S,S)-(−)-tartaric acid
unnatural isomer
L-(R,R)-(+)-tartaric acid
natural isomer
(2R,3S)-tartaric acid DL-(S,S/R,R)-(±)-tartaric acid
PubChem
EINECS number
CAS number 526-83-0 147-71-7 87-69-4 147-73-9 133-37-9

Derivatives

Important derivatives of tartaric acid include its salts, Cream of tartar (potassium bitartrate), Rochelle salt (potassium sodium tartrate, a mild laxative) and tartar emetic (antimony potassium tartrate).

Tartaric acid is a muscle toxin, which works by inhibiting the production of malic acid, and in high doses causes paralysis and death. The minimum recorded fatal dose for a human is about 7.5 grams/kg. Given this figure, it would take over 500g to kill a person weighing 70kg, and so it may be safely included in many foods, especially sour-tasting sweets. As a food additive, tartaric acid is used as an antioxidant with E number E334, tartrates are other additives serving as antioxidants or emulsifiers.

When cream of tartar is added to water, a suspension results which serves to clean copper coins very well. This is due to the fact that the tartrate solution can dissolve the layer of copper(II) oxide present on the surface of the coin. The resulting Copper(II)-tartrate complex that results is easily soluble in water.

Tartaric acid in wine

Tartaric acid may be most immediately recognizable to wine drinkers as the source of "wine diamonds," the small potassium bitartrate crystals that sometimes form spontaneously on the cork. These "tartrates" are harmless, despite sometimes being mistaken for broken glass, and are prevented in many wines through cold stabilization. The tartrates that remain on the inside of aging barrels were at one time a major industrial source of potassium bitartrate.

However, tartaric acid plays an important role chemically, lowering the pH of fermenting "must" to a level where many undesirable spoilage bacteria cannot live, and acting as a preservative after fermentation. In the mouth, tartaric acid provides some of the tartness in the wine, although citric and malic acids also play a role.

References

External links

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