|Jmol 3D model||Interactive image|
|C4H6O6 (Basic formula)
HO2CCH(OH)CH(OH)CO2H (Structural formula)
|Molar mass||150.087 g/mol|
|Density||1.79 g/mL (H2O)|
|Melting point||171 to 174 °C (340 to 345 °F; 444 to 447 K) (L or D-tartaric; pure)
206 °C (DL, racemic)
165–166 °C ("meso-anhydrous")
146–148 °C (meso-hydrous)
|1.33 kg/L (L or D-tartaric)
0.21 kg/L (DL, racemic)
|Acidity (pKa)||L(+) 25 °C :
pKa1= 2.89 pKa2= 4.40
meso 25 °C:
pKa1= 3.22 pKa2= 4.85
EU classification (DSD)
Related carboxylic acids
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Tartaric acid is a white crystalline organic acid that occurs naturally in many plants, most notably in grapes. Its salt, potassium bitartrate, commonly known as cream of tartar, develops naturally in the process of winemaking. It is commonly mixed with sodium bicarbonate and is sold as baking powder used as a leavening agent in food preparation. The acid itself is added to foods as an antioxidant and to impart its distinctive sour taste.
Tartaric acid played an important role in the discovery of chemical chirality. This property of tartaric acid was first observed 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 Potassium sodium tartrate crystals, which he found to be chiral. By manually sorting the differently shaped crystals under magnification, Pasteur was the first to produce a pure sample of levotartaric acid.
Naturally occurring tartaric acid is chiral, meaning it has molecules that are not 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 levotartaric acid. The mirror-image (enantiomeric) form, dextrotartaric acid or D-(-)-tartaric acid, and the achiral form, mesotartaric acid, can be separated through synthetic procedures. The dextro and levo prefixes are not related to the D/L configuration (which is derived rather indirectly from their structural relation to D- or L-glyceraldehyde), but to the dispersion of Circularly Polarized Light (Optical rotatory dispersion ORD), (+) = dextrorotatory, (−) = levorotatory. Sometimes, instead of capital letters, small italic d and l are used. They are abbreviations of dextro- and levo- and, nowadays, should not be used. Levotartaric and dextrotartaric acid are enantiomers or optical isomers. On the other hand, mesotartaric acid is a diastereomer (has no optical isomer) which imply no ORD is observed. The lack of optical activity is due to a mirror plane in the molecule, [segmented line in picture below]
Louis Pasteur, working with an optically inactive form of tartaric acid, DL-tartaric acid [is a 1:1 mixture of the levo and dextro forms], was able to crystallize and separate both enantiomers. The separation enentiomers in two discrete different crystals is called a "conglomerate" and the process is called resolution. Later it was concluded that these resolved crystals (see picture) rotated the Circular Polarized Light in opposite directions, because of the molecular symmetry. These two molecules are distinct from mesotartaric acid and when mixed are 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. A racemate is described as a crystal containing the two enantiomers.
Tartaric acid in Fehling's solution is used to prevent copper(II) ions from precipitating with the hydroxide ions present in solution. By mixing this solution with glucose and heating, copper(I) oxide precipitates. Copper(I) oxide is a reddish-brown solid, and is produced by the reduction of a Cu(II)-tartrate with an aldehyde.
DL-tartaric acid (racemic acid)
|Forms of tartaric acid|
|Common name||tartaric acid||Dextrotartaric acid||Levotartaric acid||mesotartaric acid||racemic acid|
|(2R,3S)-tartaric acid||DL-(S,S/R,R)-(±)-tartaric acid|
|PubChem||CID 875 from PubChem||CID 439655 from PubChem||CID 444305 from PubChem||CID 78956 from PubChem||CID 5851 from PubChem|
The L-(+)-tartaric acid isomer of tartaric acid is industrially produced in the largest amounts. It is obtained from naturally occurring component of lees, a solid byproduct of fermentations. The former byproducts mostly consist of potassium bitartrate (KHC4H4O6). This potassium salt is converted to calcium tartrate (CaC4H4O6) upon treatment with milk of lime (Ca(OH)2):
- KO2CCH(OH)CH(OH)CO2H + Ca(OH)2 → Ca(O2CCH(OH)CH(OH)CO2) + KOH + H2O
In practice, higher yields of calcium tartrate are obtained with the addition of calcium chloride. Calcium tartrate is then converted to tartaric acid by treating the salt with aqueous sulfuric acid. This process yields free L-(+)-tartaric acid:
- Ca(O2CCH(OH)CH(OH)CO2) + H2SO4 → HO2CCH(OH)CH(OH)CO2H + CaSO4
Racemic tartaric acid
- HO2CC2H2CO2H + H2O2 → OC2H2(CO2H) 2
In the next step, the epoxide is hydrolyzed to form racemic tartaric acid.
- OC2H2(CO2H)2 + H2O → (HOCH)2(CO2H)2
Meso-tartaric acid is formed via thermal isomerization. Dextro-tartaric acid is heated in water at 165 °C for about 2 days. Meso-tartaric acid can also be prepared from dibromosuccinic acid using silver hydroxide.
- HO2CCHBrCHBrCO2H + 2 AgOH → HO2CCH(OH)CH(OH)CO2H + 2 AgBr
Meso-tartaric acid can be separated from the racemic acid by crystallization, the racemate being less soluble.
L-(+)-tartaric acid, can participate in several reactions. As shown the reaction scheme below, dihydroxymaleic acid is produced upon treatment of L-(+)-tartaric acid with hydrogen peroxide in the presence of a ferrous salt.
- HO2CCH(OH)CH(OH)CO2H + H2O2 → HO2CC(OH)C(OH)CO2H + 2 H2O
Dihydroxymaleic acid can then be oxidized to tartronic acid with nitric acid.
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). Diisopropyl tartrate is used as a catalyst in asymmetric synthesis.
Tartaric acid is a muscle toxin, which works by inhibiting the production of malic acid, and in high doses causes paralysis and death. The median lethal dose (LD50) is about 7.5 grams/kg for a human, 5.3 grams/kg for rabbits, and 4.4 grams/kg for mice. Given this figure, it would take over 500 g (18 oz) to kill a person weighing 70 kg (150 lb), 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, as the tartrate solution can dissolve the layer of copper(II) oxide present on the surface of the coin. The resulting copper(II)-tartrate complex 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 or bottom of the bottle. These "tartrates" are harmless, despite sometimes being mistaken for broken glass, and are prevented in many wines through cold stabilization, although not always preferred since it can change the wine's profile. The tartrates remaining on the inside of aging barrels were at one time a major industrial source of potassium bitartrate.
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.
Tartaric acid and its derivatives have a plethora of uses in the field of pharmaceuticals. For example, tartaric acid has been used in the production of effervescent salts, in combination with citric acid, in order to improve the taste of oral medications. The potassium antimonyl derivative of the acid known as tartar emetic is included, in small doses, in cough syrup as an expectorant.
Tartaric acid also has several applications for industrial use. The acid has been observed to chelate metal ions such as calcium and magnesium. Therefore, the acid has served in the farming and metal industries as a chelating agent for complexing micronutrients in soil fertilizer and for cleaning metal surfaces consisting of aluminium, copper, iron, and alloys of these metals, respectively.
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