Tartaric Acid And Tartrate Chemistry

Tartaric acid and tartrate chemistry

Crystals frequently appear when the last glass is poured from a bottle of red wine. They are of course perfectly harmless potassium tartrate crystals, known in the wine trade as `wine diamonds`. This compound is the monopotassium salt of tartaric acid, a naturally occurring weak acid. Other salts are possible Dipotassium tartrate and Potassium sodium tartrate (Rochelle salt) also exist. These salts form in wine when tartaric acid and potassium ions, both present naturally in grapes, combine to form a crystalline salt. Tartaric acid is an organic compound, containing more than one functional group. Under IUPAC naming rules, it is classified as a carboxylic acid, since the COOH functional group is more important in the hierarchical classification than the hydroxy groups that are also contained within the molecule. The correct IUPAC name for tartaric acid, seen below, is 2,3-Dihydroxybutanedioic acid.

Tartrate salts have very limited solubility at low temperatures in aqueous solutions. These conditions are found when wine is stored, which is when the crystals form in the bottom of barrels or wine bottles. Pure tartrate salts are white crystalline solids, but in red wine the colour is absorbed. The wine industry `cold stabilise` white wine by chilling it to a few degrees C, forcing crystallisation. This apparently results in fewer disputes in restaurants, when `offensive` crystals appear in the glass. This is a fantastic real life example of recrystallisation, an important technique used to purify solid organic products. The solvent, which in wine is an ethanol in water mixture, must be carefully chosen so that the product of interest is very soluble at high temperatures, but much less soluble at low temperatures. A distinct solubility curve must exist.

Tartaric acid contains chiral carbon centres. These are carbon atoms that have four different atoms or groups directly bonded to them. These chiral carbon atoms, or chiral centres, are starred in the diagram below. The naturally occurring form of tartaric acid is referred to as the D or (-) isomer of the acid, more often now as 2S, 3S-tartaric acid. These designations give a clue as to the effect that is observed when plane polarised light travels through solutions of tartaric acid. The rotation of the plane polarised light was first observed in 1832 by Jean Baptiste Biot. The D- isomer rotates polarised light in a clockwise direction, when approaching the observer. The other, more expensive, enantiomer 2R, 3R-tartaric acid rotates polarised light in the opposite direction. The two isomers are non-superimposable mirror images.

A spectacular example of homogeneous catalysis can be observed when Potassium sodium tartrate (Rochelle salt) reacts with hydrogen peroxide. This oxidation reaction proceeds only very slowly in the absence of a catalyst. The 2,3-dihydroxybutanedioate ion is oxidised by hydrogen peroxide to form carbon dioxide and methanoate ions as products. Cobalt(II) ions catalyse this reaction in a display of variable oxidation state. Cobalt(II) ions are pink. The hydrogen peroxide oxidises the cobalt(II) ions to cobalt(III), which is an obvious green colour. The cobalt(III) ions bond to the tartrate ions, forming an intermediate transition species which is more easily oxidised. Oxidation proceeds rapidly, with an eruption of carbon dioxide gas bubbles. The cobalt(III) ions are then reduced back to pink cobalt(II) ions at the end of the reaction. The alternative reaction pathway provided by the catalyst has a lower activation energy than the uncatalysed route.