he tartaric acid is an organic compound whose molecular formula is COOH (CHOH) 2 COOH. It has two carboxyl groups; that is, it can release two protons (H + ). In other words, it is a diprotic acid. It can also be classified as an aldaric acid (acid sugar) and a derivative of succinic acid.
Its salt has been known since time immemorial and constitutes one of the by-products of winemaking. This crystallizes as a white sediment baptized as “wine diamonds”, which accumulate in the cork or in the bottom of the barrels and bottles. This salt is potassium bitartrate (or potassium acid tartrate).
The salts of tartaric acid have in common the presence of one or two cations (Na + , K + . NH 4 + , Ca 2+ , etc.) because, when releasing its two protons, it remains negatively charged with a charge of -1 (as with bitartrate salts) or -2.
In turn, this compound has been the object of study and teaching of organic theories related to optical activity, more precisely with stereochemistry.
Where is tartaric acid found?
Tartaric acid is a component of many plants and foods, such as apricots, avocados, apples, tamarinds, sunflower seeds, and grapes.
In the wine aging process, this acid —at cold temperatures— combines with potassium to crystallize as tartrate. In red wines the concentration of these tartrates is lower, while in white wines they are more abundant.
Tartrates are salts of white crystals, but when they occlude impurities from the alcoholic environment, they acquire reddish or purple tones.
Structure of tartaric acid
In the upper image the molecular structure of tartaric acid is represented. The carboxyl groups (–COOH) are located at the lateral ends and are separated by a short chain of two carbons (C 2 and C 3 ).
In turn, each of these carbons is linked to an H (white sphere) and an OH group. This structure can rotate the C 2 –C 3 bond , thus generating various conformations that stabilize the molecule.
That is, the central bond of the molecule rotates like a rotating cylinder, consecutively alternating the spatial arrangement of the groups –COOH, H and OH (Newman projections).
For example, in the image the two OH groups point in opposite directions, which means that they are in anti positions to each other. The same happens with the –COOH groups.
Another possible conformation is that of a pair of eclipsed groups, in which both groups are oriented in the same direction. These conformations would not play an important role in the structure of the compound if all the groups of the C 2 and C 3 carbons were equal.
Since the four groups are different in this compound (–COOH, OH, H, and the other side of the molecule), the carbons are asymmetric (or chiral) and exhibit the famous optical activity.
The way the groups are arranged at the C 2 and C 3 carbons of tartaric acid determines some different structures and properties for the same compound; that is, it allows the existence of stereoisomers.
Tartaric acid applications
In the food industry
It is used as a stabilizer of eulsions in bakeries. It is also used as an ingredient in yeast, jam, gelatin, and carbonated drinks. Likewise, it performs functions as an acidifying, leavening and ion sequestering agent.
Tartaric acid is found in these foods: cookies, candies, chocolates, fizzy liquids, baked goods, and wines.
In the production of wines it is used to make them more balanced, from the taste point of view, by lowering their pH.
In the pharmaceutical industry
It is used in the creation of pills, antibiotics and effervescent pills, as well as in medicines used in the treatment of heart disease.
In the chemical industry
It is used in photography as well as in electroplating and is an ideal antioxidant for industrial greases.
It is also used as a metal ion scavenger. How? By rotating your bonds in such a way that you can locate the electron-rich carbonyl group oxygen atoms around these positively charged species.
In the construction industry
It slows down the hardening process of plaster, cement and plaster, making the handling of these materials more efficient.
Tartaric acid properties
The most common applications of tartaric acid are:
- Tartaric acid is available in the form of a crystalline powder or slightly opaque white crystals. It has a pleasant taste, and this property is indicative of a good quality wine.
- It melts at 206ºC and burns at 210ºC. It is very soluble in water, alcohols, basic solutions and borax.
- Its density is 1.79 g / mL at 18ºC and it has two acidity constants: pKa 1 and pKa 2 . That is, each of the two acidic protons has its own tendency to be released into the aqueous medium.
- As it has –COOH and OH groups, it can be analyzed by infrared (IR) spectroscopy for its qualitative and quantitative determinations.
- Other techniques such as mass spectroscopy and nuclear magnetic resonance allow the previous analyzes to be carried out on this compound.
Tartaric acid was the first organic compound to develop enantiomeric resolution. What does this mean? It means that its stereoisomers could be separated manually thanks to the research work of the biochemist Louis Pasteur, in 1848.
And what are the stereoisomers of tartaric acid? These are: (R, R), (S, S) and (R, S). R and S are the spatial configurations of the C 2 and C 3 carbons .
Tartaric acid (R, R), the most “natural”, rotates polarized light to the right; tartaric acid (S, S) rotates it to the left, counterclockwise. And finally, tartaric acid (R, S) does not rotate polarized light, being optically inactive.
Louis Pasteur, using a microscope and tweezers, found and separated tartaric acid crystals showing “right-handed” and “left-handed” patterns, such as in the image above.
Thus, “right-handed” crystals are those formed by the (R, R) enantiomer, while “left-handed” crystals are those of the (S, S) enantiomer.
However, the crystals of tartaric acid (R, S) do not differ from the others, since they exhibit both right-handed and left-handed characteristics at the same time; therefore, they could not be “resolved.”