The Stereochemistry of Crystals in a Wine Barrel

Observing the deposit of Tartrare crystals in the red barrels, stereochemistry was born with the intuitions of Louis Pasteur

by Roberto Poeti
The Birth of Stereochemistry.

Rain of crystals: I kept an old barrel in the cellar of my house for many years before deciding to make wood for the fireplace. Its interior walls, in the dimly lit cellar, seemed to be covered with a blackish, amorphous deposit. But when I took it outside and it was illuminated by sunlight, I felt the thrill of discovery. That amorphous layer now appeared to be made of red shell-shaped crystalline formations, on which were set clusters of prismatic crystals, colorless transparent that shone like diamonds. It was a unique sight.

The red shell-shaped formations are of calcium tartrate, and the clear prismatic crystals are of potassium hydrogen tartrate .

The composition of crystals

I have searched for a long time, without success, on the internet images similar to these crystalline formations;  their composition on X-ray analysis is made up of an aggregate of thin sheets of single calcium tartrate crystals. And these translucent foils can be observed if you look at the sample from the side. The color is naturally given by anthocyanins, while the prismatic crystals, colorless and transparent, are made up of potassium hydrogen tartrate.

The History of Crystals

These crystal prairies must have been common when winemaking was done using traditional methods. In fact, Lucretius and Pliny the Elder were familiar with cream of tartar (the name of the deposit in barrels consisting mainly of potassium acid tartrate): it had a sour taste and burned with a violet flame. At that time it was used in a dozen remedies. It also attracted the attention of the great chemist Carl Scheele from whom he obtained and purified tartaric acid in 1769.

Crystals and light

At the beginning of the nineteenth century, the history of these crystals was intertwined with that of light. There is a period in the history of chemistry , which we can place in the first half of the nineteenth century , which is of extraordinary importance because the foundations of stereochemistry were laid, which in the following years would become “Chemistry in space” , which will be the title  of an essays by Jacobus van’t Hoff  in 1875 . The instrument of investigation, which revolutionized the image of the structure of matter, was polarized light. It was discovered by Etienne Malus in 1808; a few years later polarized light began to be used as a probe to “observe” the structure of matter.

Due to the double refraction of a calcite crystal, a laser beam passing through it splits. Malus discovers polarized light by investigating the phenomenon of double refraction.







The Importance of Quartz Crystals

Examining quartz crystals, François Arago and Jean-Baptiste Biot, pupils of Malus, discovered that there were two types of specular and non-superimposable hemihedral crystals that rotated the plane of polarization of light (Many crystals of the same substance can vary in habitus in their relative size and in the development of similar planes, however any change in the value of the angles between faces or edges is reproduced in symmetrically arranged faces and corners. In hemihedral crystals this does not happen, so they have an incomplete symmetry).

Two hemihedral quartz crystals found in a local market in Toledo, Spain









The discovery of Jean-Baptiste Biot

Biot completed the investigation by observing that solutions of organic substances including for example sugar, camphor, lactic acid and tartrates also rotated the plane of polarization, i.e.  they were optically active. While quartz lost its property when molten or dissolved, and Biot acutely hypothesized that the optical activity of organic substances, which also manifested itself in their solutions, resided in the molecules, while in quartz it depended on the whole crystalline edifice.

 Why were tartrates chosen?

But it is on tartaric acid, which was obtained from the deposits of tartrates , left by the wine in the barrels , that the investigation will focus later . Why were tartrates chosen? Tartaric acid could be obtained pure with relative ease by following the procedure set up by Carl Scheele.  The raw tartar or cream of tartar extracted from the barrels was dissolved in hot water; With the addition of calcium carbonate powder, one half of the potassium acid tartrate was precipitated as calcium tartrate, followed by a second precipitation with calcium chloride for the other half. The precipitated calcium tartrate was collected, washed and then decomposed by an excess of sulfuric acid.  The calcium sulphate, produced by the reaction, was then separated by filtration and the tartaric acid solution obtained was evaporated to obtain the deposit of tartaric acid crystals.

Pure tartrate crystals

The crystals were purified by dissolving them in hot water, decolorizing the solution with charcoal, and recrystallizing them. Large crystals were obtained if sulphuric acid was also added. It is extraordinary how this procedure with such a rational sequentiality could have been set up by Scheele in 1768, in an era still so uncertain about the knowledge of chemistry. In the end, the result was for a crystallographer the best that could be hoped for: large, regular, crystals in large quantities

Louis Pasteur’s remarks

The habitus of the crystals of tartaric acid and its different salts was always hemihedral and in the same direction, just as the plane of polarized light was rotated in the same direction. This is what that keen observer Louis Pasteur saw  , who saw his hypothesis of a close correlation between crystal morphology and rotational power confirmed, as had also been discovered for quartz crystals.  But we have to take a step back before we continue with Pasteur.

Where was tartaric acid produced?

In the region of Thann in France something happened which will condition the course of the research:

Tartaric acid and its salts were widely used, they entered as ingredients in many cosmetics and remedies such as Rochelle’s salt and emetic tartars, they were also in demand by the textile industry. So many wineries had converted into industries for the production of tartaric acid. Between 1822 and 1824 Paul Kestner, one of these tartar industrialists from Thann (France), had obtained, together with tartaric acid, another substance which he believed to be oxalic acid. The substance crystallized first from tartaric acid solutions, so the crystals could be carefully separated. In fact, he had noticed that the presence of the substance in the subsequent phase of purification of tartaric acid made the crystallization of the latter irregular. The substance produced was set aside, stored, because it was not considered to have any commercial value. Later discovering the nature of the substance, no matter how many attempts Kestner made, he could no longer obtain it.  A series of favorable conditions had occurred that he was no longer able to reproduce.

Paratartaric acid

From this incredible event, the development of chemical crystallography had an unexpected help. The substance in question was paratartaric acid. Gay-Lussac in 1826 established that it had the same composition as tartaric acid. Tartaric acid and paratartaric acid together with their salts were investigated, before Pasteur’s studies, by Biot and Eilhard Mitscherlich because they were isomeric substances that presented slight chemical differences and often their crystals were isomorphic. To understand the root of their differences, Mitschelich set out to investigate the symmetry of crystals. Of all the salts examined, two of them, sodium ammonium tartrate and sodium ammonium paratartrate, had an identical crystalline habitus. Mitschelich put it this way in a note to the Academy of Sciences in 1841:

“….here the nature and number of the atoms, their arrangement and their distances, are the same in the two bodies compared with each other.”

The Difference Between Tartaric and Paratartaric Acid

The tartrate rotated the plane of polarized light, and the paratartrate was indifferent. Surprised and confused by the result of his work, Pasteur hesitated to make them public for ten years. Pasteur recalls in his lecture given in 1860 to the Chemical Society of Paris that what Mitschelich had found called into question the very definition of chemical species, but that perhaps Mitschelich had not noticed that the crystalline form of his tartrate was hemihedral like that of all other tartrates, while that of paratartrate was not.   If this was confirmed, Mitschelich’s note was no longer extraordinary, because the two crystalline forms would not have been identical.

Pasteur’s long experimentation

After graduating from the École Normale in 1847, Pasteur was called to Strasbourg as an assistant professor of chemistry. He was twenty-five years old when he embarked on this research that would last until 1853 and whose results would have the effect of an earthquake. He will work using samples of paratartaric acid that the industrialist Kestner had given him.

Samples of paratartaric acid labeled by Pasteur, received from the industrialist Kestner. They are located in his laboratory at the Pasteur Institute in Paris.

Pasteur and the discovery of the hemihedry of crystals

He found that the crystals of potassium and ammonium tartrate exhibited hemihedria, the same as that which he had found for the other tartrates he had examined; but in the crystals of sodium and ammonium paratartrate which he had crystallized, he found that two distinct types of hemihedral crystals had separated, with the facets oriented now to the right and now to the left. The images were mirrored but not superimposable. He found that right-handed hemhedral crystals rotated the plane of polarization to the right, while those oriented to the left rotated the plane to the left. The former were identical to those of tartaric acid known until then.

Simplified models of right and left tartaric acid crystals according to Pasteur that he obtained by resolving paratartaric acid. In red the hemihedral facets

Pasteur was helped by luck twice

It was such an unexpected result that his incredulous teacher Biot asked him to repeat it in front of him, before giving an account of it to the Academy. Only a keen observer and a trained mind could arrive at these results.  However, Pasteur was helped by luck twice:

 First, because sodium ammonium paratartrate is the only one that can be resolved to the two optical isomers by crystallization.

Secondly, the temperature at which he operated was that of cold Paris rather than mild Mediterranean. Only by operating below 26°C is separation achieved.

Why does a molecule have dissymmetry?

But Pasteur was not only a very skilled experimenter, he was able to give the results of his research a revolutionary significance for that time. Although it was premature to speak of bonds between atoms and the perception of molecules in their three dimensions was still very uncertain if not opposed, he imagined that there were definite spatial relations between atoms, and he used this criterion to distinguish two isomer substances from each other.

The conclusions he drew from his research are expressed in his reading

“We do know that, on the one hand, the molecular arrangements of the two tartaric acids are dissymmetrical, and on the other, that they are strictly the same, with the only difference that they offer dissymmetries of opposite directions. Are the atoms of the right acid gathered according to the coils of a right-handed helix, or situated at the tops of an irregular tetrahedron, or arranged according to this or that definite dissymmetrical resemblance? We do not know how to answer these questions. But what cannot be the subject of doubt is that there exists an arrangement of atoms according to a dissymmetrical order in a  non-superimposable image  .”

Twenty-five years later, stereochemistry was born from Pasteur

Reading these words, we immediately think of the van’t Hoff tetrahedron and the DNA helix.  It was with van’t Hoff’s tetrahedron, after twenty-five years, that stereochemistry was born. In the history of science, coincidences, fatalities, and fortunate circumstances often accompany great achievements; The case I have examined is one of the great discoveries assisted by a series of fortunate circumstances.   We were accompanied on this brief journey through the history of chemistry by two main protagonists: polarized light and the crystals of an aged wine barrel. Both were the protagonists of the birth of 3D chemistry.

Dionisio has given us an unbelievable gift: stereochemistry!

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