The Stereochemistry in the crystals contained in a wine barrel

by Roberto Poeti

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Red crystal formations : thus was born the stereochemistry

I  have kept for many years an old cask in the cellar of my house before deciding to make firewood. Its interior walls, in the dimly lit cellar, seemed covered with blackish storage and amorphous. But when I took her out and was illuminated by sunlight I felt the thrill of discovery. The amorphous layer now appeared made of red crystal formations shaped like shells, which were arranged groups of transparent, colorless prismatic crystals that sparkled like diamonds. It was a unique show.

The composition  of Crystals

I have long sought, unsuccessfully, in internet images like these crystal formations .  Their composition, using X-holders, is made up of a cluster of thin foils of single crystals of potassium acid  tartrate . These translucent  sheets you can see if you look at the sample sideways .  The color of course is caused by anthocyanins, while the colourless and transparent prismatic crystals are made up of calcium tartrate.

The story of cristals

We shall follow briefly the story of these crystals that have been protagonists of progress of chemistry in the first half of the nineteenth century. For a series of coincidences, as we shall see, tartaric, paratartarici acids and their salts have been indispensable for discoveries that I will describe briefly. Had to be common these grasslands of crystals when winemaking was done using traditional methods. In fact, Lucretius and Pliny the elder were familiar with cream of tartar (name of sediment in barrels consisting predominantly potassium tartrate) : It tasted sour and burned with a flame violacea. At that time it was used in a dozen remedies.

The crystals and the  light

Attracted the attention of even the great chemist Carl Wilhelm Scheele from which he extracted and purified tartaric acid in 1769. In the early nineteenth century the history of these crystals is intertwined with that of light . There is a period in the history of chemistry, which we can place in the first half of the 19th century, which is of great importance because they laid the foundations of what in later years becomes “chemistry in space”, so title  an essay Jacobus van ‘t Hoff in 1875. The survey instrument, which revolutionized the image of the structure of matter, was polarized light . His discovery occurred by Etienne Malus in the year 1808, and a few years later the polarized light began to be used as a probe to “Watch” the structure of matter.

Quartz crystal mirror images in my collection

The importance of quarz cristals

Examining of quartz crystals, Francois Arago and Jean-Baptiste Biot, students of Malus, discovered that there were two types of mirrored and non-overlapping emiedrici crystals that rotated the plane of polarization of light (Many crystals of the same substance can vary in habitus in their relative sizes and developing similar plans, however whenever the value of the angles between faces or edges can be seen in the faces and corners symmetrically arranged. In emiedrici crystals this does not occur, therefore they possess a symmetry fragment.

The discovery of Jean-Baptiste Biot

BIOT completed its investigation by noting that even solutions of organic substances such as sugar, camphor, lactic acid and tartrates rotated the plane of polarization, i.e. were optically active. As the quartz lost his property when melted or dissolved, Biot pointedly suggested that the optical activity of organic substances, which also manifested in their solutions, resided in the molecules, whereas in quartz depended on the entire crystalline building.

Why were chosen the tartrates?

 But it is on the tartaric acid, which was extracted from deposits of tartrates, that wine had left in the barrels, which will focus the investigation.  Tartaric acid could be obtained with relative ease by following the procedure set by Carl Scheele. Raw Tartar or cremor tartar extracted from casks was dissolved in hot water and then with the addition of powdered calcium carbonate happened  the precipitation of a half of potassium tartrate as calcium tartrate , followed a second precipitation with calcium chloride for the other half. Precipitated calcium tartrate was collected, washed and then decomposed by a glut of sulfuric acid. Calcium sulphate, produced by the reaction, was then separated by filtration and tartaric acid solution obtained was evaporated to obtain tartaric acid crystals deposit.

Pure tartrate crystals

The crystals were purified making them dissolve again in hot water, then the solution was discoloured with charcoal, and finally was done the crystallization    . Were obtained of large crystals if you added even sulfuric acid. It’s amazing how this process with a sequence so rational may have been set by Scheele in 1768, in an age still so uncertain about knowledge of chemistry. The final result was considered, for a crystallographer, the best you could hope for: large crystals, regular, and in large quantity.

Louis Pasteur’s observations

The habitus of tartaric acid crystals and its different salts was always emiedrico and in the same direction, as in the same direction was rotated the plane of polarized light. That’s what Louis Pasteur, keen observer, saw. He saw confirmed his hypothesis of a close correlation between morphology of crystals and optical rotation, as had been discovered for quartz crystals.

Where was produced tartaric acid ?

But we need to step back before continuing with Pasteur. In the region of Thann in France something happened that will determine the course of the research  Tartaric acid and its salts had a large circulation, entered as an ingredient in many cosmetics and remedies like salt of Rochelle and the Tartars emetics, were also required by the textile industry. So many wineries were converted to industries for the production of tartaric acid. Between 1822 and 1824 Paul Kestner, an industrial of Thann (France) had gotten together with tartaric acid, another substance that had thought it oxalic acid. The substance first crystallized from solutions of tartaric acid, so you could separate out the crystals. He noted that the presence of the substance in the next stage of purification of tartaric acid made irregular crystallization of the latter. The substance produced was pushed aside, stored, because no one believed  had a commercial value. Discovery following the nature of the substance, for how many attempts Kestner did, was unable to get it. There had been a number of favorable conditions that he was no longer able to reproduce. From this incredible story development of chemical crystallography had an unexpected help.

The paratartaric acid

The substance was paratartaric acid. Gay Lussac in 1826 established that the substance  had the same composition of tartaric acid. Tartaric acid and paratartaric and their salts were investigated before the studies of Pasteur, from Biot and Eilhard Mitscherlich because  were different isomer substances presenting minor chemical differences and often their crystals were isomorphic. To understand the root of their differences Mitschelich began to investigate the symmetry of crystals . Of all the salts examined, two of them ammonium sodium tartrate and sodium ammonium paratartrato presented with a identical crystalline habitus . Thus spoke Mitschelich in a note presented to the Academy of Sciences in 1841: “ … the nature and number of atoms, their layout and their distances, are the same in the two bodies between them compared “.

The difference between paratartaric and tartaric acid

Pasteur recalls in his lecture given in 1860 to the Paris chemical society that what Mitschelich had found challenged the very definition of chemical species, but that perhaps Mitschelich had not noticed that the crystalline form of his tartrate was emiedrica like everyone else was, but not the crystalline form of paratartrato. Tartrate  rotated the plane of polarized light while the paratartrato was indifferent. Surprised and confused to the result of his work, for ten years he hesitated to make them public. Pasteur recalls in his lecture given in 1860 to the Paris chemical society that what Mitschelich had found challenged the very definition of chemical species, but maybe Mitschelich didn’t notice that the crystalline form of his tartrate was emiedrica like everyone else was, but not paratartrato.   If this was confirmed the note of Mitschelich no longer had anything extraordinary, because the two crystal forms would not be identical. Pasteur after graduating from normal school in 1847, he was called to Strasbourg as Assistant Professor of chemistry. Has twenty-five years old when he worked in this research that will continue until 1853 and the outcome of which will have the effect of an earthquake. Will work using paratartaric acid samples that the industrialist Kestner had made him have.

Paratartaric acid labelled samples from Pasteur, received by industrialist Kestner. Lie in his lab at the Pasteur Institute in Paris.

Pasteur and his discovery of the emiedria of tartrates

He found that crystals of potassium and ammonium tartrate presented emiedria, the same one that he had found for others tartrates from him examined , but in paratartrato crystals of sodium and ammonium that had crystallized noted that had split two distinct types of emiedrici crystals with facets oriented now right now left. Were mirror images but not overlapping. He found that left emiedrici crystals revolved the polarisation plane right, while those oriented to the left turn the plane left. The first were identical to those of tartaric acid hitherto known.

Simplified models of tartaric acid crystals left and right according to Pasteur who obtained by solving the acid paratartaric. In red the veneers emiedriche

Pasteur was helped by luck twice

Was an outcome so unexpected that his master Biot incredulously asked him to repeat it in front of him, before presenting it in front of the Academy. Only a keen observer and a prepared mind could reach these results, however, Pasteur was helped by luck twice:

First : because the sodium and ammonium paratartrato is the only one, with the possible exception of potassium sodium tartrate, which can be resolved into two isomers with the crystallization.

 Second : the temperature at which had operated was that of the cold Paris rather than the mild Mediterranean. Only working below 26° C is obtained the separation.

Because the molecules can be dissymmetrical ?

Pasteur was  a skilled investigator ; he  was able to give the results of its research the revolutionary significance for that time  . Although it was too early to talk about bonds between atoms and the perception of molecules in their three dimensions were still very uncertain if not opposed , he imagined that between atoms there are spatial relationships defined ; and used this criterion to distinguish two substances between them isomers.

The conclusions that he drew from his researches are expressed in his lecture

“We indeed know that, on the one hand, the provisions of tartaric acid are two molecular dissymmetrical, second, that they are all the same except to offer the dissymmetry in opposite directions. The right acid atoms are gathered according to the spire of a hand screw , or located at the top of an irregular tetrahedron, or arranged according to this or that particular dissymmetric form? To these questions we couldn’t answer. But what cannot be subject to doubt is that there is an arrangement of atoms in an order dissymmetric in non-overlapping image “.

After twenty-five years from Pasteur born the stereochemistry

Reading these words there comes to mind the tetrahedron of van ‘t Hoff and the DNA Helix by James Watson and Francis Crick. With the tetrahedron of Van’t Hoff, after twenty-five years, born the stereochemistry. In the history of science coincidences, fatality, fortunate circumstances often accompany the great achievements; the case I’ve reviewed falls right into that of discovery backed by a series of fortunate circumstances.   Two main protagonists have accompanied us in this short trip : polarized light and crystals of a wine barrel aged  . Both were the protagonists of the birth of chemistry in 3D.

Dionysius made a far-fetched gift: the stereochemistry!

This post is also available in: blankItaliano (Italian)

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