From Biot To Pasteur

BIOGF5APHIC PHOTO 2.3. Louis Pastcur. (Permission for use granted from the Edgar Pahs Smith Collection at the University of Pennsylvania Library.) 

The salt can be formed by mixing the free tartaric acid with the bases sodium hydroxide (NaOH) and ammonium hydroxide (NH4OHSNH3 H2O). Note that the hydrogens (protons) are removed but the chirality of the tetrahedral carbons is unchanged. The simple chemical reaction describing 

Unlike the situation with the melting of quartz, when crystals of sodium ammonium tartrate were dissolved in water, they also rotated the plane of polarization. This led Pasteur to the correct conclusion that in this case, and many others, the phenomenon of optical rotation was a property of the molecules themselves, and not their arrangement in the crystal. 

FIGURE 2.9. Crystal formsof (—) (a) and (+) (b) sodium ammonium tartrate. 

how many racemic mixtures crystallize in enantiomeric forms Not many Approximately 250 racemic substances 

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Pasteur was only 26 years old at the time and was unknown in scientific circles. He was concerned about the accuracy of his observations because a few years earlier, the well-known German organic chemist Eilhardt Mitscherlich had reported that crystals of the same salt were all identical. Pasteur immediately reported his findings to Jean-Baptiste Biot and repeated the experiment with Biot present. Biot was convinced that Pasteur had successfully separated the enantiomers of sodium ammonium tartrate. Pasteur s experiment also created a new chemical term. Tartaric acid is obtained from grapes, so it was also called racemic acid (racemus is Latin for a bunch of grapes ). When Pasteur found that tartaric acid was actually a mixture of enantiomers, he called it a racemic mixture. Separation of enantiomers is called the resolution of a racemic mixture. 

Little was done after Biot s discovery of optical activity until 1848, when Louis Pasteur began work on a study of crystalline tartaric acid salts derived from wine. On crystallizing a concentrated solution of sodium ammonium tartrate below 28 °C, Pasteur made the surprising observation that two distinct kinds of crystals precipitated. Furthermore, the two kinds of crystals were non-superimposable mirror images and were related in the same way that a right hand is related to a left hand. 

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