Pasteurs Discovery of Enantiomers

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. 

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 

Louis Pasteur (1822-1895) w/as born at Dole in the Jura region of France, the son ot leather tanners. After receiving his doc-toiate froiii the Ecole Nurmale Superieure at age 25, his landmark discovery of tartaric acid enantiomers was made only 1 year later. Pasteur is best known for his studies in bacteriology and for his discovery of vaccines for anthrax and rabies. 

Pasteui made flie surprisirig observation tlial two disliiiet kinds of cry tals precipitated. Furthermore, tlie two kinds of crystals were mirror images am were related in the same way that a right hand is rclaied to a left hand. 

Working carefully with tweezers, Pasteur was able to separate the crystal into two piles, one of right-handed crystals and one of left-handed crys tals like those shown in i-igure 9.6. Although the original sample, a 50 50 mix lure of right and left, was optically inactive, solutions of the crystals from cad of the sorted j iles were optically active, and their specific rotations were equa in amount but opposite in sign. 

Today we would describe Pasteur s work by saying that he had discovered enantiomers. Knantiomers, also called optical isomers, have identical physical pre perties, such as melting point and boiling point, but differ in the direction in which their solutions rotate plane-polarized light. 

Test your knowledge of Key Ideas by answering end-of-chapter exercises marked with A. 

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