Van’t Hoff-LeBel explanation

A WORD ABOUT... Pasteur s Experiments and the van t Hoff-LeBel Explanation... 

As discussed in the A Word About... Pasteur s Experiments and the van t Hoff-LeBel Explanation on pages 160-161, a critical experiment and observation made by Pasteur was that when he dissolved pure forms of the left- and right-handed crystals, he found that each solution was optically active, even though each set of crystals was obtained from crystallization of the racemic acid. The optical rotations of (R,R)- and (S,S)-tartaric acid are provided in Figure 5.14. Predict the specific rotation of a solution that contains 75% (R,R) and 25% (S,S) crystals of tartaric acid. 

It is this fourth unresolvable inactive tartaric acid which gives to tartaric acid its especial interest and importance in connection with the theory of stereo-isomerism. This acid, like the other three, has been fully explained in accordance with the tetra-hedral theory of van t Hoff and LeBel. The explanation rests upon the fact that there is a second asymmetric carbon atom in tartaric acid. We may construct, by models, or, by drawings, space-formulas for tartaric acid. According to the tetra-hedral theory, the dextro, levo and racemic inactive forms will be as follows, analogous to the corresponding formulas for the three lactic acids. The meso-tartaric acid is represented by the third drawing. 

From the chirality standpoint the next fundamental development occurred in 1874, when the tetrahedral carbon atom was proposed as a basis for molecular chirality by the Dutch and French chemists Jacobus Henricus van t Hoff (1852— 1911) [47, 48] and Joseph Achille LeBel (1847-1930) [49], respectively, independently and almost simultaneously. The discovery of the asymmetric carbon atom (van t Hoff s terminology) finally provided the explanation for the existence of optical isomers and for the chiral nature of the molecules of optically active substances, including many drugs. In his original 1874 pamphlet proposing the tetrahedron [47] van t Hoff listed camphor as a chiral molecule, but the structure he gave (19) was incorrect. 

Van t Hoff s theory offered an explanation for many hitherto mysterious cases of "absolute isomerism," including that of the two acids from wine tartar and the two lactic acids. The theory also validated and made sense of Pasteur s work of 1848 and his speculative notions of 1860. The first publication of the theory was in a self-published twelve-page Dutch pamphlet, dated 5 September 1874 and printed immediately thereafter a French translation of the pamphlet appeared almost simultaneously in a relatively obscure Dutch journal. Two months later (5 November 1874), van t Hoff s former labmate in Wurtz s research group, LeBel, published a substantially similar theory in the Bulletin de la Societe Chimique. There has never been any evidence that either man knew of the other s work in advance of his own publication. Since van t Hoff had published his theory so obscurely, it was in LeBeTs paper that most chemists first read of these ideas. But because LeBeTs... 

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