H

Figure 10 Tetrahedral model for lactic acid enantiomers (carbon atom is at the center of the tetrahedron) as envisioned by van't Hoff.

pounds: ethylidene lactic acid (now called a-hydroxypropionic acid), as-partic acid, asparagine, malic acid, glutaric acid, tartaric acid, sugars and glucosides, camphor, bomeol, and camphoric acid.

Two compounds from this list are worthy of note: tactic acid (Fig, 10) and tartaric acid (Fig, 11). Wislicenus (Fig. 12) extensively investigated the isomers of lactic acid between 1863 and 1873, and was convinced that the number of isomers exceeded that allowed by the existing structural theory (12). However, due to experimental difficulties in obtaining pure samples of the isomers, in addition to the limits of the structural theory then known to him, he ended up going around in circles, van't Hoff studied the publications of Wislicenus on lactic acids and they led him to his own stereochemical ideas. In fact, lactic acid was the first concrete example of an optically active compound that van't Hoff discussed after his theoretical introduction. He pointed out that ethylidene lactic acid contains an asymmetric carbon. Therefore, it can exist as two pure enantiomers or a racemic mixture, which nicely cleared up the confusion surrounding the lactic acid

Figure 11 Structures for three tartaric acid isomers that are representative of the tetrahedral models used by van't Hoff.

Figure 11 Structures for three tartaric acid isomers that are representative of the tetrahedral models used by van't Hoff.

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