FIGURE 9 Spatial models for a tetrasubstituted methane. Top, planar; middle, pyramidal; bottom, tetrahedral.

van't Hoff then introduced the concept of the asymmetric carbon atom as follows: "When the four affinities of the carbon atom are satisfied by four univalent groups differing among themselves, two and not more than two different tetrahedrons are obtained, one of which is the reflected image of the other, they cannot be superposed; that is, we have to deal with two structural formulas isomeric in space." van't Hoff proposed that all carbon compounds that in solution rotate the plane of polarization possess an asymmetric carbon atom. He illustrated this for a great number of com cm ch3

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

cm ch3

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