Mirror Plane

figure 1 Symmetrical and asymmetrical letters of the English alphabet and their mirror images, dimensional figure whose axis of dissymmetry lies between the front and back sides. The mirror image of the human body is also reversed, for example, the mirror image of the right hand appears to be on the left side of the body, etc. In addition, these mirror images cannot be made identical by simple spatial manipulations. The mirror image of an asymmetrical two-dimensional figure such as the letter R can be lifted out of the plane of the paper, turned over, and placed exactly on top of the original figure. A mirror image of a right hand cannot be placed exactly on top of a right hand (with both palms up or both palms down). Three-dimensional figures can exist as nonsuperimposable mirror images.

The spatial relationships that exist for the human body also exist at the molecular level because the molecules of nature exist as three-dimensional symmetrical and asymmetrical figures. One of the most common asymmetric molecules is a tetravalent carbon atom with four different ligands attached to it. The spatial arrangement of the atoms in this molecule is shown in Fig. 2. The carbon atom depicted in Fig. 2 is the asymmetric center of the molecule, and the molecule is a chiral stereoisomer. If the molecule and its mirror image are nonsuperimposable, the relationship between the two molecules is enantiomeric, and the two stereoisomers are enantiomers. Carbon is not the only atom that can act as an asymmetric center. Phosphorus, sulfur, and nitrogen are among some of the other atoms that can form chiral molecules.

Figure 2 An asymmetric tetravalent carbon atom and its mirror image.

Figure 2 An asymmetric tetravalent carbon atom and its mirror image.

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