Stereochemistry

Symmetry is a term used to describes molecules with equal parts. When a molecule is symmetrical it has portions that correspond in shape, size, and structure so Figure 1. Illustration by Hans & Cassidy. Courtesy of Gale Group. Figure 2. Illustration by Hans & Cassidy. Courtesy of Gale Group.

that they could be mapped or transposed on one another. Bilateral symmetry means that a molecule can be divided into two corresponding parts. Radial symmetry means that if a molecule is rotated about an axis that a certain number of degrees rotation (always less than 360°) it looks identical to the molecule prior to rotation.

A molecule is said to be symmetrical if it can be divided into equal mirror image parts by a line or a plane. Humans are roughly bilaterally symmetrical. Draw a line down the middle of the human body and the line divides the body into two mirror image halves. If a blob of ink were placed on a piece of paper, and then the paper was folded over and then unfolded again, you would find two ink spots—the original and the image—symmetrical about the fold in the paper. Molecules and complexes can have more than just two planes of symmetry.

Human hands provide an excellent example of the concept of molecular handedness. The right and left hands are normally mirror images of each other, the only major difference between them being in the direction one takes to go from the thumb to the fingers. This sense of direction is termed handedness, that is, whether a molecule or complex has a left and right orientation. Two molecules can be mirror images of each other, alike in every way except for their handedness.

Handedness can have profound implications. Some medicines are vastly more effective in their left-handed configuration than in their right-handed configuration. In some cases biological systems make only one of the forms. In some cases only one of the forms is effective in cellular chemical reactions.

Figure 3. Illustration by Hans & Cassidy. Courtesy of Gale Group.

Figure 4. Illustration by Hans & Cassidy. Courtesy of Gale Group.

A molecule that is not symmetric, that is, a molecule without a plane of symmetry, is termed an asymmetric molecule. Asymmetric molecules can have another property termed chirality.