Stereochemistry

The three-dimensional structure of a molecule determines its physical properties, such as the temperature at which it turns from a liquid to a gas (boiling point) and the temperature at which it changes from a solid to a liquid (melting point). The geometric structure of a molecule is also responsible for its chemical properties, such as its strength as an acid or base. The compound trans-1,2-dichloroethene Figure 5. Illustration by Hans & Cassidy. Courtesy of Gale Group. becomes a gas at a much higher temperature than the structurally similar cis-1,2-dichloroethene. The compound cis-3-phenylpropenoic acid is a stronger acid than trans-3-phenylpropenoic acid only because the hydrogen atoms are connected to the doubly bonded carbon atoms differently.

The geometric structure of a molecule can also have a dramatic effect on how that molecule tastes or how it functions as a drug. The antibacterial drug chloramphenicol is commercially produced as a mixture of the two compounds in Figure 5. One three-dimensional arrangement of atoms is an active drug, the other geometric structure is ineffective as an antibacterial agent.

In most cases the energy of a molecule or a compound, that is, the particular energy level of its electrons depends upon the relative geometry of the atoms comprising the molecule or compound. Nuclear geometry means the geometrical or spatial relationships between the nucleus of the atoms in a compound or molecule (e.g., the balls in a ball and stick model). When a molecule or compound's energy is related to its shape this is termed a stereoelectronic property.

Stereoelectronic effects arise from the different alignment of electronic orbitals with different arrangements of nuclear geometry. It is possible to control the rate or products of some chemical reactions by controlling the stereoelectronic properties of the reactants.