Stereochemistry

Sometimes it is difficult to tell whether or not two molecules or complexes will exhibit stereochemical properties. If two molecules or complexes have the same molecular formula they are candidates for stereochemical analysis. The first step is to determine if the two molecules or complexes are superimposable. If they are then are identical structures and will not exhibit stereochemical properties. The second step is to determine if the atoms are connected to each other in the same order. If the atoms are not connected in the same order then the molecules or complexes are constitutional isomers and will not exhibit stereochemical properties. If the atoms are connected in the same order then they are stereoisomers. The next step is to see if the stereoisomers can be made identical by rotating them around a single bond in the molecule or complex then they are called conformational isomers. Stereoisomers that can not be so rotated are called configurational isomers. The last step is to analyze the configurational isomers to determine whether they are enantiomers, diastereomers, or cis-trans isomers. Those that are mirror images are enantiomers. Those stereoisomers that are not mirror images of each other are diastereomers (the prefix dia indicated opposite or across from as in diagonal) or cis-trans isomers. Stereoisomers can also be characterized as cis (Latin for "on this side") or trans (Latin for "across") when they differ in the positions of atoms or groups relative to a reference plane. They are cis-isomers if the atoms are on the same side of the plane or trans-isomers if they are on opposite sides of the reference plane.

If the molecule has a double bond in its chemical for mula—for example, formaldehyde, O=CH₂—then the three-dimensional structure of the molecule is somewhat different. To translate formaldehyde into its geometric structure, one must know its chemical formula indicates a central carbon atom that has a double bond to an oxygen atom (C=O) and two single bonds to hydrogen atoms (CH). In the geometric arrangement of a carbon atom that has a double bond to another atom, there is a 120° angle between any two bonds, and each bond points away from the central carbon atom. If the bonded atoms are connected by imaginary lines, they represent the corners of an equilateral triangle (see Figure 2). In molecules that contain two carbon atoms connected by a double bond and each of which is bonded to a hydrogen atom and another atom, then the geometric isomer that has both hydrogen atoms on the same side is in a cis configuration. The molecule with the hydrogen atoms on opposite sides of the double bond is designated as the trans configuration. For example, cis-1,2-dichloroethene has the hydrogen atoms on the same side of the double bond, where as trans-1,2-dichloroethene has them on opposite sides. Both of these compounds have the same chemical formula (ClHC=CHCl), but their geometric representations are different (see Figure 3).

The only other type of bond a carbon atom can have is a triple bond—that is, three bonds to the same atom. Acetylene (HCCH) is a molecule that contains a triple bond between the two carbon atoms, and each carbon atom is bonded to a hydrogen atom (C-H). A carbon atom with a triple bond to another atom is geometrically straight or linear.