Stereochemistry

A molecule is said to be chiral if it lacks symmetry and its mirror images are not superimposable. To be chiral a molecule must lack symmetry, that is, a chiral molecule can not have any type or symmetry.

Carbon atoms with four sp³ hybridized orbitals can enter into up to four different bonds about the central carbon atom. When the central carbon bonds with differing atoms or groups of atoms the carbon is termed an asymmetric carbon atom. Bromochlorofluoromethane is an example of such a molecule. The central carbon, with four sp³ bonds oriented (pointing) to the corners of a tetrahedron, is bonded to a bromine, chlorine, fluorine and methane atoms. There is no symmetry to this molecule.

Chiral carbon atoms are also assigned an R and S designation. Although the rules for determining this designation can be complex, for simple molecules and compounds with chiral carbons the determination is easily accomplished with the help of a model of the molecule. The four different bonded groups are assigned a priority. When assigning priority to groups, atoms that are directly bonded to the central chiral carbon atom have their priority based upon their atomic number. The atom with the highest atomic number has highest priority and atom with the lowest atomic number the lowest priority. As a result, hydrogen atoms bonded to the chiral molecule have the lowest priority. If isotopes are bonded then the isotope with the largest mass has the higher priority. The molecule is then turned so that the lowest priority group is farthest away from view. If one must take a counterclockwise path from the highest to lowest priority group the chiral configuration is said to be sinister (S). If the path from highest to lowest priority groups is clockwise then the chiral molecule is said to be rectus (R).

The compound carvone has two three-dimensional structures, one S and the other R (see Figure 4).

The compounds differ in their three-dimensional structure by the position of the indicated hydrogen atom. In S-Carvone, only the hydrogen atom is pointed into the paper, while in the R compound, the hydrogen atom is coming out of the paper. S-Carvone has a caraway flavor when tasted, whereas the R compound has the flavor of spearmint.

The rectus (R) and sinister (S) property relates to the structure of an individual molecule. In contrast, dextro (+) and levo (-) properties are based on the properties of a large collection of the molecules or complex.

Because a molecule can have more than one chiral carbon. The number of stereoisomers can be determined by the 2n rule, where n = the number of chiral carbons. Thus, if one chiral carbon is present there are two possible stereoisomers, with two chiral carbons there are four possible stereoisomers. Any chemical reaction that yields predominantly one stereoisomer out of several stereoisomer possibilities is said to be a stereoselective reaction.