In stereochemistry
Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms within molecules. An important branch of stereochemistry is the study of chiral molecules....

, prochiral molecules are those that can be converted from achiral to chiral
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....

 in a single step.

If two identical substituent
In organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon...

s are attached to a sp3
Orbital hybridisation
In chemistry, hybridisation is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties. Hybridised orbitals are very useful in the explanation of the shape of molecular orbitals for molecules. It is an integral part...

-hybridized atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

, the descriptors pro-R and pro-S are used to distinguish between the two. Promoting the pro-R substituent to higher priority than the identical alternative would result in an R chirality center at the original sp3-hybridized atom, and vice versa.

A trigonal planar sp2-hybridized atom can be converted to a chiral center when a substituent is added to the re or si face of the molecule. These faces are labelled re if the substituents at the trigonal atom are disposed in decreasing Cahn-Ingold-Prelog priority order in a clockwise order, and si if the priorities decrease in counter-clockwise order; but the designation of the resulting chiral centre as s or r depends on the priority of the incoming group.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.