stereochemistry Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms that form the structure of molecules and their manipulation. The study of stereochemistry focuses on the relationships between stereoi ...

, prochiral molecules are those that can be converted from achiral to chiral in a single step. An achiral species which can be converted to a chiral in two steps is called proprochiral. If two identical

substituent A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and ''functional group'', as well as '' side ...

s are attached to a sp³-hybridized

atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, a ...

, the descriptors ''pro''-R and ''pro''-S are used to distinguish between the two. Promoting the ''pro''-R substituent to higher priority than the other identical substituent results in an ''R'' chirality center at the original sp³-hybridized atom, and analogously for the ''pro''-S substituent. A

trigonal planar In chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of an equilateral triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands ...

sp²-hybridized atom can be converted to a chiral center when a substituent is added to the ''re'' or ''si'' () face of the molecule. A face is labeled ''re'' if, when looking at that face, the substituents at the trigonal atom are arranged in increasing Cahn-Ingold-Prelog priority order (1 to 2 to 3) in a clockwise order, and ''si'' if the priorities increase in anti-clockwise order; note that the designation of the resulting chiral center as ''S'' or ''R'' depends on the priority of the incoming group. The concept of prochirality is necessary for understanding some aspects of enzyme stereospecificity. Alexander Ogston pointed out that when a symmetrical molecule is placed in an asymmetric environment, such as the surface of an

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...

, supposedly identically placed groups become distinguishable. In this way he showed that earlier exclusion of non-chiral

citrate Citric acid is an organic compound with the chemical formula HOC(CO2H)(CH2CO2H)2. It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in the ...

as a possible intermediate in the tricarboxylate cycle was mistaken.

References

{{Reflist Stereochemistry