Prochirality
In stereochemistry, 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 substituents are attached to a sp-hybridized atom, 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 sp-hybridized atom, and analogously for the "pro"-S substituent. A trigonal planar sp-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 terms "re" and "si" originate from the Latin "rectus" (right) and "sinister" (left), in reference to this ordering . 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, supposedly identically placed groups become distinguishable. In this way he showed that earlier exclusion of non-chiral citrate as a possible intermediate in the tricarboxylate cycle was mistaken.