https://doi.org/10.1351/goldbook.P04859
This term is used in different, sometimes contradictory ways; four are listed below.
- The geometric property of an achiralobject (or spatial arrangement of points or atoms) which is capable of becomingchiralin a singledesymmetrizationstep. Anachiralmolecular entity, or a part of it considered on its own, is thus called prochiral if it can be madechiralby the replacement of an existing atom (orachiralgroup) by a different one. Anachiralobject which is capable of becomingchiralin twodesymmetrizationsteps is sometimes described as proprochiral. For example the proprochiral CH3CO2H becomes prochiral as CH2DCO2H andchiralas CHDTCO2H.
- The term prochirality also applies to an achiralmolecule or entity which contains a trigonal system and which can be madechiralby the addition to the trigonal system of a new atom orachiralgroup. For example addition of hydrogen to one of theenantiotopicfaces of the prochiral ketone CH3CH2COCH3 gives one of the enantiomers of thechiralalcohol CH3CH2CHOHCH3; the addition of CN− to one of thediastereotopicfaces of thechiralaldehyde shown below converts it into one of thediastereoisomersof the cyanohydrin. The two faces of the trigonal system may be described as Re and Si.
- The term prochiral also applies to a tetrahedral atom of an achiralorchiralmolecule which is bonded to twostereoheterotopicgroups. For example, the prochiral molecule CH3CH2OH can be converted into thechiralmolecule CH3CHDOH by the isotopic replacement of one of the twoenantiotopichydrogen atoms of themethylenegroup. The carbon atom of themethylenegroup is called prochiral. The prochiral molecule HO2CCH2CHOHCH2CO2H can be converted into achiralproduct by esterification of one of the twoenantiotopic–CH2CO2H groups. The carbon atom of the CHOH group is called prochiral. Thechiralmolecule CH3CHOHCH2CH3 can be converted into one of thediastereoisomersof CH3CHOHCHDCH3 by the isotopic replacement of one of the twodiastereotopichydrogen atoms of themethylenegroup. The carbon atom of themethylenegroup is called prochiral. Thestereoheterotopicgroups in these cases may be described as pro-R or pro-S. Reference to the twostereoheterotopicgroups themselves as prochiral, although common, is strongly discouraged.See:chirality centre
- The term prochirality is also applied to the enantiotopicfaces of a trigonal system.