This term is used in different, sometimes contradictory ways; four are listed below.
- The geometric property of an @A00069@ object (or spatial arrangement of points or atoms) which is capable of becoming @C01057@ in a single @D01623@ step. An @A00069@ molecular entity, or a part of it considered on its own, is thus called prochiral if it can be made @C01057@ by the replacement of an existing atom (or @A00069@ group) by a different one. An @A00069@ object which is capable of becoming @C01057@ in two @D01623@ steps is sometimes described as proprochiral. For example the proprochiral CH3CO2H becomes prochiral as CH2DCO2H and @C01057@ as CHDTCO2H.
- The term prochirality also applies to an @A00069@ molecule or entity which contains a trigonal system and which can be made @C01057@ by the addition to the trigonal system of a new atom or @A00069@ group. For example addition of hydrogen to one of the @E02083@ faces of the prochiral ketone CH3CH2COCH3 gives one of the enantiomers of the @C01057@ alcohol CH3CH2CHOHCH3; the addition of CN− to one of the @D01685@ faces of the @C01057@ aldehyde shown below converts it into one of the @D01679@ of the cyanohydrin. The two faces of the trigonal system may be described as Re and Si. P04859.png
- The term prochiral also applies to a tetrahedral atom of an @A00069@ or @C01057@ molecule which is bonded to two @S05981@ groups. For example, the prochiral molecule CH3CH2OH can be converted into the @C01057@ molecule CH3CHDOH by the isotopic replacement of one of the two @E02083@ hydrogen atoms of the @M03881@ group. The carbon atom of the @M03881@ group is called prochiral. The prochiral molecule HO2CCH2CHOHCH2CO2H can be converted into a @C01057@ product by esterification of one of the two @E02083@–CH2CO2H groups. The carbon atom of the CHOH group is called prochiral. The @C01057@ molecule CH3CHOHCH2CH3 can be converted into one of the @D01679@ of CH3CHOHCHDCH3 by the isotopic replacement of one of the two @D01685@ hydrogen atoms of the @M03881@ group. The carbon atom of the @M03881@ group is called prochiral. The @S05981@ groups in these cases may be described as pro-R or pro-S. Reference to the two @S05981@ groups themselves as prochiral, although common, is strongly discouraged.See:chirality centre
- The term prochirality is also applied to the @E02083@ faces of a trigonal system.