https://doi.org/10.1351/goldbook.P04859
This term is used in different, sometimes contradictory ways; four are listed below.
  1. The geometric property of an @[email protected] object (or spatial arrangement of points or atoms) which is capable of becoming @[email protected] in a single @[email protected] step. An @[email protected] molecular entity, or a part of it considered on its own, is thus called prochiral if it can be made @[email protected] by the replacement of an existing atom (or @[email protected] group) by a different one. An @[email protected] object which is capable of becoming @[email protected] in two @[email protected] steps is sometimes described as proprochiral. For example the proprochiral CH3CO2H becomes prochiral as CH2DCO2H and @[email protected] as CHDTCO2H.
  2. The term prochirality also applies to an @[email protected] molecule or entity which contains a trigonal system and which can be made @[email protected] by the addition to the trigonal system of a new atom or @[email protected] group. For example addition of hydrogen to one of the @[email protected] faces of the prochiral ketone CH3CH2COCH3 gives one of the enantiomers of the @[email protected] alcohol CH3CH2CHOHCH3; the addition of CN to one of the @[email protected] faces of the @[email protected] aldehyde shown below converts it into one of the @[email protected] of the cyanohydrin. The two faces of the trigonal system may be described as Re and Si.
    P04859.png
  3. The term prochiral also applies to a tetrahedral atom of an @[email protected] or @[email protected] molecule which is bonded to two @[email protected] groups. For example, the prochiral molecule CH3CH2OH can be converted into the @[email protected] molecule CH3CHDOH by the isotopic replacement of one of the two @[email protected] hydrogen atoms of the @[email protected] group. The carbon atom of the @[email protected] group is called prochiral. The prochiral molecule HO2CCH2CHOHCH2CO2H can be converted into a @[email protected] product by esterification of one of the two @[email protected] –CH2CO2H groups. The carbon atom of the CHOH group is called prochiral. The @[email protected] molecule CH3CHOHCH2CH3 can be converted into one of the @[email protected] of CH3CHOHCHDCH3 by the isotopic replacement of one of the two @[email protected] hydrogen atoms of the @[email protected] group. The carbon atom of the @[email protected] group is called prochiral. The @[email protected] groups in these cases may be described as pro-R or pro-S. Reference to the two @[email protected] groups themselves as prochiral, although common, is strongly discouraged.
    See:
    chirality centre
  4. The term prochirality is also applied to the @[email protected] faces of a trigonal system.
Source:
PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2213 [Terms] [Paper]