This term is used in different, sometimes contradictory ways; four are listed below.
The geometric property of an achiral object (or spatial arrangement of points or atoms) which is capable of becoming chiral in a single
desymmetrization step. An achiral molecular entity, or a part of it considered on its own, is thus called prochiral if it can be made chiral by the replacement of an existing atom (or achiral group) by a different one. An achiral object which is capable of becoming chiral in two desymmetrization steps is sometimes described as proprochiral. For example the proprochiral
becomes prochiral as
and chiral as
The term prochirality also applies to an achiral molecule or entity which contains a trigonal system and which can be made chiral by the addition to the trigonal system of a new atom or achiral group. For example addition of hydrogen to one of the enantiotopic faces of the prochiral ketone
gives one of the enantiomers of the chiral alcohol
the addition of
to one of the diastereotopic faces of the chiral aldehyde shown below converts it into one of the diastereoisomers of 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 achiral or chiral molecule which is bonded to two
stereoheterotopic groups. For example, the prochiral molecule
can be converted into the chiral molecule
by the isotopic replacement of one of the two enantiotopic hydrogen atoms of the methylene group. The carbon atom of the methylene group is called prochiral. The prochiral molecule
can be converted into a chiral product by
esterification of one of the two enantiotopic
groups. The carbon atom of the
group is called prochiral. The chiral molecule
can be converted into one of the diastereoisomers of
by the isotopic replacement of one of the two
hydrogen atoms of the methylene group. The carbon atom of the methylene group is called prochiral. The stereoheterotopic groups in these cases may be described as pro-R or pro-S. Reference to the two stereoheterotopic groups themselves as prochiral, although common, is strongly discouraged.
The term prochirality is also applied to the enantiotopic faces of a trigonal system.
PAC, 1996, 68, 2193
(Basic terminology of stereochemistry (IUPAC Recommendations 1996))
on page 2213
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook