An atom or group (charged or uncharged) that becomes detached from an atom in what
is considered to be the residual or main part of the
substrate in a specified reaction. For example, in the heterolytic
solvolysis of benzyl bromide in acetic acid:
the leaving group is
Br−; in the reaction:
the leaving group is
NMe3 ; in the
electrophilic nitration of benzene, it is
H+.
The term has meaning only in relation to a specified reaction. The leaving group is
not, in general, the same as the
substituent group present in the substrate (e.g. bromo and trimethylammonio in the substrates of the
first two examples above.) A slightly different usage of the term prevails in the
(non-mechanistic) naming of transformations, where the actual substituent group present
in the substrate (and also in the product) is referred to as the leaving group.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1134
