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 @S06082@ in a specified reaction. For example, in the heterolytic @S05762@ of benzyl bromide in acetic acid:
the leaving group is Br; in the reaction:
the leaving group is NMe3; in the @E02020@ 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 @S06076@ 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.
See also:
entering group
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1134 [Terms] [Paper]