In the representation of stereochemical relationships 'anti' means 'on opposite sides' of a reference plane, in contrast to 'syn' which means 'on the same side', as in the following examples.
- Two substituents attached to atoms joined by a single bond are anti if the torsion angle (dihedral angle) between the bonds to the substituents is greater than 90°, or syn if it is less than 90°. (A further distinction is made between antiperiplanar, synperiplanar, anticlinal and synclinal.)
- In the older literature the terms anti and syn were used to designate stereoisomers of oximes and related compounds. That usage was superseded by the terms 'trans' and 'cis' or E and Z, respectively.
- When the terms are used in the context of chemical reactions or transformations, they designate the relative orientation of substituents in the substrate or product:
- Addition to a carbon-carbon double bond:
- Alkene-forming elimination:
A00381-4.png In the examples described under (1) and (2) anti processes are always antarafacial, and syn processes are suprafacial.
See also: endo, exo, syn, anti
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1084 (https://doi.org/10.1351/pac199466051077)
PAC, 1996, 68, 2193. 'Basic terminology of stereochemistry (IUPAC Recommendations 1996)' on page 2199 (https://doi.org/10.1351/pac199668122193)