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Antarafacial ( Woodward-Hoffmann symbol a) and suprafacial (s) are two topological concepts in organic chemistry describing the relationship between two simultaneous chemical bond making and/or bond breaking processes in or around a reaction center. The reaction center can be a p- or sp''n-''orbital (Woodward-Hoffmann symbol ω), a 
conjugated system
In theoretical chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in a molecule, which in general lowers the overall energy of the molecule and increases stability. It is conventionally represented as ...
(π) or even a sigma bond (σ).
* The relationship is ''antarafacial'' when opposite faces of the π system or isolated orbital are involved in the process (think ''anti''). For a σ bond, it corresponds to involvement of one "interior" lobe and one "exterior" lobe of the bond.
* The relationship is ''suprafacial'' when the same face of the π system or isolated orbital are involved in the process (think ''syn''). For a σ bond, it corresponds to involvement of two "interior" lobes or two "exterior" lobes of the bond.
The components of all pericyclic reactions, including sigmatropic reactions and cycloadditions, and electrocyclizations, can be classified as either suprafacial or antarafacial, and this determines the stereochemistry
Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms that form the structure of molecules and their manipulation. The study of stereochemistry focuses on the relationships between stereois ...
. In particular, antarafacial topology corresponds to inversion of configuration for the carbon atom of a , ''n''
The comma is a punctuation mark that appears in several variants in different languages. It has the same shape as an apostrophe or single closing quotation mark () in many typefaces, but it differs from them in being placed on the baseline ...
sigmatropic rearrangement, and conrotation for electrocyclic ring closure, while suprafacial corresponds to retention and disrotation.
An example is the ,3hydride shift, in which the interacting frontier orbitals are the allyl
In organic chemistry, an allyl group is a substituent with the structural formula , where R is the rest of the molecule. It consists of a methylene bridge () attached to a vinyl group (). The name is derived from the scientific name for garlic, ...
free radical and the hydrogen 1s orbitals. The suprafacial shift is symmetry-forbidden because orbitals with opposite algebraic signs overlap. The symmetry allowed antarafacial shift would require a strained transition state and is also unlikely. In contrast a symmetry allowed and suprafacial ,5hydride shift is a common event.F.A. Carey, R.J. Sundberg, ''Advanced Organic Chemistry'' Part A {{ISBN, 0-306-41087-7
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