Antarafacial - suprafacial

Antarafacial and suprafacial 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 ^[1]. The reaction center can be a p-orbital, a conjugated system or even a sigma bond.

This relationship is antarafacial when opposite faces are involved (think anti).
It is suprafacial when both changes occur at the same face.

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Many sigmatropic reactions and cycloadditions can be either suprafacial or antarafacial and this determines the stereochemistry.

An example is the [1,3]-hydride shift in which the interacting frontier orbitals are the allyl 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 [1,5]-hydride shift is a common event ^[2].