Preferred Label : antarafacial;
IUPAC definition : When a part of a molecule ('molecular fragment') undergoes two changes in bonding
(bond-making or bond-breaking), either to a common centre or to two related centres,
external to itself, these bonding changes may be related in one of two spatially different
ways. These are designated as 'antarafacial' if opposite faces of the molecular fragment
are involved, and 'suprafacial' if both changes occur at the same face. The concept
of 'face' is clear from the diagrams in the cases of planar (or approximately planar)
frameworks with isolated or interacting p-orbitals [diagrams (a) and (b)]. The terms
antarafacial and suprafacial are, however, also employed in cases in which the essential
part of the molecular fragment undergoing changes in bonding comprises two atoms linked
only by a σ-bond. In these cases it is customary to refer to the phases of the local
σ-bonding orbital: occurrence of the two bonding changes at sites of like orbital
phase is regarded as suprafacial, whereas that at two sites of opposite phase is antarafacial.
The possibilities are shown for C–C and C–H σ-bonds in diagrams (c) and (d). There
may be two distinct and alternative stereochemical outcomes of a suprafacial process
involving a σ-bond between saturated carbon atoms, i.e. either retention or inversion
at both centres. The antarafacial process results in inversion at one centre and retention
at the second. For examples of the use of these terms, see cycloaddition, sigmatropic
rearrangement.;
Origin ID : A00377;
See also
When a part of a molecule ('molecular fragment') undergoes two changes in bonding
(bond-making or bond-breaking), either to a common centre or to two related centres,
external to itself, these bonding changes may be related in one of two spatially different
ways. These are designated as 'antarafacial' if opposite faces of the molecular fragment
are involved, and 'suprafacial' if both changes occur at the same face. The concept
of 'face' is clear from the diagrams in the cases of planar (or approximately planar)
frameworks with isolated or interacting p-orbitals [diagrams (a) and (b)]. The terms
antarafacial and suprafacial are, however, also employed in cases in which the essential
part of the molecular fragment undergoing changes in bonding comprises two atoms linked
only by a σ-bond. In these cases it is customary to refer to the phases of the local
σ-bonding orbital: occurrence of the two bonding changes at sites of like orbital
phase is regarded as suprafacial, whereas that at two sites of opposite phase is antarafacial.
The possibilities are shown for C–C and C–H σ-bonds in diagrams (c) and (d). There
may be two distinct and alternative stereochemical outcomes of a suprafacial process
involving a σ-bond between saturated carbon atoms, i.e. either retention or inversion
at both centres. The antarafacial process results in inversion at one centre and retention
at the second. For examples of the use of these terms, see cycloaddition, sigmatropic
rearrangement.