antiperiplan

Antiperiplan, more correctly antiperiplanar orientation, is a term used in conformational analysis in organic chemistry to describe the arrangement of substituents around a carbon–carbon bond in which the substituents lie on opposite sides of the bond, giving a dihedral angle of about 180 degrees. In a Newman projection, an antiperiplanar relationship means the bonds or substituents on adjacent carbons are anti to each other, maximizing opposite orientation across the bond.

The standard term is antiperiplanar (also written as anti-periplanar); antiperiplan is a less common shortened form found in some texts. Antiperiplanar describes a geometric relationship rather than a specific reaction, and it is closely related to the concept of anti conformation in simple alkanes like butane, where the two methyl groups can be oriented 180 degrees apart.

In open-chain molecules, rotation around a single bond can yield multiple conformations, including antiperiplanar arrangements where substituents on neighboring carbons are trans to each other across the bond. In cyclic systems such as cyclohexanes, antiperiplanar relationships often appear as diaxial interactions in certain chair conformations, which can influence the feasibility of reactions.

Significance in reactions is notable: the antiperiplanar arrangement is particularly important for bimolecular eliminations (E2), where the leaving group and a β-hydrogen must adopt an antiperiplanar geometry to achieve favorable orbital overlap. When such alignment is not accessible, reactions may proceed more slowly or through alternative pathways (such as syn-elimination or different conformers).

Examples include the antiperiplanar arrangement in butane’s anti conformer and the axial-axial alignments in cyclohexane derivatives that enable E2 elimination.