Illustrated Glossary of Organic Chemistry

Periplanar: Atoms, groups, bonds having a torsional angle between 0^o and 30^o (syn-periplanar) or between 150^o and 180^o (anti-periplanar). Coplanar refers to objects lying in the same plane (torsional angle = 0^o or 180^o).

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In this eclipsed conformation, the two chlorine atoms are syn-periplanar, as are the two hydrogen atoms and the two bromine atoms. The Cl-C-C-Cl torsional angle is 0^o (i.e., lying in the same plane and pointing in the same direction). Verify with a molecular model.

Bond rotation converts the eclipsed conformation into an anti-staggered conformation, in which the chlorine atoms are anti-periplanar. The Cl-C-C-Cl torsional angle is 180^o (i.e., lying in the same plane but pointing in opposite directions.) Verify with a molecular model.