Planarchiral

Planarchiral, or planar chirality, is a form of stereoisomerism in which the chiral element is a plane rather than a stereogenic center or an axis. In planarchiral molecules, two mirror-image arrangements cannot be interconverted by any allowed rotation without breaking bonds, making them non-superimposable enantiomers.

The most familiar examples arise in rigid, bridged or partially bridged polycyclic systems such as paracyclophanes and other cyclophane frameworks, where the arrangement of substituents relative to the plane of the ring creates chirality. Planar chirality also appears in substituted ferrocene and other metallocene systems, where bulky substituents on the cyclopentadienyl rings produce a fixed asymmetry with respect to the metal center. In addition, certain rigid biaryl systems with restricted rotation can exhibit planar chirality when the two rings adopt a non-superimposable arrangement that defines a chiral plane.

Identification and analysis of planarchiral compounds typically involve spectroscopic methods sensitive to chirality, such as circular dichroism, as well as chiral chromatography and X-ray crystallography. The enantiomeric forms can often be separated and individually studied, revealing differences in properties relevant to catalysis, sensing, or material science.

Nomenclature and usage: the term planarchiral is used in stereochemical literature to describe planar-chiral systems; some authors prefer the phrase “planar chirality” as the descriptor, while planarchiral serves as an adjectival form in certain texts. The concept is an important part of discussions on stereochemical diversity beyond conventional stereocenters and axes.