An allene has a unit whose terminal substituents lie in perpendicular planes. In , with and , the two terminal pairs can define enantiomers without any tetrahedral carbon stereocenter. The groups need be different within each end pair, not all four mutually different. This is axial chirality.[ref]
Axial Chirality. Basic Terminology of Stereochemistry, entry Axial Chirality: nonplanar pairs about an axis, including allenes
chirality produced by distinguishable terminal substituents arranged about the constrained axis of a spiro framework.
Axial stereoisomerism in suitably substituted alkylidenecyclohexanes, where the constrained ring and alkene distinguish two handed arrangements.
Stereoisomerism arising from restricted rotation about a bond. A hindered biaryl can make otherwise similar methylene hydrogens diastereotopic on the NMR timescale.
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Axial chirality refers to a type of chirality where a molecule's asymmetry arises not from a center of chirality (like a chiral carbon atom), but from a difference in spatial arrangement around an axis. This form of chirality is commonly observed in certain types of molecules, including biphenyls, terphenyls, and helicenes, where two or more substituents or groups are rotated relative to each other.