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Incoming links: Axial chirality
Allene 2026-10-14
An allene contains the unit with two consecutive carbon–carbon covalent bonds of order two. Suitable terminal substituents give axial chirality.
Axial chirality 2026-10-14
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 (chemistry) Created 2026-10-13 Updated 2026-10-14
Chirality means that a molecule cannot be superposed on its mirror image. A carbon attached to four different groups is a common source of chirality; the definition concerns the whole molecule, rather than requiring this particular structural feature.
The test is spatial: apply a translation and a rotation to the entire molecule until corresponding chemical atoms coincide, keeping their identities and chemical bonds. A mirror operation is a reflection; physically turning a model is a rotation. They are different operations. If no such superposition exists, the two forms are enantiomers; if one succeeds, the molecule is achiral. A different-looking flat drawing alone establishes neither conclusion.
For a simple molecule with exactly one tetrahedral stereocenter and four constitutionally different substituents, the mirror forms cannot superpose. Compare entire attached groups, including paths around a ring, rather than just the first chemical atom. Several tetrahedral stereocenters require a whole-molecule check: a meso compound can be achiral. Conversely, axial chirality needs no tetrahedral stereocenter.[ref]
5.2 The Reason for Handedness in Molecules: Chirality. McMurry, Organic Chemistry, §5.2: whole-molecule chirality, four different groups and equivalent ring paths
Spatial mirror pair with four distinct substituents
. Ideal tetrahedral molecular geometry with distinct substituents A, B, C and D. The two models use the same viewing direction and the reflection . Their opposite signed determinants exclude superposition by any label-preserving rotation. The letters do not denote R and S configuration priorities. Details.
Why care about this distinction? Two enantiomers can fit differently into an already chiral molecule, just as two opposite hands fit differently into one glove. Thus a protein's spatial arrangement can distinguish a mirror pair even when an achiral measurement cannot. Polarimetry provides a different kind of test: it measures a sample's net optical rotation, which depends on the populations of its enantiomers.