Delocalization

A phenomenon in which the most loosely held bonding electrons of some molecules serve to bind not two but several atoms. This contrasts with localization, a characteristic of ordinary single bonds, as in the normal paraffins, for example, methane (CH₄); in ethane, (C₂H₆); or in the molecules water (H₂O) and ammonia (NH₃). See also Chemical bonding.

The prototype completely delocalized system is the ideal crystalline metal, in which the valence electrons are spaced uniformly over a periodic lattice of positive-ion cores. This confers a special stability to the metal, and it also accounts for its high electrical conductivity and other metallic properties.

The aromatic and conjugated molecules of organic chemistry contain delocalized electronic systems. The benzene molecule (C₆H₆) is considered the archetypical aromatic molecule. Benzene possesses an underlying single-bonded planar framework (C₆H₆)⁶⁺ plus six additional electrons. Available for these electrons are six 2p orbitals, one on every carbon atom, each perpendicular to the molecular plane. The six electrons, called pi electrons, are ascribable to the six carbon atoms in such a way that neither structure (1) nor structure (2) is an accurate representation,

but instead a structure that can be described in the valence bond language as a resonance hybrid of the two, schematically written as (3), where the circle stands for the delocalized electrons. All molecules containing such rings possess an extra stability associated with the delocalization phenomenon. As a consequence, they also have a low propensity for chemical reactivity. See also Molecular orbital theory; Resonance (molecular structure).

---