Delocalized electron

Benzene, with the delocalization of the electrons indicated by the circle.

In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or one covalent bond. ^[1] Delocalized electrons are contained within an orbital that extends over several adjacent atoms. Classically, delocalized electrons can be found in conjugated systems and mesoionic compounds. It is increasingly appreciated that electrons in sigma bonding levels are also delocalized. For example, in methane, the bonding electrons are shared by all five atoms equally. Pervasive existence of delocalization is implicit in molecular orbital theory.

Examples

In the simple aromatic ring of benzene the delocalization of six π electrons over the C₆ ring is often graphically indicated by a circle. The fact that the six C-C bonds are equidistant is one indication of this delocalization. In valence bond theory, delocalization in benzene is represented by resonance structures.

Delocalized electrons also exist in the structure of solid metals, where the d-subshell interferes with the above s-subshell. Metallic structure consist of aligned positive ions (cations) in a "sea" of delocalized electrons. This means that the electrons are free to move throughout the structure, and gives rise to properties such as conductivity.

In diamond all four outer electrons of each carbon atom are 'localized' between the atoms in covalent bonding. The movement of electrons is restricted and diamond does not conduct an electric current. In graphite, each carbon atom uses only 3 of its 4 outer energy level electrons in covalently bonding to three other carbon atoms in a plane. Each carbon atom contributes one electron to a delocalized system of electrons that is also a part of the chemical bonding. The delocalized electrons are free to move throughout the plane. For this reason, graphite conducts electricity along the planes of carbon atoms, but does not conduct in a direction at right angles to the plane.

In the case of ions it is common to speak about delocalized charge (charge delocalization) when meaning delocalized electrons. An example of delocalized electrons (delcocalized charge) in ions can be found in the carboxylate group, wherein the negative charge is centered equally on the two oxygen atoms. Charge delocalization in anions is an important factor determining their reactivity (generally: the higher the extent of delocalization the lower the reactivity) and, specifically, the acid strength of their conjugate acids. As a general rule, the better delocalized is the charge in an anion the stronger is its conjugate acid. For example, the negative charge in perchlorate anion (ClO₄^-) is evenly distributed among the symmetrically oriented oxygen atoms (and a part of it is also kept by the central chlorine atom). This excellent charge delocalization combined with the high number of oxygen atoms (four) and high electronegativity of the central chlorine atom leads to perchloric acid being one of the strongest known acids (with pK_a values cited in the range of -7 .. -10). The extent of charge delocalization in an anion can be quantitatively expressed via the WAPS parameter.^[2]

Delocalization in reactions

Delocalized electrons are important for several reasons. One, an expected chemical reaction may not occur because the electrons delocalize to a more stable configuration, resulting in a reaction that happens at a different location. An example attempting the Friedel–Crafts alkylation of benzene with 1-chloro-2-methylpropane; the carbocation rearranges to a tert-butyl group stabilized by hyperconjugation, a particular form of delocalization.

See also

• Conjugated system
• Molecular orbital
• Aromatic ring current

References

1. ^ IUPAC Gold Book delocalization
2. ^ Kaupmees K., Kaljurand I., Leito I. (2010). "Influence of Water Content on the Acidities in Acetonitrile. Quantifying Charge Delocalization in Anions". J. Phys. Chem. A 114: 11788–11793. doi:10.1021/jp105670t.