離域電子
维基百科，自由的百科全书
離域電子，泛稱自由電子，是在分子中沒有組成原子或共價鍵的電子。 Delocalized electrons are part of a pi electron system that extends over several adjacent atoms. Delocalized electrons can be found in conjugated systems of double bonds and in aromatic and mesoionic systems. A case of delocalized electrons occurs also in solid metals, where the d-subshell interferes with the above s-subshell, and contributes to the properties of a metal.
In the simple aromatic ring benzene the delocalization of six pi electrons over the whole ring is often indicated by a circle. This shows that the bond lengths are equal for all the C-C bonds.Alternatively, benzene can be represented by resonance structures.
Another example of a delocalized electron can be found in a carboxylic acid. When dissolved in an aqueous solution, the hydroxyl group releases a proton and the electrons are then delocalized between the two oxygen atoms in the carboxylate anion.
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 Fridel-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.
Delocalized electrons also exist in the structure of metals. 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.
