离域Delocalized electron

苯分子中 6 个 π 电子分布在整个环上，6 个 C–C 键长度相同（139 pm），介于单键和双键之间。乙酸根离子 CH₃COO⁻ 中负电荷平均分布在两个氧原子上，两个 C–O 键等长。石墨每层中每个碳原子贡献一个电子形成离域 π 体系，因此石墨能沿层导电。金属中的价电子在整个晶体中离域，形成金属键。

In benzene the six π electrons spread round the whole ring, and all six C–C bonds are the same length (139 pm), between single and double bonds. In the ethanoate ion CH₃COO⁻ the negative charge is shared equally by both oxygens and the two C–O bonds are equal. In each layer of graphite every carbon contributes one electron to a delocalised π system, so graphite conducts along the layers. In metals the valence electrons are delocalised through the whole crystal, giving metallic bonding.

环己烯氢化放热约 120 kJ mol⁻¹，若苯是含三个独立双键的结构，氢化应放热约 360 kJ mol⁻¹，实测只有约 208 kJ mol⁻¹。苯比假想结构稳定约 150 kJ mol⁻¹，这部分称为离域能。离域使苯难以发生会破坏 π 体系的加成反应，而倾向于发生取代反应。

Hydrogenating cyclohexene releases about 120 kJ mol⁻¹. If benzene had three separate double bonds its hydrogenation should release about 360 kJ mol⁻¹, but only about 208 kJ mol⁻¹ is measured. Benzene is therefore about 150 kJ mol⁻¹ more stable than the hypothetical structure, the delocalisation energy. This is why benzene resists addition, which would destroy the π system, and favours substitution.

羧酸的酸性强于醇，原因之一是羧酸根中负电荷离域在两个氧原子上而稳定；苯酚的酸性强于乙醇，是因为酚氧负离子的负电荷可离域到苯环上。苯胺的碱性弱于脂肪胺，是因为氮上的孤对电子部分离域到苯环中，不易接受质子。

Carboxylic acids are stronger acids than alcohols partly because the carboxylate's negative charge is delocalised over two oxygens; phenol is more acidic than ethanol because the phenoxide's charge spreads into the ring. Phenylamine is a weaker base than alkylamines because the nitrogen lone pair is partly delocalised into the ring and less available to accept a proton.