苯Benzene

每个碳原子 sp² 杂化，形成 C–C 与 C–H σ 键骨架；剩余的 p 轨道垂直于环平面，侧面重叠成环状 π 体系。六个 C–C 键长相等（139 pm），介于单键（154 pm）与双键（134 pm）之间。苯的氢化热比假想的“环己三烯”低约 150 kJ/mol，这部分即共振能（离域能）。

Each carbon is sp² hybridized, forming the σ framework of C–C and C–H bonds; the remaining p orbitals stand perpendicular to the ring and overlap into a cyclic π system. All six C–C bonds are 139 pm, between a single (154 pm) and a double bond (134 pm). Benzene's heat of hydrogenation is about 150 kJ/mol lower than that of a hypothetical cyclohexatriene; this is the resonance (delocalization) energy.

法拉第于 1825 年从照明煤气的油状残余物中分离出苯。苯的 C:H = 1:1 却难以发生加成反应，这一矛盾促使凯库勒在 1865 年提出单双键交替的六元环结构，并于 1872 年补充“振荡假说”，以解释邻位二取代苯只有一种。

Faraday isolated benzene in 1825 from the oily residue of illuminating gas. Its 1:1 C:H ratio paired with a reluctance to undergo addition was a puzzle. Kekulé proposed in 1865 a six-membered ring of alternating single and double bonds, and in 1872 added an oscillation hypothesis to explain why only one ortho-disubstituted benzene exists.

杜瓦、拉登堡等人曾提出杜瓦苯、棱晶烷等替代结构。1929 年朗斯代尔用 X 射线证明六甲基苯环是平面且键长相等。1930 年代鲍林以共振论描述苯，休克尔则用分子轨道法提出 4n+2 规则，将芳香性推广到离子与杂环体系。

Dewar, Ladenburg and others proposed alternative structures such as Dewar benzene and prismane. In 1929 Lonsdale's X-ray work showed the hexamethylbenzene ring to be planar with equal bonds. In the 1930s Pauling described benzene with resonance theory, while Hückel's molecular-orbital treatment gave the 4n+2 rule, extending aromaticity to ions and heterocycles.

苯是生产乙苯（苯乙烯）、异丙苯（苯酚、丙酮）、环己烷（尼龙）和硝基苯（苯胺）的基础原料。苯被 IARC 列为 1 类致癌物，长期接触可导致再生障碍性贫血和白血病，实验中常以甲苯替代。

Benzene is the feedstock for ethylbenzene (styrene), cumene (phenol and acetone), cyclohexane (nylon) and nitrobenzene (aniline). IARC classifies it as a Group 1 carcinogen; chronic exposure causes aplastic anaemia and leukaemia, so toluene is often used as a laboratory substitute.