苯的证据与芳香族取代
引入| English | 释义 | 拼音 |
|---|---|---|
| delocalised/dɪˈlɒkəlaɪzd/ | 离域的 | lí yù de |
| nitronium | 硝基正离子 | xiāo wēng |
| intermediate/ˌɪntəˈmiːdɪət/ | 中间体 | zhōng jiān tǐ |
| curly arrows | 曲箭头 | qū jiàn tóu |
Why does benzene resist the alkene test?
- Benzene has a delocalised 离域的 ring system. Addition would destroy that stabilisation.
- It resists ordinary bromine-water addition conditions. Substitution can replace H and then restore the ring.
Compare the evidence
- X-ray evidence shows equal carbon–carbon bond lengths between typical single and double lengths.
- Benzene hydrogenation is less exothermic than three isolated double bonds predict. This supports a lower-enthalpy delocalised starting structure.
三个孤立双键预测氢化放热为 −354 kJ/mol；苯的实测值为 −206。求其稳定化能（kJ/mol）。
难度 2/5 · 今日首次完成可获 140 金币及少量额外奖励
稳定化能 = (−206) − (−354) = 148 kJ/mol。苯的起始结构比相同氢化产物的假想定域模型低出该数值。
哪两项观察结果支持苯的离域结构？
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键长和焓变证据独立限制了该结构。
Generate the electrophile first
- Concentrated nitric and sulfuric acids generate NO₂⁺, the nitronium 硝基正离子 electrophile.
- The ring electron pair forms a bond to NO₂⁺. A positive intermediate 中间体 has H and NO₂ on the same ring carbon.
Three isolated hydrogenations at −120 each predict −360; a benzene value of −208 differs by 152 kJ per mole.
硝化反应中的亲电试剂是什么？
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浓酸混合物生成硝酰阳离子 NO₂⁺。
Restore the ring
- A C–H bond electron pair returns to the ring as H⁺ leaves. Delocalisation is restored in nitrobenzene.
- Draw curly arrows 曲箭头 from electron pairs, not from the positive hydrogen ion.
表示电子流向的弯曲箭头应从质子开始。
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它从一对电子开始，例如环恢复过程中的 C–H 键电子对。
Choose conditions for the actual ring
- Benzene bromination needs a halogen carrier such as FeBr₃. Phenol reacts with bromine water without that catalyst.
- Phenol gives white 2,4,6-tribromophenol. Its oxygen lone pair increases ring electron density, making electrophile attack easier.
A delocalised ring is not three ordinary double bonds rapidly changing positions.
Use delocalisation and reaction conditions to explain aromatic evidence. Draw electron-pair arrows and show the ring losing then regaining aromaticity during substitution.
苯酚与溴水反应生成 _____ 沉淀。
难度 1/5 · 今日首次完成可获 100 金币及少量额外奖励
产物是白色 2,4,6-三溴苯酚。