Benzene evidence and aromatic substitution
Introduced| English | Meaning |
|---|---|
| delocalised/dɪˈlɒkəlaɪzd/ | Describing electrons that are spread over several atoms rather than confined to one bond or atom. |
| nitronium | The positively charged ion NO2+, an electrophile used in aromatic nitration. |
| intermediate/ˌɪntəˈmiːdɪət/ | A species formed in one step of a reaction mechanism and consumed in a later step. |
| curly arrows | Mechanism arrows showing electron movement: double-barbed for pairs, fishhook for single electrons. |
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.
Three isolated bonds predict hydrogenation −354 kJ per mole; benzene gives −206. Find stabilisation magnitude in kJ per mole.
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Stabilisation magnitude = (−206) − (−354) = 148 kJ per mole. The benzene starting structure lies this much below the hypothetical localised model for the same hydrogenation products.
Which two observations support delocalised benzene?
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Bond-length and enthalpy evidence constrain the structure independently.
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.
What is the nitrating electrophile?
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Concentrated acid mixture generates nitronium, 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.
A curly arrow showing electron flow should start at a proton.
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It starts at an electron pair, such as the C–H bond pair during ring restoration.
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.
Phenol with bromine water gives a _____ precipitate.
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The product is white 2,4,6-tribromophenol.