Aromatic chemistry centres around benzene, a remarkably stable ring-shaped molecule... Show more
Chemistry280 views·Updated May 12, 2026·2 pagesUnderstanding Aromatic Chemistry for AQA A-Level Students
jessica@jessicaa7
1
of 2
Benzene Structure and Stability
Ever wondered why benzene (C₆H₆) is so special compared to other hydrocarbons? This cyclic, planar molecule has each carbon bonded to two other carbons and one hydrogen, with the remaining electron sitting in a p-orbital above and below the ring.
What makes benzene truly unique is its delocalised electron structure. Unlike what you'd expect from alternating single and double bonds, all C-C bonds are exactly the same length (139pm). This delocalisation creates extraordinary stability.
Scientists prove benzene's stability by comparing enthalpy of hydrogenation values. If benzene had three separate double bonds like cyclohexatriene, we'd expect -360 kJ/mol, but it's actually much less negative, showing benzene needs more energy to break its bonds.
Key Insight: Benzene's stability comes from delocalised electrons forming a 'ring of charge' above and below the carbon atoms.
Arenes (aromatic compounds) can be named in two ways: either as substituted benzenes (like bromobenzene) or with benzene as a phenyl functional group (like phenylamine for C₆H₅NH₂). The naming depends on which part of the molecule is more significant.
2
of 2
Reactions of Benzene: Making the Unreactive React
Benzene's stability creates a problem - it's too stable to react easily! However, chemists have developed clever ways to make it react through electrophilic substitution rather than addition reactions that would destroy the stable ring.
Friedel-Crafts acylation uses a halogen carrier catalyst like AlCl₃ to create a powerful electrophile. The catalyst polarises an acyl chloride (RCOCl), forming a carbocation that's strong enough to attack benzene's electron-rich ring. Under reflux with dry ether, this produces phenylketones.
The reaction mechanism involves the delocalised electrons attacking the electrophile, temporarily breaking the ring's stability. A hydrogen is then removed, reforming the aromatic ring with the new acyl group attached.
Exam Tip: Remember that benzene undergoes substitution, not addition - this preserves the stable aromatic ring structure.
Nitration of benzene creates nitrobenzene, essential for making explosives and dyes. Concentrated nitric and sulfuric acids react to form the nitronium ion (NO₂⁺), a powerful electrophile. Keep temperatures below 55°C to avoid multiple substitutions - too hot and you'll get unwanted products!
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Chemistry280 views·Updated May 12, 2026·2 pagesUnderstanding Aromatic Chemistry for AQA A-Level Students
jessica@jessicaa7
Aromatic chemistry centres around benzene, a remarkably stable ring-shaped molecule that forms the backbone of countless everyday products. Understanding how benzene's unique structure makes it both incredibly stable and reactive in specific ways is crucial for grasping organic chemistry.
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Benzene Structure and Stability
Ever wondered why benzene (C₆H₆) is so special compared to other hydrocarbons? This cyclic, planar molecule has each carbon bonded to two other carbons and one hydrogen, with the remaining electron sitting in a p-orbital above and below the ring.
What makes benzene truly unique is its delocalised electron structure. Unlike what you'd expect from alternating single and double bonds, all C-C bonds are exactly the same length (139pm). This delocalisation creates extraordinary stability.
Scientists prove benzene's stability by comparing enthalpy of hydrogenation values. If benzene had three separate double bonds like cyclohexatriene, we'd expect -360 kJ/mol, but it's actually much less negative, showing benzene needs more energy to break its bonds.
Key Insight: Benzene's stability comes from delocalised electrons forming a 'ring of charge' above and below the carbon atoms.
Arenes (aromatic compounds) can be named in two ways: either as substituted benzenes (like bromobenzene) or with benzene as a phenyl functional group (like phenylamine for C₆H₅NH₂). The naming depends on which part of the molecule is more significant.
2
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Reactions of Benzene: Making the Unreactive React
Benzene's stability creates a problem - it's too stable to react easily! However, chemists have developed clever ways to make it react through electrophilic substitution rather than addition reactions that would destroy the stable ring.
Friedel-Crafts acylation uses a halogen carrier catalyst like AlCl₃ to create a powerful electrophile. The catalyst polarises an acyl chloride (RCOCl), forming a carbocation that's strong enough to attack benzene's electron-rich ring. Under reflux with dry ether, this produces phenylketones.
The reaction mechanism involves the delocalised electrons attacking the electrophile, temporarily breaking the ring's stability. A hydrogen is then removed, reforming the aromatic ring with the new acyl group attached.
Exam Tip: Remember that benzene undergoes substitution, not addition - this preserves the stable aromatic ring structure.
Nitration of benzene creates nitrobenzene, essential for making explosives and dyes. Concentrated nitric and sulfuric acids react to form the nitronium ion (NO₂⁺), a powerful electrophile. Keep temperatures below 55°C to avoid multiple substitutions - too hot and you'll get unwanted products!
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