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Summary 4.2 Aromaticity
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Summary booklet on the content needed for 4.2 WJEC A level Chemistry. Useful for learning the content and creating flashcards. Includes - Benzene structure - Kekules Theory - Reactions of Benzene - C-Cl bond strength - Phenol acidity

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  • April 5, 2023
  • 7
  • 2022/2023
  • Summary

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4.2 Aromaticity



Benzene

• Planar Molecule
• All bond angles are 120
• Each C is bonded to two other carbons and one Hydrogen by sigma (σ)
bonds
• The fourth e- in the 2p orbital above and below the plane is delocalised as
the p orbitals overlap
• Creates a pi (π) electron ring system which is very stable
• Does not undergo addition reactions easily as it would disrupt the πe- ring system




Kekulés Theory

Problem 1 – Reactions of Benzene

• Would expect Kekulés structure of Benzene to react through
electrophilic addition due to double bonds
• This would cause Bromine water to decolourise but it does not
C6H6 + 3Br2 C6H6Br6
• Benzene is relatively unreactive under normal conditions so suggests
no double bonds present




Problem 2 – Thermostability of Benzene

• Enthalpy of hydrogenation to Cyclohexene is -120 KJmol-1




• Enthalpy of hydrogenation to Benzene should be -360 KJmol-1




• Actual enthalpy of hydrogenation of Benzene is -152 KJmol-1 so it is 208 KJmol-1 more stable

, Problem 3 – Bond Lengths

• The bond length of Benzene is between a single and double
Carbon bond suggesting the bonds alternate between carbons
causing resonance.




Reactions of Benzene

• Cannot undergo addition reactions unless vigorous conditions
• πe- ring system is very stable and would be disrupted
• Benzene reacts through electrophilic substitution

Electrophile – An electron deficient species/lone pair acceptor e.g. NO2+, Cl+, CH3+



General Mechanism for Electrophilic Substitution of Benzene




1. The πe- ring system is an area of high e- density and so is attracted to positive electrophile
2. The πe- ring system is disrupted (less stable) with original H and electrophile bonded to C
3. The H leaves and gives the bonded e- back to the πe- ring which is reformed, the H has been
substituted

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