In benzene carbon only forms 3 bonds. what bout the 4th?
Final lone elcetron is in a p-orbital which sticks out above and below the ring to form a delocalised ring
Why are all C–C bonds in benzene the same length?
Due to delocalisation of π electrons over the whole ring, forming a delocalised π system above and below the plane.
Explain the stability of Benzene using the enthalpy of hydrogenation (5 points)
- if we hydrogenate cyclohexene it has an enthalpy chnage of -120kjmol^-1
- cyclohexene has 1 double bond
- therefore if benzene had 3 double bonds we’d expect an enthalpy change of hydrogenation of - 360
kjmol^-1 - howver when we measure enthalpy change of hydrogenation of benzene its experimaental value is only -208kjmol^-1
- this suggests that more nergy is needed to break bonds in benzene vz cyclohexa 1,3, tri-ene so it is more stable and this is due to its delocalised structure
what are Arenes?
aromatic compounds/molecules that contain a benzene ring
What electrons are delocalised in benzene?
The six π electrons, one from each carbon’s unhybridised p orbital.
Why is benzene more stable than cyclohexa-1,3,5-triene?
Delocalisation lowers the molecule’s energy, giving extra stability compared to a structure with localised double bonds.
How does enthalpy of hydrogenation show benzene’s extra stability?
Expected ΔH (3 × C=C) ≈ –360 kJ mol⁻¹
Actual ΔH ≈ –208 kJ mol⁻¹
Difference (-152 kJ mol⁻¹) is the delocalisation energy
Why does benzene undergo substitution rather than addition reactions?
Addition would destroy the delocalised π system, losing stability; substitution preserves delocalisation.
What type of reactions does benzene undergo?
Electrophilic substitution reactions, typically limited to monosubstitution.
Why is benzene susceptible to electrophiles?
The electron-rich delocalised π system attracts electrophiles.
What happens in the first step of electrophilic substitution?
The π system attacks an electrophile, forming a sigma complex (arenium ion).
Why is the sigma complex unstable?
Delocalisation is temporarily lost; the ring is no longer fully aromatic.
Q: How is aromaticity restored?
A proton is lost, and the π system reforms.
Reagents and conditions for nitration of benzene?
Conc. HNO₃ + conc. H₂SO₄
Temperature < 55°C
What is the electrophile in nitration?
The nitronium ion, NO₂⁺
How is the nitronium ion generated?
HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O
Role of sulfuric acid in nitration?
Acts as a strong acid catalyst, generating the electrophile NO₂⁺.
Key steps in the nitration mechanism?
Formation of NO₂⁺
Attack on π system → sigma complex
Loss of H⁺ → nitrobenzene formed
Why must temperature be controlled in nitration?
Higher temperatures cause multiple substitutions, reducing yield.
What type of substitution is Friedel–Crafts acylation?
Electrophilic substitution introducing an acyl group (–CO–R).
Reagents for Friedel–Crafts acylation?
Acyl chloride
AlCl₃ catalyst
What electrophile is formed in acylation?
The acylium ion (RCO⁺)
How is the acylium ion generated?
RCOCl + AlCl₃ → RCO⁺ + AlCl₄⁻
Why is AlCl₃ described as a Lewis acid?
It accepts a lone pair from the acyl chloride.
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3.1 Atomic structure26
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3.1.2 Amount of substance7
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3.1.3 Bonding38
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3.1.4 Energetics18
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3.1.6 Chemical equilibria10
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3.1.7 Oxidation, reduction and redox equations0
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3.1.8 Thermodynamics (evrythng frm hr=A-level only0
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3.1.9 Rate equations22
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Optical isomerism A219
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ALL of A1 Chemistry - tuition flashcards101
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Aldehydes and ketones28
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Periodicity104
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Transition metals A246
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Aromatic chemistry40
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enthalpy definitions13
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Amines51
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aromatics, amines and biochemistry44
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Organic synthesis21
