Aromatic Chemistry
Benzene (C₆H₆) is the simplest aromatic compound. Its unusual stability arises from the delocalisation of pi electrons across the ring. Aromatic compounds undergo electrophilic substitution rather than addition, preserving the stable delocalised ring system. This lesson covers the structure of benzene, evidence for delocalisation, and the mechanisms of key electrophilic substitution reactions.
The Structure of Benzene
Kekule Model (Historical)
Friedrich August Kekule proposed in 1865 that benzene was a ring of six carbon atoms with alternating single and double bonds: cyclohex-1,3,5-triene. He suggested the structure rapidly interconverted between two forms.
Problems with the Kekule model:
- Enthalpy of hydrogenation: Cyclohexene + H₂ → cyclohexane; ΔH = −120 kJ mol⁻¹. If benzene were cyclohex-1,3,5-triene (three C=C), we would expect ΔH = 3 × (−120) = −360 kJ mol⁻¹. The actual value is −208 kJ mol⁻¹. Benzene is 152 kJ mol⁻¹ more stable than predicted. This extra stability is the delocalisation (resonance) energy.
- Bond lengths: In the Kekule structure, we would expect alternating C=C (0.134 nm) and C–C (0.154 nm) bonds. In reality, all six C–C bonds in benzene are the same length (0.140 nm) — intermediate between a single and double bond.
- Reactivity: If benzene had three C=C double bonds, it should readily undergo addition reactions (like alkenes). In practice, benzene is resistant to addition and prefers substitution reactions (which preserve the ring).
- Electron density map: X-ray diffraction shows a symmetrical, uniform distribution of electron density around the ring, inconsistent with alternating bonds.
Delocalised Model (Modern)
The modern model describes benzene as:
- A planar, regular hexagon of six carbon atoms.
- Each carbon is sp² hybridised (trigonal planar, 120° bond angles).
- Each carbon forms three σ bonds (two to adjacent C atoms, one to H) using sp² hybrid orbitals.
- Each carbon has one unhybridised p-orbital perpendicular to the plane of the ring.
- The six p-orbitals overlap sideways to form a delocalised π system — the six π electrons are shared equally across all six carbons.
- This is represented by a circle inside the hexagon.
The delocalisation of the π electrons makes benzene thermodynamically very stable. It does not readily undergo addition reactions because this would disrupt the delocalised system.
Key Definition: Delocalisation energy (resonance energy) is the difference between the predicted enthalpy of hydrogenation (based on the Kekule model) and the actual enthalpy of hydrogenation. For benzene, it is approximately 152 kJ mol⁻¹, showing that the delocalised structure is more stable.
Electrophilic Substitution — General Pattern
Benzene reacts with electrophiles, but it undergoes substitution (not addition), because substitution preserves the stable delocalised ring.
General mechanism:
- An electrophile (E⁺) is generated (often with the help of a catalyst/halogen carrier).
- The electrophile is attracted to the electron-rich delocalised π system.
- A curly arrow from the π system of the ring to the electrophile forms a new C–E bond.
- This creates a positively charged intermediate (sometimes called a Wheland intermediate or arenium ion) in which the delocalised system is partially disrupted.
- A hydrogen atom is lost from the same carbon. A curly arrow from the C–H bond to the ring restores the delocalised system. The H leaves as H⁺.
- The stable aromatic ring is regenerated.
Reaction 1: Nitration
Equation: C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O (benzene → nitrobenzene)
Reagents: Concentrated nitric acid (conc. HNO₃) + concentrated sulfuric acid (conc. H₂SO₄)
Temperature: Below 55°C (to prevent di- and tri-nitration)
Electrophile: The nitronium ion, NO₂⁺
Generation of the electrophile:
HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O
The H₂SO₄ protonates the HNO₃, which then loses water to form NO₂⁺.
Mechanism:
- Curly arrow from the delocalised π system of benzene to the N of NO₂⁺ (forming a C–N bond).
- Intermediate: a cyclohexadienyl cation (aromatic system partially disrupted, ring carries a + charge).
- Curly arrow from the C–H bond back into the ring (restoring aromaticity). H⁺ is released.
- The H⁺ regenerates H₂SO₄ (catalyst): H⁺ + HSO₄⁻ → H₂SO₄.