In simple terms
A friendly intro before the formal notes — no formulas yet.
Benzene's Stable Secret
Benzene has a unique flat ring structure with a cloud of shared electrons, making it unusually stable. This stability dictates its chemical reactions, favouring substitution over addition.
Imagine six friends holding hands in a circle to form a strong framework (the σ bonds). Now, instead of passing a single ball (a localised π bond) between just two people at a time, they are all juggling a shared cloud of six balls together, above and below their hands (the delocalised π system). It's much harder to break this cooperative group apart (high stability) than to just intercept one ball from a simple pair.
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Benzene C₆H₆ — planar hexagon, 120° bond angles.
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σ bonds: C–C and C–H framework; π cloud above/below ring.
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Electrophilic substitution preserves aromaticity.
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Resonance structures — average bond order 1.5.
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Full topic notes
Formal explanation with the rigour you need for the exam.
The Shape and Structure of Benzene
Experimental evidence, primarily from X-ray diffraction studies, reveals that benzene is a perfectly flat, or planar, molecule. The six carbon atoms are arranged in a regular hexagon. All the carbon-carbon bond lengths are identical at 0.139 nm, and all bond angles, both C-C-C and H-C-C, are 120°. This uniformity is a key feature that the earlier Kekulé model of alternating single and double bonds could not explain.
Formula: C₆H₆
Shape: Planar, regular hexagon
Bond angles: 120° throughout
C-C bond lengths: All are 0.139 nm (intermediate between single and double bonds)
Orbital Overlap: The σ and π Framework
To achieve a trigonal planar geometry with 120° bond angles, each carbon atom in the benzene ring is sp² hybridised. Each carbon atom forms three σ (sigma) bonds. Two of these are formed by the head-on overlap of its sp² hybrid orbitals with those of its two neighbouring carbon atoms. The third σ bond is formed by the head-on overlap of its remaining sp² hybrid orbital with the 1s orbital of a hydrogen atom. This network of σ bonds creates a strong, planar C₆H₆ framework.
After forming the σ framework, each carbon atom has one unhybridised 2p orbital remaining. These p-orbitals are oriented perpendicular (at 90°) to the plane of the ring. The six p-orbitals are close enough to overlap sideways with their neighbours on both sides. This continuous sideways overlap merges the six individual p-orbitals into two ring-shaped electron clouds, one above and one below the plane of the σ framework. The six electrons (one from each carbon's p-orbital) are no longer associated with a single carbon atom but are shared amongst all six. This is the delocalised π system.
When asked to describe the bonding in benzene, always discuss the formation of the σ framework first, then explain the formation of the delocalised π system from the remaining p-orbitals. Marks are often allocated for both parts separately.
Evidence for the Delocalised Model
The delocalised model of benzene is not just a theory; it is supported by robust experimental evidence. The three main lines of evidence are thermochemical data, bond length measurements, and chemical reactivity.
Worked examples
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The enthalpy of hydrogenation of cyclohexene (C₆H₁₀), which has one C=C bond, is -120 kJ mol⁻¹. The experimental enthalpy of hydrogenation of benzene (C₆H₆) to cyclohexane (C₆H₁₂) is -208 kJ mol⁻¹. (a) Predict the enthalpy of hydrogenation for the hypothetical cyclohexa-1,3,5-triene. (b) Calculate the delocalisation energy of benzene. (c) Explain what this value indicates about the stability of benzene.
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(a) The hypothetical cyclohexa-1,3,5-triene has three localised C=C bonds. Predicted enthalpy change = 3 × (enthalpy change for one C=C bond) = 3 × (-120 kJ mol⁻¹) = -360 kJ mol⁻¹.
Explain why the C–O bond in phenol (C₆H₅OH) is shorter than the C–O bond in cyclohexanol (C₆H₁₁OH), with reference to orbital overlap.
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In phenol, the oxygen atom is bonded to an sp² hybridised carbon of the benzene ring. One of the lone pairs on the oxygen atom is in a p-orbital that is parallel to the p-orbitals of the ring.
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Glossary
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Revision flashcards
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What is the hybridisation of the carbon atoms in a benzene molecule?
sp² hybridisation. Each carbon uses three sp² hybrid orbitals to form σ bonds and has one unhybridised p-orbital.
Key takeaways
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Formula: C₆H₆
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Shape: Planar, regular hexagon
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Bond angles: 120° throughout
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C-C bond lengths: All are 0.139 nm (intermediate between single and double bonds)
Practice — then mark it
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Practice Questions: Shapes of Aromatic Molecules
Practice Questions: Shapes of Aromatic Molecules
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