IB DP Chemistry - S2.2.12 Benzene- Study Notes - New Syllabus - 2026, 2027 & 2028
IB DP Chemistry – S2.2.12 Benzene- Study Notes – New Syllabus
IITian Academy excellent Study Notes and effective strategies will help you prepare for your IB DP Chemistry exam.
- IB DP Chemistry 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Chemistry 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
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Structure 2.2.12 — Benzene and Resonance
Structure 2.2.12 — Benzene and Resonance
Benzene (\( \text{C}_6\text{H}_6 \)) is an aromatic compound whose bonding cannot be represented by a single Lewis structure. It exhibits resonance and is best described as a resonance hybrid with delocalized π electrons.
Lewis Structures of Benzene
Benzene can be drawn with alternating single and double bonds around a hexagonal ring:
These two resonance structures differ in the position of the double bonds, but neither accurately reflects the true structure.
Physical and Experimental Evidence for Resonance in Benzene
1. Bond Length Evidence:
X-ray diffraction studies show that all C–C bonds in benzene are equal in length, about 0.139 nm. This is intermediate between a typical C–C single bond (0.154 nm) and C=C double bond (0.134 nm).
2. Bond Energy:
The enthalpy of hydrogenation of benzene is less exothermic than expected. Theoretical hydrogenation of three double bonds in cyclohexatriene (not real benzene) would release about -360 kJ/mol, but benzene releases only -208 kJ/mol. The difference (~152 kJ/mol) is called the resonance energy, showing extra stability due to delocalization.
3. Chemical Reactivity:
Benzene does not undergo typical addition reactions of alkenes. Instead, it undergoes electrophilic substitution reactions, preserving the aromatic ring. This suggests a stable delocalized π system that resists disruption.
4. Spectroscopy:
UV-Vis spectroscopy of benzene shows strong absorption in the ultraviolet range due to π to π* transitions, confirming the presence of delocalized π electrons across the ring.
Resonance Hybrid and Delocalization
The actual structure of benzene is a resonance hybrid of the two Lewis structures. The six π electrons from the p orbitals of carbon atoms are delocalized around the ring, forming a continuous electron cloud above and below the plane of the ring.
This delocalization gives benzene its exceptional stability and unique chemical behavior.Summary of Properties Explained by Resonance
- All C–C bonds are equal in length and strength.
- Benzene is less reactive than alkenes, resisting addition reactions.
- Undergoes substitution to retain aromatic character.
- Extra stability due to resonance energy.
Example
Benzene (\( \text{C}_6\text{H}_6 \)) does not undergo typical electrophilic addition reactions like ethene does. Explain this behavior using the concept of resonance and delocalization.
▶️Answer/Explanation
Ethene has a localized π bond that can easily break and react with electrophiles, allowing addition reactions.
In contrast, benzene has a delocalized π electron cloud formed by the overlap of six p orbitals. This delocalization creates a stable resonance hybrid, and breaking this system would require energy and disrupt the stability of the ring.
Therefore, benzene prefers electrophilic substitution reactions, where one hydrogen is replaced and the delocalized π system is preserved.
Example
The experimental enthalpy of hydrogenation of benzene is −208 kJ/mol. Theoretically, if benzene had three isolated C=C bonds like cyclohexatriene, the enthalpy of hydrogenation would be −360 kJ/mol. What does this difference tell you about the bonding in benzene?
▶️Answer/Explanation
The difference between the expected and actual hydrogenation enthalpy is:
\( \Delta H = -360 – (-208) = -152 \text{ kJ/mol} \)
This value is known as the resonance energy. It indicates that benzene is more stable than a structure with three isolated C=C bonds.
The extra stability arises from the delocalization of π electrons in the ring, confirming the existence of resonance and a resonance hybrid structure in benzene.