Benzene: Structure and Stability

Learning Objectives (OCR Spec 6.1.1)

By the end of this lesson you should be able to:

• Describe the Kekule (alternating single/double bonds) structure of benzene and explain why it is inadequate
• Describe the delocalised model of benzene with 6 pi electrons
• State the evidence for the delocalised model: bond lengths, enthalpy of hydrogenation, resistance to addition
• Use the IUPAC nomenclature for benzene derivatives
• Explain why benzene is especially stable compared with a hypothetical cyclohexatriene
• Draw benzene using both the Kekule and delocalised (circle) representations

The Discovery of Benzene

Benzene was first isolated in 1825 by Michael Faraday from a by-product of whale-oil lamp fuel. It was soon found to have molecular formula C6H6 - an extraordinarily unsaturated molecule with a hydrogen-to-carbon ratio of only 1:1. By comparison, hexane C6H14 has 2.33:1 and cyclohexane C6H12 has 2:1. Benzene therefore has the degree of unsaturation of a ring plus three double bonds - but it does NOT behave like a compound with three normal C=C double bonds.

For fifty years after Faraday's isolation, chemists struggled to write a structural formula that explained benzene's properties. The problem was solved in 1865 by August Kekule, who proposed a cyclic structure with alternating single and double bonds:

       H
       |
   H   C   H
    \ / \ /
     C   C
     ||   ||
     C   C
    / \ / \
   H   C   H
       |
       H

This is the Kekule structure - a six-membered ring with three alternating C-C and C=C bonds. It accounts for the formula C6H6 and explains why benzene is unsaturated. Kekule said he saw the structure in a dream of a snake biting its own tail.

Why Kekule Is Wrong

Kekule's structure was a great leap forward, but within a few years chemists realised it did not fit all the evidence:

1. Bond Lengths

In a C-C single bond, the bond length is about 154 pm (1.54 angstroms). In a C=C double bond, it is about 134 pm (1.34 angstroms). In the Kekule structure you would expect alternating short-long-short-long bonds. However, X-ray diffraction shows that all six C-C bonds in benzene are identical, with a length of 139 pm - intermediate between single and double. This is incompatible with three localised double bonds.

2. Enthalpy of Hydrogenation

When cyclohexene (one C=C) is hydrogenated to cyclohexane:

C6H10 + H2 -> C6H12, delta_H = -120 kJ mol-1

Extrapolating, a hypothetical cyclohexatriene (three C=C, Kekule benzene) should release 3 x (-120) = -360 kJ mol-1 on hydrogenation. But the observed enthalpy of hydrogenation of benzene is:

C6H6 + 3 H2 -> C6H12, delta_H = -208 kJ mol-1

This is 152 kJ mol-1 less exothermic than expected. Benzene is therefore 152 kJ mol-1 more stable than cyclohexatriene would be. This extra stability is called the resonance / delocalisation energy of benzene.

3. Resistance to Addition

Alkenes readily undergo electrophilic addition: C=C + Br2 -> C-Br-C-Br (bromine decolourises). However, benzene does not react with bromine water or cold dilute KMnO4 at room temperature. It resists addition because addition would break the stable aromatic ring. Benzene undergoes substitution instead of addition (see lesson 11), preserving the ring.

4. Shape

Benzene is planar with all bond angles 120 degrees, consistent with sp2 hybridisation of all six carbons. The six p-orbitals perpendicular to the ring form a continuous pi-system above and below the plane.

graph TD
  A[C6H6 molecular formula] --> B{Kekule 1,3,5-cyclohexatriene<br/>alternating single and double bonds}
  B --> C[Predicts C-C 154 pm<br/>C=C 134 pm alternating]
  B --> D[Predicts enthalpy of hydrogenation<br/>about -360 kJ mol-1]
  B --> E[Predicts addition with Br2]
  C --> F[Observed: all 139 pm<br/>contradiction]
  D --> G[Observed: -208 kJ mol-1<br/>contradiction]
  E --> H[Observed: no reaction<br/>contradiction]
  F --> I[Delocalised model needed]
  G --> I
  H --> I

The Delocalised Model

The accepted model of benzene is the delocalised pi-system: