Topic 18 · page 1 of 10

Benzene and the energy evidence

Benzene, C₆H₆, does not behave like three C=C bonds. Its enthalpy of hydrogenation is 152 kJ mol⁻¹ less exothermic than a Kekulé ring predicts, so the ring is unusually stable.

14 min read18.1

Benzene and two ways to draw it 18.1

Benzene is C₆H₆: a ring of six carbon atoms, each bonded to one hydrogen atom, all in one flat plane. It is the simplest arene.

In 1865 Kekulé suggested a ring with alternating single and double bonds, cyclohexa-1,3,5-triene. Check it fits the formula: each carbon has four bonds, and the ring uses one to H, one to each neighbour and one more for a double bond. That gives three C–C, three C=C and six C–H bonds, which is C₆H₆.

The modern model replaces the double bonds with a ring of delocalised electrons, drawn as a circle.

Two ways to draw benzene: the Kekulé ring with alternating double bonds, and the ring with a circle for the delocalised electrons.
The Kekulé structure and the delocalised structure of benzene.

The problem with Kekulé's structure 18.1

If benzene really had three C=C bonds, it should behave like an alkene. It should decolourise bromine water at once and add hydrogen easily. It does neither.

Three kinds of data, thermochemical, X-ray diffraction and infrared, show that benzene does not have the Kekulé structure. This page covers the first. The next page covers the other two.

Evidence 1: enthalpy of hydrogenation 18.1

Hydrogenation adds hydrogen across a C=C bond. For cyclohexene, which has one C=C bond:

CX6HX10(l)+HX2(g)→CX6HX12(l)ΔH=−120 kJ mol−1 \ce{C6H10(l) + H2(g) -> C6H12(l)} \qquad \Delta H = -120\ \text{kJ mol}^{-1}

Prediction. If Kekulé's benzene has three separate C=C bonds, hydrogenating all three should release three times as much energy:

3×(−120)=−360 kJ mol−1 3 \times (-120) = -360\ \text{kJ mol}^{-1}

Measurement. The real reaction gives the same product, cyclohexane:

CX6HX6(l)+3 HX2(g)→CX6HX12(l)ΔH=−208 kJ mol−1 \ce{C6H6(l) + 3H2(g) -> C6H12(l)} \qquad \Delta H = -208\ \text{kJ mol}^{-1}

The measured value is 360 − 208 = 152 kJ mol⁻¹ less exothermic than predicted.

Enthalpy level diagram. Hypothetical Kekulé benzene plus 3H2 sits at the top. Real benzene plus 3H2 is 152 kJ per mole lower. Cyclohexane is at the bottom, 360 kJ per mole below the Kekulé level and 208 kJ per mole below real benzene.
Drawn to one scale. Both routes end at cyclohexane, so the gap at the top is the extra stability of real benzene.

Both routes end at the same product, so the difference must be at the start. Real benzene begins lower in energy than the Kekulé structure, so less energy is released on the way down to cyclohexane.

Worked exampleHow much more stable is benzene?

The enthalpy of hydrogenation of cyclohexene is −120 kJ mol⁻¹. That of benzene is −208 kJ mol⁻¹. Calculate the extra stability of benzene.

  1. Predicted for Kekulé benzene: 3 × (−120) = −360 kJ mol⁻¹
  2. Measured: −208 kJ mol⁻¹
  3. Difference: −208 − (−360) = +152, so benzene releases 152 kJ mol⁻¹ less energy, which means it is 152 kJ mol⁻¹ more stable.

Check yourself

  1. How many C–C single bonds, C=C double bonds and C–H bonds are there in the Kekulé structure of benzene?
  2. The enthalpy of hydrogenation of cyclohexene is −120 kJ mol⁻¹. Predict the value for the Kekulé structure of benzene, and for cyclohexa-1,4-diene, which has two isolated C=C bonds.
  3. The measured enthalpy of hydrogenation of benzene is −208 kJ mol⁻¹. What does the difference from the predicted value show?
  4. Calculate the energy released when 3.90 g of benzene is hydrogenated to cyclohexane. How much would be released if benzene had the Kekulé structure?
  5. A student says: "−208 kJ mol⁻¹ is more exothermic than −120 kJ mol⁻¹, so benzene is less stable than cyclohexene." Explain the error.

Answers cover these until you have tried the questions

  1. Three C–C, three C=C and six C–H bonds.
  2. Kekulé benzene: 3 × (−120) = −360 kJ mol⁻¹. Cyclohexa-1,4-diene: 2 × (−120) = −240 kJ mol⁻¹.
  3. The measured value is 360 − 208 = 152 kJ mol⁻¹ less exothermic. Real benzene starts 152 kJ mol⁻¹ lower in energy than the Kekulé structure, so it is more stable. The extra stability is because the π electrons are delocalised.
  4. M(C₆H₆) = 6 × 12.0 + 6 × 1.0 = 78.0 g mol⁻¹. n = 3.90 ÷ 78.0 = 0.0500 mol. Energy released = 0.0500 × 208 = 10.4 kJ. Kekulé structure: 0.0500 × 360 = 18.0 kJ.
  5. Benzene reacts with three moles of H₂, cyclohexene with one, so the values are not comparable. The correct comparison is with three times the cyclohexene value, −360 kJ mol⁻¹. Benzene releases 152 kJ mol⁻¹ less than that, so it is more stable than a Kekulé ring.