Benzene (HL) (DP IB Chemistry): Revision Note

Benzene

• Benzene is a six-carbon ring with the molecular formula C₆H₆

• The structure was first proposed by Kekulé, who suggested it contained alternating single and double carbon–carbon bonds

• This model predicted that benzene would behave like an alkene in chemical reactions

  • However, this is not the case

Structure of benzene

• Benzene is a cyclic molecule with a regular hexagonal shape

• All six carbon atoms:

  • Are sp² hybridised

  • Form three sigma (σ) bonds:

    • Two to neighbouring carbons

    • One to a hydrogen atom

• Each carbon atom also has an unhybridised p orbital perpendicular to the ring plane

• These p orbitals overlap sideways to form a delocalised π system above and below the ring

• The six π electrons are shared equally across all six carbon atoms

• This delocalisation gives benzene its unusual stability and explains why all carbon–carbon bonds are:

  • Equal in length

  • Intermediate between single and double bonds

• Benzene is planar with bond angles of 120°

Experimental evidence for delocalisation

• This evidence of the bonding in benzene is provided by data from:

  • Enthalpy changes of hydrogenation

  • Carbon-carbon bond lengths from X-ray diffraction

  • Saturation tests

  • Infrared spectroscopy

Enthalpy changes of hydrogenation

• The hydrogenation of the one C=C bond in cyclohexene has an enthalpy change of –120 kJ mol^-1:

C₆H₁₀ + H₂ → C₆H₁₂   ΔH^ꝋ = -120 kJ mol^-1

• Based on the Kekulé model, the hydrogenation of benzene with three C=C bonds should release 3 × –120 = –360 kJ mol^-1:

C₆H₆ + 3H₂ → C₆H₁₂     ΔH^ꝋ expected = –360 kJ mol^-1

• The actual enthalpy change of benzene is only –208 kJ mol⁻¹

  • This is 152 kJ mol⁻¹ less exothermic than predicted

• This difference is called the delocalisation energy

  • It shows that benzene is more stable than the Kekulé model suggests

Carbon-carbon bond lengths

• X-ray diffraction shows all C–C bond lengths in benzene are 140 pm

• This is intermediate between:

  • A single C–C bond = 154 pm

  • A double C=C bond = 134 pm

• This supports a structure with delocalised bonding

  • The Kekulé structure would produce alternating bond lengths:

    • 134 pm for double bonds

    • 154 pm for single bonds

  • It would not produce the uniform 140 pm seen experimentally

Saturation tests

• Cyclohexene decolourises bromine water via electrophilic addition across the C=C bond

  • If benzene had double bonds (as in the Kekulé structure), it would also react

• Benzene does not decolourise bromine water

  • This confirms it does not contain isolated double bonds, further supporting delocalisation

Infrared spectroscopy

• Cyclohexene shows a strong absorption at 1620 – 1680 cm^-1

  • This is due to C=C stretching

• Based on Kekulé’s model, benzene should show similar peaks

  • However, benzene does not absorb in this range

  • Benzene shows weaker peaks at:

    • 1450 cm^-1

    • 1500 cm^-1

    • 1580 cm⁻¹

  • These peaks are characteristic of delocalised π systems in arenes