Benzene (OCR A Level Chemistry A): Revision Note

Comparing Models of Benzene

• The structure of benzene (C₆H₆) was a long-standing puzzle in chemistry

• Two models are used to describe its structure:

  1. The early Kekulé model

  2. The modern delocalised model

The Kekulé model

• This model proposed a hexagonal ring of six carbon atoms with alternating single and double C-C bonds

• It suggests that the π-electrons in benzene are localised within these three double bonds

The delocalised model

• This is the accepted modern model

• It proposes that the p-orbitals of all six carbon atoms overlap sideways, both above and below the plane of the ring

• This overlap forms a continuous ring of electron density, creating a delocalised π-system where the six π-electrons are spread over the entire ring

Evidence for the delocalised model

• There are three key pieces of experimental evidence that support the delocalised model over the Kekulé model

1. Bond lengths

• X-ray diffraction analysis shows that all six carbon-carbon bonds in benzene are identical in length at 0.140 nm

• This contradicts the Kekulé model

  • The Kekulé model would have alternating short C=C double bonds (0.134 nm) and long C–C single bonds (0.154 nm)

• The measured bond length is intermediate between a single and double bond, supporting the delocalised model

2. Molecular shape & bond angles:

• X-ray diffraction also shows that benzene is a perfectly planar (flat) regular hexagon

• All C–C–C bond angles are identical at 120°

• This contradicts the distorted ring of alternating 120° and 109.5° angles that a strict Kekulé model would imply

• The perfect hexagonal shape is only possible if all six C-C bonds are identical

3. Enthalpy of hydrogenation:

• The hydrogenation of cyclohexene (one C=C bond) has an enthalpy change of -120 kJ mol^-1

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

• The Kekulé structure (with three C=C bonds) would therefore be expected to have an enthalpy of hydrogenation of 3 × (-120) = -360 kJ mol^-1

C₆H₆ + 3H₂ → C₆H₁₂   ΔH^Θ = 3 x -120 kJ mol^-1 = -360 kJ mol^-1

• However, the experimental value for benzene is only -208 kJ mol^-1

• This means benzene is significantly more stable (less exothermic) than the Kekulé structure suggests, due to the energy of the delocalised π-system.

Summary of Kekulé vs. delocalised models

Benzene Resistance to Halogenation

• This is a key piece of chemical evidence for the delocalised model

Halogenation in alkenes

• Alkenes (like cyclohexene) have a localised region of high electron density in their C=C double bond

• This is strong enough to polarise an approaching Br₂ molecule

  • This results one δ+ bromine atom and one δ- bromine atom 

• So, a rapid electrophilic addition reaction occurs at room temperature

Halogenation in benzene

• In benzene, the electron density of the six π-electrons is delocalised and spread out over the entire ring

• This means the electron density at any one point is lower than in an alkene's C=C bond

  • So, it is not strong enough to polarise the Br₂ molecule

• Therefore, benzene does not react with bromine under normal conditions

• Benzene resists addition reactions as this would disrupt the very stable delocalised ring

• Instead, it undergoes substitution reactions

  • This requires a halogen carrier catalyst, such as AlBr₃

Nomenclature of Aromatic Compounds

• In chemistry, aromatic compounds are those that contain one or more benzene rings

• The IUPAC rules for naming simple substituted benzenes involve using the substituent name as a prefix, followed by "-benzene"

  • For example:

• Some common aromatic compounds have accepted trivial names that you should know

  • For example:

• For multiple substituents, numbers are used to indicate their positions on the ring

  • The goal is to use the lowest possible numbers

  • For example: