Describe the delocalised structure of benzene and the evidence for it, explain why benzene undergoes electrophilic substitution rather than addition, and describe the mechanisms of nitration and halogenation

What this dot point is asking

SEAB wants you to describe the delocalised structure of benzene and the evidence for it, explain why benzene undergoes electrophilic substitution rather than addition, and set out the mechanisms of nitration and halogenation. The delocalisation evidence and the nitration mechanism (including generating the electrophile) are reliable high-mark questions.

The answer

The structure of benzene

Benzene, $\text{C}_6\text{H}_6$, is a planar, regular hexagon. Each carbon is $sp^2$ hybridised and forms three sigma bonds (to two carbons and one hydrogen). The remaining p orbital on each carbon, perpendicular to the ring, overlaps sideways with its neighbours to form a continuous ring of delocalised pi electrons above and below the plane.

Evidence for delocalisation

The delocalised model is supported by three lines of evidence against the Kekule alternating-double-bond structure:

• Equal bond lengths. All six C-C bonds are the same length (about $0.139$ nm), between a single and a double bond. Kekule would predict alternating lengths.
• Enthalpy of hydrogenation. Benzene's hydrogenation is much less exothermic than three times that of cyclohexene. The difference (the delocalisation or resonance energy) shows benzene is more stable than the hypothetical Kekule molecule.
• Resistance to addition. Benzene does not readily add bromine or decolourise bromine water, unlike an alkene, because addition would destroy the stable delocalised ring.

Why substitution, not addition

The delocalised ring is energetically very stable. An addition reaction would break the delocalisation and lose that stability. Substitution replaces a hydrogen with another group while preserving the delocalised ring, so it is much more favourable. Benzene therefore undergoes electrophilic substitution.

Nitration mechanism

Conditions: concentrated nitric acid and concentrated sulfuric acid (catalyst), about $50$ degrees Celsius.

1. Generate the electrophile. Sulfuric acid protonates nitric acid, which loses water to form the nitronium ion: $\text{HNO}_3 + 2\text{H}_2\text{SO}_4 \rightarrow \text{NO}_2^+ + 2\text{HSO}_4^- + \text{H}_3\text{O}^+$.
2. Attack. The delocalised pi electrons attack $\text{NO}_2^+$, forming an unstable positively charged intermediate with disrupted delocalisation.
3. Restore the ring. The intermediate loses a proton, restoring the delocalised ring and giving nitrobenzene; the proton regenerates the sulfuric acid catalyst.

Halogenation mechanism

Conditions: chlorine (or bromine) with a halogen-carrier catalyst such as $\text{AlCl}_3$ or $\text{FeCl}_3$, at room temperature.

1. The catalyst generates the electrophile: $\text{Cl}_2 + \text{AlCl}_3 \rightarrow \text{Cl}^+ + \text{AlCl}_4^-$.
2. The pi electrons attack $\text{Cl}^+$, forming the intermediate.
3. Loss of a proton restores the ring (giving chlorobenzene) and regenerates the catalyst.

Examples in context

Example 1. Making explosives and dyes. Repeated nitration of benzene rings produces compounds such as TNT, and the substituted aromatic products are intermediates for dyes and pharmaceuticals. SEAB uses these applications to motivate the nitration mechanism and to test whether candidates can predict products of further substitution.

Example 2. Comparing benzene with cyclohexene. A common Paper 2 question gives bromine water and asks candidates to distinguish benzene from cyclohexene. Cyclohexene rapidly decolourises bromine water by electrophilic addition, while benzene does not react under the same conditions, directly demonstrating the stability conferred by delocalisation.

Try this

Q1. State two pieces of evidence for the delocalised structure of benzene. [2 marks]

• Cue. Equal C-C bond lengths; enthalpy of hydrogenation less exothermic than expected (delocalisation energy). (Resistance to addition also acceptable.)

Q2. Write the equation for the formation of the electrophile in the nitration of benzene. [1 mark]

• Cue. $\text{HNO}_3 + 2\text{H}_2\text{SO}_4 \rightarrow \text{NO}_2^+ + 2\text{HSO}_4^- + \text{H}_3\text{O}^+$.

Q3. State the reagents and catalyst for the chlorination of benzene, and name the product. [2 marks]

• Cue. Chlorine with an $\text{AlCl}_3$ (or $\text{FeCl}_3$) catalyst; product chlorobenzene.