Reactivity 3.4.13 – Electrophilic Substitution Reactions of Benzene

Benzene is an aromatic compound with a delocalised π electron system across six carbon atoms in a planar hexagonal ring. Despite being unsaturated, benzene does not typically undergo addition reactions like alkenes. Instead, it undergoes electrophilic substitution reactions to preserve the stability of the aromatic ring.

Why Benzene Resists Addition:

• Benzene’s π electrons are delocalised, forming a stable resonance hybrid.
• Addition reactions would disrupt this aromatic stability by breaking the delocalised system.
• Substitution reactions allow benzene to maintain aromaticity after reaction.

Electrophilic substitution is a type of reaction where an electrophile replaces a hydrogen atom on the benzene ring.

General Characteristics of Electrophilic Substitution in Benzene:

• Occurs under specific conditions (often using catalysts like AlCl₃, FeCl₃, or H₂SO₄).
• Involves highly reactive electrophiles (E⁺) that attack the delocalised π electrons in the benzene ring.
• Proceeds via a two-step mechanism: formation of an arenium ion (carbocation intermediate) and loss of a proton to reform the aromatic ring.

Common Electrophilic Substitution Reactions of Benzene:

• Nitration: Benzene + HNO₃ (conc.) with H₂SO₄ catalyst → nitrobenzene
• Halogenation: Benzene + Cl₂ or Br₂ with FeCl₃ or FeBr₃ → chlorobenzene or bromobenzene
• Friedel-Crafts Alkylation: Benzene + R-Cl with AlCl₃ → alkylbenzene
• Friedel-Crafts Acylation: Benzene + RCOCl with AlCl₃ → aryl ketone

Example – Nitration of Benzene:

\( \text{C}_6\text{H}_6 + \text{HNO}_3 \xrightarrow{\text{H}_2\text{SO}_4} \text{C}_6\text{H}_5\text{NO}_2 + \text{H}_2\text{O} \)

Aromatic Stability Preservation:

After the substitution, the π electron system is re-established, allowing benzene to retain its aromaticity and thermodynamic stability.

Energy Profile:

• The activation energy is high due to the need to temporarily break aromaticity.
• The intermediate (arenium ion) is less stable, but the final product regains aromaticity, making the reaction favourable overall.

Mechanism of the Reaction Between Benzene and a Charged Electrophile (E⁺)

Electrophilic substitution in benzene occurs via a two-step mechanism:

Step 1 – Formation of the arenium ion (carbocation intermediate):

The electrophile (E⁺) is attracted to the high electron density of the delocalised π system in benzene. The π electrons from the ring form a bond with the electrophile, resulting in a non-aromatic carbocation intermediate (also called a sigma complex or arenium ion).

Step 2 – Restoration of aromaticity:

A base removes a proton (H⁺) from the carbon where substitution occurred. The electrons from the C-H bond are returned to the π system, regenerating the aromatic ring.

General Curly Arrow Mechanism:

• Reactants: Benzene and an electrophile (E⁺)
• Intermediate: Arenium ion (with one carbon sp³ hybridised)
• Products: Substituted benzene (with E group) and a proton (H⁺)

Curly Arrow Notation (Steps):

1. The curved arrow originates from the delocalised π electrons and points toward the electrophile (E⁺), forming a new σ bond.
2. This forms the arenium ion, where the positive charge is delocalised across the ring (resonance forms).
3. Another curved arrow shows the loss of a proton (H⁺) from the substituted carbon, with the electrons from the C-H bond returning to the ring to restore aromaticity.

Example: Bromination of Benzene

Step 1: Generation of electrophile (not assessed but occurs via catalyst)

\( \text{Br}_2 + \text{FeBr}_3 \rightarrow \text{Br}^+ + \text{FeBr}_4^- \)

Step 2: Benzene reacts with \( \text{Br}^+ \)

1. π electrons attack \( \text{Br}^+ \), forming a sigma complex (arenium ion)
2. The arenium ion loses \( \text{H}^+ \) and reforms the aromatic system

Overall Reaction:

\( \text{C}_6\text{H}_6 + \text{Br}_2 \xrightarrow{\text{FeBr}_3} \text{C}_6\text{H}_5\text{Br} + \text{HBr} \)

Energy Considerations:

• The reaction temporarily disrupts aromaticity, making the intermediate less stable.
• Aromaticity is fully restored in the final step, driving the reaction forward thermodynamically.