Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution (EAS)

Overview

• Definition: A chemical reaction where an electrophile replaces a hydrogen atom on an aromatic ring.

• Importance: Key mechanism in organic chemistry for synthesizing substituted aromatic compounds.

Mechanism

1. Formation of Electrophile:

   • Electrophiles can be generated from various reagents (e.g., halogens, nitrating agents).

   • Common electrophiles include:

     • Halogens (Br₂, Cl₂ with FeBr₃, FeCl₃)

     • Nitronium ion (NO₂⁺ from HNO₃ and H₂SO₄)

     • Sulfonium ion (SO₃H⁺ from SO₃)

2. Electrophilic Attack:

   • The aromatic ring donates a pair of π electrons to the electrophile, forming a non-aromatic carbocation intermediate (arenium ion).

   • This step is rate-determining.

3. Deprotonation:

   • A base (often the conjugate base of the acid used) removes a proton from the carbocation, restoring aromaticity.

   • The product is a substituted aromatic compound.

Types of EAS Reactions

1. Halogenation:

   • Substitution of a hydrogen atom with a halogen (e.g., bromination).

   • Requires a halogen and a Lewis acid catalyst.

2. Nitration:

   • Introduction of a nitro group (NO₂).

   • Uses a mixture of concentrated nitric acid and sulfuric acid.

3. Sulfonation:

   • Introduction of a sulfonyl group (SO₃H).

   • Involves sulfur trioxide (SO₃) or fuming sulfuric acid.

4. Alkylation (Friedel-Crafts Alkylation):

   • Addition of an alkyl group.

   • Requires an alkyl halide and a Lewis acid (e.g., AlCl₃).

5. Acylation (Friedel-Crafts Acylation):

   • Introduction of an acyl group (RCO-).

   • Uses an acyl chloride and a Lewis acid.

Regioselectivity

• Ortho/Para vs. Meta Directing:

  • Electron-donating groups