Chapter 18: Electrophilic Aromatic Substitution Reactions

Benzene and EAS

• Benzene does not undergo addition rxns because benzene will lose its aromaticity.

• Benzene undergoes substitution

• Benzene acts as a nucleophile

Electrophilic Aromatic Substitution (EAS)

• Substitution reaction

• Benzene is a nucleophile

• An electrophile (E⁺) will replace one of the protons on benzene

Halogenation

• X₂= Br₂ or Cl₂

• Catalyst (Lewis Acid): FeCl_3,AlCl₃,AlBr₃

Nitration

• Electrophile: NO₂⁺ (Nitronium Ion)

• Catalyst: H₂SO₄

• Source of EP: HNO₃

NO₂ Reduction

• NO₂ , nitro group, can be reduced to NH₂ using Fe/Zn in HCl followed by NaOH.

• Is reduced into analine (NH₂)

Sulphuration

• Produces SO₃H

• Electrophile: SO₃

• Catalyst: H₂SO₄

• Fuming H₂SO₄ can also produce SO₃H addition

Friedel Craft Alkylation

• Electrophile: Carbocation generated from the RX

• Catalyst: AlCl₃ - Lewis acid

Limitations of Friedel Craft

1. Possibility of carbocation rearrangement

   • The carbocation could possibly rearrange through a hydride shift.

   • The most stable carbocation will be the major product.

2. Presence of a Strong Electron-Withdrawing Group

   • A strong EWG deactivates the ring - thus there is no reaction.

   • Ex. Nitro group (NO₂)

3. The leaving group can not be an sp² carbon.

4. Possibility of polyalkylation

   • More than 1 alkyl is added

Friedel Craft Acylation

• Electrophile: Acylium Ion (RCO⁺)

• R is an aryl group

• Catalyst: AlCl₃ (Lewis Acid)

Clemmensen Reduction

• Reduces the aryl on the benzyl

• Converts ketones and aldehydes to alkanes

• Reduces the carbon oxygen double bond.