15

by

Electrophilic Aromatic Substitution (EAS)

Electrophilic aromatic substitution (EAS) is a type of organic reaction in which an electrophile (an electron-deficient species) replaces a hydrogen atom on an aromatic ring (e.g., benzene). This reaction is characteristic of aromatic compounds due to their stability and electron-rich π-system.

General Mechanism of EAS:

  1. Formation of the Electrophile:
  • The electrophile ((E^+)) is generated using a catalyst or reagent.
  1. Attack on the Aromatic Ring:
  • The aromatic ring donates π-electrons to the electrophile, forming a carbocation intermediate (called the arenium ion or sigma complex).
  1. Deprotonation:
  • A base removes a proton ((H^+)) from the carbocation intermediate, restoring the aromaticity of the ring and forming the final substituted product.

General Reaction:

[
\text{Aromatic Ring} + E^+ \rightarrow \text{Substituted Aromatic Ring} + H^+
]

Examples of EAS Reactions:

1. Nitration:

  • Electrophile: Nitronium ion ((NO_2^+)).
  • Reagent: (HNO_3) (nitric acid) + (H_2SO_4) (sulfuric acid).
  • Example: [ C_6H_6 + HNO_3 \xrightarrow{H_2SO_4} C_6H_5NO_2 + H_2O ]
    • Benzene reacts with nitric acid to form nitrobenzene.

2. Halogenation:

  • Electrophile: Halogen cation ((X^+)).
  • Reagent: (X_2) (halogen) + (FeX_3) (Lewis acid catalyst).
  • Example: [ C_6H_6 + Br_2 \xrightarrow{FeBr_3} C_6H_5Br + HBr ]
    • Benzene reacts with bromine to form bromobenzene.

3. Sulfonation:

  • Electrophile: Sulfur trioxide ((SO_3)).
  • Reagent: (H_2SO_4) (sulfuric acid) or (SO_3).
  • Example: [ C_6H_6 + SO_3 \xrightarrow{H_2SO_4} C_6H_5SO_3H ]
    • Benzene reacts with sulfur trioxide to form benzenesulfonic acid.

4. Friedel-Crafts Alkylation:

  • Electrophile: Carbocation ((R^+)).
  • Reagent: Alkyl halide ((R-X)) + (AlX_3) (Lewis acid catalyst).
  • Example: [ C_6H_6 + CH_3Cl \xrightarrow{AlCl_3} C_6H_5CH_3 + HCl ]
    • Benzene reacts with methyl chloride to form toluene.

5. Friedel-Crafts Acylation:

  • Electrophile: Acylium ion ((RCO^+)).
  • Reagent: Acyl chloride ((RCOCl)) + (AlCl_3) (Lewis acid catalyst).
  • Example: [ C_6H_6 + CH_3COCl \xrightarrow{AlCl_3} C_6H_5COCH_3 + HCl ]
    • Benzene reacts with acetyl chloride to form acetophenone.

Key Features of EAS:

  1. Aromaticity is Preserved:
  • The aromatic ring remains stable throughout the reaction.
  1. Electrophile is Generated:
  • The electrophile is often generated in situ using a catalyst or reagent.
  1. Substituents Affect Reactivity:
  • Electron-donating groups (e.g., (-OH), (-CH_3)) activate the ring and direct substitution to the ortho and para positions.
  • Electron-withdrawing groups (e.g., (-NO_2), (-COOH)) deactivate the ring and direct substitution to the meta position.

Mechanism of EAS (Example: Nitration of Benzene):

  1. Formation of the Electrophile:
    [
    HNO_3 + H_2SO_4 \rightarrow NO_2^+ + HSO_4^- + H_2O
    ]
  • Nitric acid and sulfuric acid generate the nitronium ion ((NO_2^+)).
  1. Attack on the Aromatic Ring:
    [
    C_6H_6 + NO_2^+ \rightarrow C_6H_5NO_2^+
    ]
  • The benzene ring donates π-electrons to the nitronium ion, forming a carbocation intermediate.
  1. Deprotonation:
    [
    C_6H_5NO_2^+ + HSO_4^- \rightarrow C_6H_5NO_2 + H_2SO_4
    ]
  • A base ((HSO_4^-)) removes a proton, restoring aromaticity and forming nitrobenzene.

Summary of EAS Reactions:

ReactionElectrophileReagentProduct
Nitration(NO_2^+)(HNO_3 + H_2SO_4)Nitrobenzene ((C_6H_5NO_2)).
Halogenation(X^+)(X_2 + FeX_3)Halobenzene ((C_6H_5X)).
Sulfonation(SO_3)(H_2SO_4) or (SO_3)Benzenesulfonic acid ((C_6H_5SO_3H)).
Friedel-Crafts Alkylation(R^+)(R-X + AlX_3)Alkylbenzene ((C_6H_5R)).
Friedel-Crafts Acylation(RCO^+)(RCOCl + AlCl_3)Aryl ketone ((C_6H_5COR)).

Practice Problem:

Predict the product of the following EAS reaction:
[
C_6H_6 + CH_3CH_2Cl \xrightarrow{AlCl_3} ?
]

  • Answer: Ethylbenzene ((C_6H_5CH_2CH_3)).

EAS is a fundamental reaction in organic chemistry, widely used to synthesize substituted aromatic compounds. Understanding the mechanism and directing effects of substituents is crucial for predicting reaction outcomes.

Leave a Comment