16

Halogenation, Nitration, and Sulfonation

These are three important types of electrophilic aromatic substitution (EAS) reactions, where an electrophile replaces a hydrogen atom on an aromatic ring (e.g., benzene). Below is a detailed explanation of each reaction, including their mechanisms, reagents, and examples.

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1. Halogenation

• Definition:
• Halogenation is the introduction of a halogen atom (e.g., (Cl), (Br)) onto an aromatic ring.
• Electrophile: Halogen cation ((X^+)).
• Reagents:
• Halogen ((X_2), e.g., (Cl_2), (Br_2)).
• Lewis acid catalyst (e.g., (FeCl_3), (FeBr_3)).
• Mechanism:

1. Formation of the Electrophile:
   [
   X_2 + FeX_3 \rightarrow X^+ + FeX_4^-
   ]
   • The Lewis acid ((FeX_3)) generates the halogen cation ((X^+)).
2. Attack on the Aromatic Ring:
   [
   C_6H_6 + X^+ \rightarrow C_6H_5X^+
   ]
   • The benzene ring donates π-electrons to the halogen cation, forming a carbocation intermediate.
3. Deprotonation:
   [
   C_6H_5X^+ + FeX_4^- \rightarrow C_6H_5X + HX + FeX_3
   ]
   • A base ((FeX_4^-)) removes a proton, restoring aromaticity and forming the halogenated product.

• Example:
  [
  C_6H_6 + Br_2 \xrightarrow{FeBr_3} C_6H_5Br + HBr
  ]
• Benzene reacts with bromine to form bromobenzene.

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2. Nitration

• Definition:
• Nitration is the introduction of a nitro group ((-NO_2)) onto an aromatic ring.
• Electrophile: Nitronium ion ((NO_2^+)).
• Reagents:
• Nitric acid ((HNO_3)).
• Sulfuric acid ((H_2SO_4)).
• Mechanism:

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^+)).
2. 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.
3. 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.

• Example:
  [
  C_6H_6 + HNO_3 \xrightarrow{H_2SO_4} C_6H_5NO_2 + H_2O
  ]
• Benzene reacts with nitric acid to form nitrobenzene.

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3. Sulfonation

• Definition:
• Sulfonation is the introduction of a sulfonic acid group ((-SO_3H)) onto an aromatic ring.
• Electrophile: Sulfur trioxide ((SO_3)).
• Reagents:
• Sulfuric acid ((H_2SO_4)) or sulfur trioxide ((SO_3)).
• Mechanism:

1. Formation of the Electrophile:
   [
   H_2SO_4 \rightarrow SO_3 + H_2O
   ]
   • Sulfuric acid generates sulfur trioxide ((SO_3)).
2. Attack on the Aromatic Ring:
   [
   C_6H_6 + SO_3 \rightarrow C_6H_5SO_3^+
   ]
   • The benzene ring donates π-electrons to sulfur trioxide, forming a carbocation intermediate.
3. Deprotonation:
   [
   C_6H_5SO_3^+ + HSO_4^- \rightarrow C_6H_5SO_3H + H_2SO_4
   ]
   • A base ((HSO_4^-)) removes a proton, restoring aromaticity and forming benzenesulfonic acid.

• Example:
  [
  C_6H_6 + SO_3 \xrightarrow{H_2SO_4} C_6H_5SO_3H
  ]
• Benzene reacts with sulfur trioxide to form benzenesulfonic acid.

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Summary of Halogenation, Nitration, and Sulfonation:

Reaction	Electrophile	Reagents	Product
Halogenation	(X^+)	(X_2 + FeX_3)	Halobenzene ((C_6H_5X)).
Nitration	(NO_2^+)	(HNO_3 + H_2SO_4)	Nitrobenzene ((C_6H_5NO_2)).
Sulfonation	(SO_3)	(H_2SO_4) or (SO_3)	Benzenesulfonic acid ((C_6H_5SO_3H)).

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Practice Problems:

1. Halogenation:

• Predict the product of the reaction: [ C_6H_6 + Cl_2 \xrightarrow{FeCl_3} ? ]
  • Answer: Chlorobenzene ((C_6H_5Cl)).

1. Nitration:

• Predict the product of the reaction: [ C_6H_6 + HNO_3 \xrightarrow{H_2SO_4} ? ]
  • Answer: Nitrobenzene ((C_6H_5NO_2)).

1. Sulfonation:

• Predict the product of the reaction: [ C_6H_6 + SO_3 \xrightarrow{H_2SO_4} ? ]
  • Answer: Benzenesulfonic acid ((C_6H_5SO_3H)).

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These reactions are fundamental in organic chemistry for introducing functional groups onto aromatic rings, enabling the synthesis of a wide range of compounds.