17

Friedel-Crafts Alkylation and Acylation

Friedel-Crafts reactions are a class of electrophilic aromatic substitution (EAS) reactions that introduce alkyl or acyl groups onto an aromatic ring. These reactions are named after the chemists Charles Friedel and James Crafts, who developed them in the late 19th century. Below is a detailed explanation of both Friedel-Crafts alkylation and acylation, including their mechanisms, reagents, and examples.

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1. Friedel-Crafts Alkylation

• Definition:
• Friedel-Crafts alkylation introduces an alkyl group ((R)) onto an aromatic ring.
• Electrophile: Carbocation ((R^+)).
• Reagents:
• Alkyl halide ((R-X), e.g., (CH_3Cl), (C_2H_5Br)).
• Lewis acid catalyst (e.g., (AlCl_3), (FeCl_3)).
• Mechanism:

1. Formation of the Electrophile:
   [
   R-X + AlCl_3 \rightarrow R^+ + AlCl_4^-
   ]
   • The Lewis acid ((AlCl_3)) generates the carbocation ((R^+)).
2. Attack on the Aromatic Ring:
   [
   C_6H_6 + R^+ \rightarrow C_6H_5R^+
   ]
   • The benzene ring donates π-electrons to the carbocation, forming a carbocation intermediate.
3. Deprotonation:
   [
   C_6H_5R^+ + AlCl_4^- \rightarrow C_6H_5R + HCl + AlCl_3
   ]
   • A base ((AlCl_4^-)) removes a proton, restoring aromaticity and forming the alkylated product.

• Example:
  [
  C_6H_6 + CH_3Cl \xrightarrow{AlCl_3} C_6H_5CH_3 + HCl
  ]
• Benzene reacts with methyl chloride to form toluene.

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2. Friedel-Crafts Acylation

• Definition:
• Friedel-Crafts acylation introduces an acyl group ((RCO)) onto an aromatic ring.
• Electrophile: Acylium ion ((RCO^+)).
• Reagents:
• Acyl chloride ((RCOCl), e.g., (CH_3COCl), (C_6H_5COCl)).
• Lewis acid catalyst (e.g., (AlCl_3), (FeCl_3)).
• Mechanism:

1. Formation of the Electrophile:
   [
   RCOCl + AlCl_3 \rightarrow RCO^+ + AlCl_4^-
   ]
   • The Lewis acid ((AlCl_3)) generates the acylium ion ((RCO^+)).
2. Attack on the Aromatic Ring:
   [
   C_6H_6 + RCO^+ \rightarrow C_6H_5COR^+
   ]
   • The benzene ring donates π-electrons to the acylium ion, forming a carbocation intermediate.
3. Deprotonation:
   [
   C_6H_5COR^+ + AlCl_4^- \rightarrow C_6H_5COR + HCl + AlCl_3
   ]
   • A base ((AlCl_4^-)) removes a proton, restoring aromaticity and forming the acylated product.

• Example:
  [
  C_6H_6 + CH_3COCl \xrightarrow{AlCl_3} C_6H_5COCH_3 + HCl
  ]
• Benzene reacts with acetyl chloride to form acetophenone.

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Key Differences Between Alkylation and Acylation:

Feature	Friedel-Crafts Alkylation	Friedel-Crafts Acylation
Electrophile	Carbocation ((R^+)).	Acylium ion ((RCO^+)).
Reagents	Alkyl halide ((R-X)).	Acyl chloride ((RCOCl)).
Product	Alkylbenzene ((C_6H_5R)).	Aryl ketone ((C_6H_5COR)).
Rearrangement	Possible (due to carbocation stability).	No rearrangement (acylium ion is stable).
Limitations	Polyalkylation can occur.	No polyacylation (ketone deactivates the ring).

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Important Points:

1. Friedel-Crafts Alkylation:

• Advantages:
  • Simple and effective for introducing alkyl groups.
• Limitations:
  • Carbocations can rearrange, leading to unexpected products.
  • Polyalkylation can occur (multiple alkyl groups added).

1. Friedel-Crafts Acylation:

• Advantages:
  • No rearrangement of the acylium ion.
  • Ketone products are less reactive, preventing polyacylation.
• Limitations:
  • Requires an additional step (Clemmensen or Wolff-Kishner reduction) to convert the ketone to an alkyl group.

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Practice Problems:

1. Friedel-Crafts Alkylation:

• Predict the product of the reaction: [ C_6H_6 + CH_3CH_2Cl \xrightarrow{AlCl_3} ? ]
  • Answer: Ethylbenzene ((C_6H_5CH_2CH_3)).

1. Friedel-Crafts Acylation:

• Predict the product of the reaction: [ C_6H_6 + C_6H_5COCl \xrightarrow{AlCl_3} ? ]
  • Answer: Benzophenone ((C_6H_5COC_6H_5)).

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Summary:

• Friedel-Crafts Alkylation: Introduces alkyl groups ((R)) onto aromatic rings.
• Friedel-Crafts Acylation: Introduces acyl groups ((RCO)) onto aromatic rings.
• Both reactions are essential for synthesizing substituted aromatic compounds in organic chemistry.

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2:- Hyperconjugation bi

3:-Wurtz reaction
Wurtz fitting reaction
Cory-house synthesis

4:- Elimination reaction all 3 types

5:-SN1 , SN2 reaction

6:-Nucleophile definition, examples and Nucleophilicity, Electrophile definition, examples and electrophilicity

6:-R/S nomenclature

7:-E/Z
9:-CIp rule

10:- Seytzeff and Hoffman products

11:- Markownikof rule , Antimarkownikof rule (Kharash effect)

12:-What is Chiral and chirality with examples

13:- Enantiomers, Diastereomer and meso compounds

14:-What do you mean by Huckle rule of aromaticity

15:- Electrophilic Aromatic Substitution definition with examples

16:- Holognation, Nitration, Sulphonation

17:- Fridel-Craft alkylation And Acylation Important