Electrophilic substitution

In nitration, a hydrogen atom in the benzene ring is replaced by a nitro, -NO₂, group in an electrophilic substitution reaction.

The reaction requires a mixture of concentrated nitric and sulfuric acids and a temperature of 25-60°C. Sulfuric acid acts as a catalyst.

For example, nitrobenzene is produced from the nitration of benzene using HNO₃ and H₂SO₄:

C₆H₆ + HNO₃ ➔ C₆H₅NO₂ + H₂O

In halogenation, a hydrogen atom in the benzene ring is replaced by a halogen atom in an electrophilic substitution reaction.

The reaction requires a halogen (e.g. Br₂ or Cl₂) and and a halogen carrier catalyst (e.g. AlBr₃ or AlCl₃).

For example, chlorobenzene is produced from the halogenation of benzene using chlorine and AlCl₃:

C₆H₆ + Cl₂ ➔ C₆H₅Cl + HCl

A halogen carrier is a catalyst that facilitates the halogenation of arenes by reacting with the halogen to generate a more reactive electrophilic species in situ.

Examples of halogen carriers include AlCl₃, FeCl₃, AlBr₃ and FeBr₃.

In Friedel-Crafts alkylation, a hydrogen atom in the benzene ring is replaced by an alkyl group in an electrophilic substitution reaction.

The reaction requires a halogenoalkane, heat and an AlCl₃ catalyst.

For example, ethylbenzene is produced from the alkylation of benzene using chloroethane:

C₆H₆ + C₂H₅Cl ➔ C₆H₅C₂H₅ + HCl

In Friedel-Crafts acylation, a hydrogen atom in the benzene ring is replaced by an acyl group in an electrophilic substitution reaction.

The reaction requires an acyl chloride, heat and an AlCl₃ catalyst.

For example, phenylethanone is produced from the acylation of benzene using ethanoyl chloride:

C₆H₆ + CH₃COCl ➔ C₆H₅COCH₃ + HCl

Friedel-Crafts reactions are crucial in organic synthesis for carbon-carbon bond formation. These reactions allow for the introduction of alkyl or acyl groups onto aromatic rings, thereby increasing the number of carbon atoms in the compound and forming new C-C bonds.

1The nitronium ion, NO₂⁺, is generated: 

HNO₃ + H₂SO₄ ➔ NO₂⁺ + HSO₄^- + H₂O

2Benzene undergoes electrophilic substitution with the electrophile NO₂⁺:

3The catalyst, H₂SO₄ is regenerated:                                                          

H⁺ + HSO₄^- ➔ H₂SO₄

1The carbocation, RCO⁺, is generated: 

RCOCl + AlCl₃ ➔ RCO⁺ + AlCl₄^-

2Benzene undergoes electrophilic substitution with the electrophile RCO⁺:

3The catalyst, AlCl₃ is regenerated:                                                            

H⁺ + AlCl₄^- ➔ AlCl₃ + HCl

1The bromonium ion, Br⁺, is generated: 

Br₂ + AlBr₃ ➔ Br⁺ + AlBr₄^-

2Benzene undergoes electrophilic substitution with the electrophile Br⁺:

3The catalyst, AlBr₃ is regenerated:                                                        

H⁺ + AlBr₄^- ➔ AlBr₃ + HBr

1The carbocation, RCH₂⁺, is generated: 

RCH₂Cl + AlCl₃ ➔ RCH₂⁺ + AlCl₄^-

2Benzene undergoes electrophilic substitution with the electrophile RCH₂⁺:

3The catalyst, AlCl₃ is regenerated:                                                            

H⁺ + AlCl₄^- ➔ AlCl₃ + HCl