Electrophilic Substitution Reactions in Benzene

This lesson covers: 

1. The mechanism of electrophilic substitution in arenes
2. How halogen carriers produce reactive electrophiles
3. Halogenation of benzene
4. Nitration of benzene
5. Friedel-Crafts alkylation reactions to form C-C bonds
6. Friedel-Crafts acylation reactions to form C-C bonds

Benzene undergoes electrophilic substitution

Electrophilic substitution reactions of benzene involve a hydrogen atom being replaced by an electrophile.

The mechanism unfolds in two stages:

1. Addition of the electrophile (E⁺) to the benzene ring - This breaks the aromaticity and forms a positively charged intermediate.
2. Loss of H⁺ - This step reforms the aromatic benzene ring with the electrophile substituted in place of hydrogen.

Halogen carriers help make good electrophiles

An electrophile has to have a pretty strong positive charge to be able to attack the stable benzene ring. Most compounds just aren't polarised enough — but some can be made into stronger electrophiles using a catalyst called a halogen carrier.

A halogen carrier accepts a lone pair of electrons from a halogen atom on an electrophile. As the lone pair of electrons is pulled away, the polarisation in the molecule increases and sometimes a carbocation forms. This makes the electrophile stronger.

Halogen carriers include aluminium halides, iron halides and iron.

For example, AlCl₃ can accept a lone pair from chlorine in Cl₂, generating the more electrophilic Cl⁺ species:

AlCl₃ + Cl₂ ➔ Cl⁺ + AlCl₄^-

Halogenation of benzene

Halogenation refers to the electrophilic substitution of a halogen atom onto a benzene ring in place of hydrogen. It can be carried out by reacting benzene with halogen molecules in the presence of halogen carrier catalysts like aluminium chloride (AlCl₃) or iron chloride (FeCl₃).

The chlorination of benzene using Cl₂ and an AlCl₃ catalyst occurs as:

1. Chlorine reacts with the aluminium chloride catalyst to form the electrophilic chloronium (Cl⁺) ion:

Cl₂ + AlCl₃ ➔ Cl⁺ + AlCl₄^-

1. Benzene undergoes electrophilic substitution with the electrophile Cl⁺, displacing a proton and forming chlorobenzene:

3.   The displaced proton reacts with AlCl4- to regenerate the AlCl₃ catalyst:

H⁺ + AlCl₄^- ➔ AlCl₃ + HCl

A similar process occurs for the bromination of benzene using bromine and catalysts like AlBr₃ or FeBr₃.

Nitration of benzene

Nitration refers to the electrophilic substitution of a nitro group (-NO₂) onto a benzene ring in place of hydrogen. It can be carried out by reacting benzene with concentrated nitric acid in the presence of concentrated sulfuric acid catalyst a temperature of 60°C or less.

The nitration of benzene using HNO₃ and H₂SO₄ occurs as:

1. Nitric acid reacts with the sulfuric acid catalyst to generate the electrophilic nitronium (NO₂⁺) ion:

HNO₃ + H₂SO₄ ➔ NO₂⁺ + HSO₄^- + H₂O

1. Benzene undergoes electrophilic substitution with the electrophile NO₂⁺, displacing a proton and forming nitrobenzene:

3.   The displaced proton reacts with HSO₄^- to regenerate the H₂SO₄ catalyst:

H⁺ + HSO₄^- ➔ H₂SO₄

Controlling nitration

Cooling the nitration reaction to less than 60°C ensures that only mononitration occurs, producing nitrobenzene as the major product.

Friedel-Crafts reactions form C-C bonds

Friedel-Crafts reactions are really useful for forming C–C bonds in organic synthesis. They are carried out by refluxing benzene with a halogen carrier and either a halogenoalkane or an acyl chloride.

There are two types:

1. Friedel-Crafts alkylation puts any alkyl group onto a benzene ring using a haloalkane and a halogen carrier.
2. Friedel-Crafts acylation substitutes an acyl group for an H atom on benzene using an acyl chloride and halogen carrier. This produces phenylketones or benzaldehyde.

Friedel-Crafts alkyation mechanism

The alkylation of benzene using chloromethane and AlCl₃ proceeds via:

1. Formation of the electrophilic methyl carbocation (CH₃⁺) intermediate:

CH₃Cl + AlCl₃ ➔ CH₃⁺ + AlCl₄^-

1. Electrophilic substitution occurs on benzene with CH₃⁺, displacing H⁺ and forming methylbenzene:

3.   The proton reacts with AlCl₄^-, regenerating the AlCl₃ catalyst:

H⁺ + AlCl₄^- ➔ AlCl₃ + HCl

Friedel-Crafts acylation mechanism

The acylation of benzene using ethanoyl chloride and AlCl₃ proceeds via:

1. Formation of the electrophilic acetyl cation (CH₃CO⁺) intermediate:

CH₃COCl + AlCl₃ ➔ CH₃CO⁺ + AlCl₄^-

1. Electrophilic substitution occurs on benzene with CH₃CO⁺, displacing H⁺ and forming phenylethanone:

3.   The proton reacts with AlCl₄^-, regenerating the AlCl₃ catalyst:

H⁺ + AlCl₄^- ➔ AlCl₃ + HCl