Reactions of Arenes

This lesson covers: 

1. The mechanism of electrophilic substitution in arenes
2. How halogen carriers produce reactive electrophiles
3. Nitration of benzene
4. 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 CH₃COCl, generating the more electrophilic CH₃CO⁺ species:

AlCl₃ + CH₃COCl ➔ CH₃CO⁺ + AlCl₄^-

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.

Nitration reactions are useful

1. Nitro compounds like nitrobenzene can be catalytically reduced to form aromatic amines. These are key intermediates used to manufacture dyes, drugs and polymers.
2. Some nitrated compounds are useful explosives, such as 2,4,6-trinitrotoluene (TNT). The three nitro groups make TNT very unstable and easily detonated.

Friedel-Crafts acylation forms C-C bonds

Friedel-Crafts acylation is a useful reaction for forming C-C bonds in organic synthesis. It is carried out by refluxing benzene with an acyl chloride and a halogen carrier, such as anhydrous aluminium chloride (AlCl₃).

In this reaction, an acyl group (-COR) is substituted for a hydrogen atom on the benzene ring. The products of Friedel-Crafts acylation are phenylketones (if the acyl chloride contains an alkyl group) or benzaldehyde (if the acyl chloride is formyl chloride, HCOCl).

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