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Substitution Reactions of Benzene and Other Aromatic Compounds

The chemical reactivity of benzene contrasts with that of the alkenes in that substitution reactions occur in preference to addition reactions, as illustrated in the following diagram (some comparable reactions of cyclohexene are shown in the green box).

 

 

Many other substitution reactions of benzene have been observed, the five most useful are listed below (chlorination and bromination are the most common halogenation reactions). Since the reagents and conditions employed in these reactions are electrophilic, these reactions are commonly referred to as Electrophilic Aromatic Substitution. The catalysts and co-reagents serve to generate the strong electrophilic species needed to effect the initial step of the substitution. The specific electrophile believed to function in each type of reaction is listed in the right hand column.

 

Reaction Type

Typical Equation

Electrophile   E(+)

Halogenation:

C6H6

+   Cl2 & heat
    FeCl3 catalyst

  ——>

C6H5Cl   +   HCl
Chlorobenzene

Cl(+) or Br(+)

Nitration:

C6H6

+   HNO3 & heat
    H2SO4 catalyst

  ——>

C6H5NO2   +   H2O
Nitrobenzene

NO2(+)

Sulfonation:

C6H6

+   H2SO4 + SO3
    & heat

  ——>

C6H5SO3H   +   H2O
Benzenesulfonic acid

SO3H(+)

Alkylation:
Friedel-Crafts

C6H6

+   R-Cl & heat
    AlCl3 catalyst

  ——>

C6H5-R   +   HCl
An Arene

R(+)

Acylation:
Friedel-Crafts

C6H6

+   RCOCl & heat
    AlCl3 catalyst

  ——>

C6H5COR   +   HCl
An Aryl Ketone

RCO(+)

Contributors

William Reusch, Professor Emeritus (Michigan State U.), Virtual Textbook of Organic Chemistry