Skip to Main Content

Organic Chemistry Text Book (CHEM 3401 and 3402)

18.1 Electrophilic Aromatic Substitution Reactions

The remarkable stability of the unsaturated hydrocarbon benzene has been discussed in an earlier section. 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). 


A demonstration of bromine substitution and addition reactions is helpful at this point, and a virtual demonstration may be initiated by clicking here.


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
Cl(+) or Br(+)
Nitration: C6H6 +   HNO3 & heat
    H2SO4 catalyst
  ——>   C6H5NO2   +   H2O
Sulfonation: C6H6 +   H2SO4 + SO3
    & heat
  ——>   C6H5SO3H   +   H2O
Benzenesulfonic acid
C6H6 +   R-Cl & heat
    AlCl3 catalyst
  ——>   C6H5-R   +   HCl
An Arene
C6H6 +   RCOCl & heat
    AlCl3 catalyst
  ——>   C6H5COR   +   HCl
An Aryl Ketone