Some of the substituent groups common on benzene cannot be introduced directly by electrophilic substitution. However, many groups can be introduced by modifying a substituent already present on a benzene ring. Three examples of substituent modification are shown below.
Reduction of a nitro group
This reaction is referred to as a reduction because the product has fewer O and more H than the reactant.
Metals (electron source) in the presence of a proton source are commonly used as reductants
Oxidation of alkyl groups on benzene rings
KMnO4 is a common strong oxidant.
The product has more O and fewer H than the reactant.
Note that the benzene ring is much less susceptible to oxidation than the alkyl substituent.
Chlorination of alkyl groups on benzene rings
Reaction with chlorine in the presence of UV light results in substitution of a hydrogen on the methyl group. This reaction occurs by the same radical mechanism as the reaction of halogens with alkanes. This contrasts with the reaction of methylbenzene with Cl2 in the presence of FeCl3 which results in substitution of one of the hydrogens on the aromatic ring.
Reduction of a nitro group
 | Fe/H3O+  |  |
This reaction is referred to as a reduction because the product has fewer O and more H than the reactant.
Metals (electron source) in the presence of a proton source are commonly used as reductants
Oxidation of alkyl groups on benzene rings
 | KMnO4/heat |  |
KMnO4 is a common strong oxidant.
The product has more O and fewer H than the reactant.
Note that the benzene ring is much less susceptible to oxidation than the alkyl substituent.
Chlorination of alkyl groups on benzene rings
| Cl2 UV light |  |
Reaction with chlorine in the presence of UV light results in substitution of a hydrogen on the methyl group. This reaction occurs by the same radical mechanism as the reaction of halogens with alkanes. This contrasts with the reaction of methylbenzene with Cl2 in the presence of FeCl3 which results in substitution of one of the hydrogens on the aromatic ring.