Modification of substituents on benzenes
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. Two examples of substituent modification are shown below. You will become familiar with even more reactions which modify substituents on benzene rings in your study of oxygen-containing functional groups.
This reaction is referred to as a reduction since the product has fewer O and more H than the reactant. Note that metals (electron source) in the presence of a proton source are used as reductants in other reactions as well.
This reaction is referred to as an oxidation because the product has more O and fewer H than the reactant. Note that the benzene ring is much less susceptible to oxidation than the saturated carbon chain. KMnO4 is a common strong oxidant.