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16.0 Introduction

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  • Objective

    After completing this section, you should be able to identify electrophilic substitution as the single most important reaction of aromatic compounds.

    Key Terms

    Make certain that you can define, and use in context, the key terms below.

    • acylation
    • alkylation
    • electrophilic substitution
    • halogenation
    • hydroxylation
    • nitration
    • sulfonation

    Study Notes

    In this chapter, you will study all of the reactions shown in the Reaction Type table. In addition to these five reaction types, we also add a sixth common electrophilic substitution known as hydroxylation.

    benzene converts to phenol

    It is important that you recognize the similarities between these reactions to minimize the amount you must memorize.

    The six pi electrons obey Huckel's rule so benzene is especially stable.  This means that the aromatic ring want to be retained during reactions.  Because of this benzene does not undergo addition like other unsaturated hydrocarbons. 

    Benzene can undergo electrophilic aromatic substitution because aromaticity is maintained.


    Other Examples of Electophilic Aromatic Substitution

    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