Aromatic Substitution Part I Attack of the Electrophiles

In this chapter we focus on the substitution attack on an aromatic molecule by an electrophile. Throughout the chapter we show you how these electrophilic substitutions occur, first by using benzene as an example. Once you become familiar with electrophilic substitution on benzene, you re ready to see what happens when substituted aromatic molecules replace the benzene molecule. 

Don t try to memorize the mechanisms in this chapter as separate unrelated entities. These mechanisms are closely related and fit together quite nicely. Look for the relationships concentrate on understanding how and why the reactions occur as they do and avoid simple memorization. 

In this section we use benzene as a typical aromatic compound to study three basic reactions halogenation, sulfonation, and nitration. In the case of halogenations, the electrophile is the X ion (X = Cl or Br). In sulfonation and nitration, the electrophiles are SO3 and NO2, respectively. In each case, part of the mechanism involves the generation of the electrophile. 

Don t try to memorize a nitration mechanism as a sepcirate mechanism from halogenations or sulfonation. Let your understanding of one mechanism reinforce your understanding of other mechanisms. 

The reaction works equally well with AICI3, AlBrj, FeClj, FeBrj, and a number of other Lewis bases. Some catalysts can also be generated through reactions like 2 FeBr2(s) + Br2G) 2 FeBr3(s). 

AM romatic systems are pretty stable they resist reacting. Nevertheless, W a number of reactions involving aromatic systems can be carried out. However, with the exception of combustion, the conditions required by the anti-aromatic systems for reactions that you studied in your first semester organic course are different than the conditions for aromatic systems. 

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Chapter 7 Aromatic Substitution Part I Attack of the Electrophiles 95... 

In pyrrole (C4H4NH), the lone pair of electrons on nitrogen is part of the aromatic (six Jt-electron) ring system. The incorporation of a lone pair of electrons activates the ring, and electrophilic substitution is faster than that for benzene. Substitution occurs principally at the 2-position, as attack at the 3-position produces a less stable cation intermediate (i.e. with two, rather than three, resonance structures). 

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