Electrophilic Substitution at a Trigonal Planar Center

The most common example of electrophilic substitution at a trigonal planar center is electrophilic aromatic substitution, which will serve as our archetype. Aromaticity was covered in Section 1.9, and the reactivity trends of aromatics will be covered in detail in Chapter 5. Chapter 8 has additional reaction examples. Aromatic rings are usually poor nucleophiles therefore excellent electrophiles are needed for the reaction to proceed. 

Groups present on the ring before the electrophile attacks dictate not only the reactivity of the ring but also the position of attack. Any group that has a resonance 

When two donors are meta to each other, attack at the position between them is extremely difficult because of steric hindrance. 

Often the most difficult part of electrophilic aromatic substitution mechanisms is working out the generation of the reactive electrophile. The first task is always to map changes on a balanced reaction. The medium is almost always acidic because reactive electrophiles are present. The electrophilic addition to the aromatic ring is just a two-step process, Ag then Dg (usually a proton). Make sure you draw the arrows correcdy, keep track of charge balance, and use the known electron flow paths. 

This is the synthesis of a somewhat controversial food preservative called BHT. It is an antioxidant that has been put in packaging to keep foods from becoming rancid from air oxidation. A balanced reaction would require two molecules of the alkene for each of the starting aromatic. The aromatic ring is relatively electron rich with two donors attached to it. With sulfuric acid present, the medium is definitely acidic. The first step is to generate the excellent electrophile needed for electrophilic aromatic substitution. For simplicity, let s symbolize sulfuric acid as H-A. The Markovnikov addition of a proton to isobutylene gives the tert-butyl carbocation, an excellent electrophile. 

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This reaction was introduced in Section 4.6, Electrophilic Substitution at a Trigonal Planar Center. The electrophile adds to the pi bond of the aromatic ring, path Ag, followed by deprotonation of the cation formed, path Dg, restoring aromatic stabilization. See Section 5.6 Aromatic Rings, for a discussion of electrophilic aromatic substitution on heteroaromatics like pyridine and on condensed aromatics like naphthalene. 

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