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L_15_09 - 318N Krische Lecture 15 Tu Electrophilic Aromatic...

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318N – Krische, Lecture 15: Tu - 03/10/09, Electrophilic Aromatic Substitution We begin by reviewing the EAS reactions covered last lecture: H-D exchange, Sulfonation, Nitration, and Halogenation. We considered the general mechanism of EAS and noted that for each of the reactions that we’ve covered, all we’ve done is simply vary the electrophilic partner. Friedel-Crafts Alkylation of Benzene : Isopropyl bromide does not react with benzene. However, we may activate isopropyl bromide as an electrophile by adding a Lewis acid such as aluminum tribromide (AlBr 3 ). Aluminum tribromide complexes isopropyl bromide and promotes the formation of isopropyl cation. This secondary carbocation represents an “activated electrophile” of sufficient reactivity to participate in EAS with benzene. The product of EAS is isopropylbenzene (a.k.a. “cumene”). For the Friedel-Crafts alkylation of benzene, THE SAME EAS MECHANISM is in operation. Tertiary halides also work well in the Friedel-Crafts alkylation. Thus, exposure of benzene to a mixture of tert-butyl chloride and aluminum chloride gives tert-butyl cation, a tertiary carbocation. This tertiary carbocation represents an
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