Ch_322b_15.01

Ch_322b_15.01 - Chapter 15.1 lecture note

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Chapter 15 Reactions of Aromatic Compounds Aromatic compounds are often called arenes . The aryl group used to symbolize the aromatic ring is Ar , so arene compounds are ArH and substituted arenes are ArX . Ar- ArH ArX aryl group generic arene substituted arene
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Electrophilic Aromatic Substitution This general reaction of arenes may be used to introduce a variety of substitutents into the structure: Ar-H + E + Ar- E + H + electrophile For the case of benzene, + E + E + H + electrophile Note: In these reactions, an electrophile substitutes for an H in the aromatic ring. This substitution reaction contrasts with the addition of an electrophilic reagent to an alkene. In the former case, the very stable aromatic ! -system is retained . In the latter, the weaker ! -bond is replaced by stronger " -bonds .
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Some Examples of Electrophilic Aromatic Substitution Reactions X 2 , FeX 3 X = Cl, Br X Halogenation HONO 2 H 2 SO 4 NO 2 Nitration SO 3 H 2 SO 4 SO 3 H Sulfonation RCl AlCl 3 R Friedel-Crafts Alkylation RCCl O = AlCl 3 R O C Friedel-Crafts Acylation These reactions are commonly used synthetic procedures for modifying the structures of arenes. They proceed by a general mechanism initiated by addition of an electrophile, E + on the aromatic ! -system forming a nonaromatic carbocation intermediate called an arenium ion .
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A General Two-Step Mechanism The aromatic ! -electrons are found above and below the plane of the
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Ch_322b_15.01 - Chapter 15.1 lecture note

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