EAS - and represents a strong thermodynamic driving force....

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Electrophilic Aromatic Substitution : EAS reactions all occur by the same mechanism: (1) an electrophilic species attacks the aromatic ring, (2) a cationic intermediate is formed, and (3) the cation eliminates via an E1 mechanism to reestablish aromaticity. Hydrogen-Deuterium Exchange Note that upon electrophilic attack, aromaticity is destroyed. Highly activated electrophiles are required to destroy aromaticity. Therefore, in this mechanism, electrophilic attack is the rate-limiting step. In the elimination step, aromaticity is regained. Professor M. J. Krische, UT Austin
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Question : Why isn’t in the cationic intermediate in EAS captured in an S N 1 reaction? In principle, anionic species present in the reaction mixture could capture the cation. However, the products would not be aromatic. Remember that benzene has 36 Kcal/mol of extra “resonance stabilization energy.” Thus, “re-aromatization” is highly exothermic
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Unformatted text preview: and represents a strong thermodynamic driving force. Professor M. J. Krische, UT Austin Chlorine itself is not sufficiently electrophilic to react with benzene. In order to enhance chlorines electrophilicity, we must weaken the Cl-Cl bond via complexation of a Lewis acid. The Cl-Cl-FeCl 3 complex is much more electrophilic and will react readily with benzene. EAS : Halogenation of Benzene Professor M. J. Krische, UT Austin EAS : Friedel-Crafts Alkylation of Benzene Same Mechanism : 1) electrophile is activated via Lewis acid complexation, 2) benzene attacks the activated electrophile to yield a cationic intermediate, which 3) re-aromatized via E 1 -type elimination. Professor M. J. Krische, UT Austin EAS : Be sure you are able to write mechanisms to account for the following EAS reactions. Professor M. J. Krische, UT Austin...
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EAS - and represents a strong thermodynamic driving force....

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