Ch_322b_15.11 - Chapter 15.11 lecture note

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The Orientation of Electrophilic Aromatic Substitution Meta Substitution: When S is Electron-Withdrawing Meta substitution arises with substituents that are electron-withdrawing . When an electrophile adds meta to the substituent, the formed arenium ion has the most positive charge at positions ortho and para to the carbon of attachment. This distribution avoids placing additional positive charge on a ring carbon already electropositive because of S . CF 3 E + meta addition CF 3 + H E arenium ion CF 3 H E CF 3 H E + + CF 3 H E + CF 3 E + para addition CF 3 + H E arenium ion CF 3 H E + CF 3 H E + CF 3 H E + additional positive charge on electropositive carbon
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Ortho addition also leads to an arenium ion with additional positive charge on the electropositive carbon bonded to the trifluoromethyl group. CF 3 E + ortho addition CF 3 + H E arenium ion Based on the resonance structure descriptions of the distributions of the positive charge among the carbons centers, the stabilities of the arenium ions are: CF 3 + H E CF 3 + H E CF 3 + H E more stable less stable
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Energetics of the Competitive Reactions for
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