Chapter 16 lecture note

Chapter 16 lecture note - Chemistry 0320 Spring 2008 Ch 16...

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Chemistry 0320 — Spring 2008 Ch 16 — Electrophilic Attack on Derivatives of Benzene page 1 Chapter 16: Electrophilic Attack on Derivatives of Benzene Substituents control regioselectivity. 16-1 Activation or Deactivation by Substituents on a Benzene Ring The identity of the substituent on a monsubstituted benzene affects the reactivity and regioselectivity of a subsequent electrophilic substitution reaction. Substituents on benzene can be grouped into: Activators : Electron donors which generally direct a second electrophilic attack to the ortho and para positions; Deactivators : Electron acceptors which generally direct a second electrophilic attack to the meta positions. The electronic influence of any substituent is determined by two factors, inductance and resonance . Inductance occurs through the σ framework, tapers off rapidly with distance and is governed mostly by the relative electronegativity of the atoms. Resonance takes place through π bonds, is longer range and is particularly strong in charged systems.
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Chemistry 0320 — Spring 2008 Ch 16 — Electrophilic Attack on Derivatives of Benzene page 2 Resonating effects of some substituents on the benzene ring through π frame: Inductance and resonance oppose each other for –NR 2 , -OR, X (halogen). The effect that wins out depends upon the relative electronegativity of the heteroatoms, and the ability of their respective p-orbitals to overlap the π system. -NR 2 , -OR: resonance>inductance X: resonance<inductance Groups such as carbonyl, cyano, nitro and sulfonyl are all electron withdrawing through resonance. They all contain a polarized double or triple bond whose partially positive end is attached to the benzene nucleus.
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Chemistry 0320 — Spring 2008 Ch 16 — Electrophilic Attack on Derivatives of Benzene page 3 Nucleophilicity: NR 2 OR R X B A , > > > > activating groups deactivating groups For example: X HNO 3 , H 2 SO 4 X NO 2 16-2 Directing Inductive Effects of Alkyl Groups Groups that donate electrons by induction activate and direct ortho and para. Nitration, sulfonation and Friedel-Crafts reactions of methylbenzene all give similar results: mainly ortho and para substitutions. The regioselectivity depends upon the nature of the substituent, not on the reagent. The methyl substituent is said to be activating and ortho- and para-directing . Ortho and para isomers are often not obtained in equal amounts, primarily due to steric effects. Para products often predominate over their ortho isomers.
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Chemistry 0320 — Spring 2008 Ch 16 — Electrophilic Attack on Derivatives of Benzene page 4 The intermediates for addition at the ortho, meta and para positions account for the differences in regioselectivity: Ortho attack (E + = electrophile) Para attack Meta attack
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Chemistry 0320 — Spring 2008 Ch 16 — Electrophilic Attack on Derivatives of Benzene page 5 Groups that withdraw electrons inductively are deactivating and meta-directing. The trifluoromethyl group is electron-withdrawing due to its strongly electronegative fluorine atoms.
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Chapter 16 lecture note - Chemistry 0320 Spring 2008 Ch 16...

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