lecture 22 - Reactions of Benzene and its Derivatives...

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Reactions of Reactions of Benzene and its Benzene and its Derivatives Derivatives Chapter 22 p. 840 2,6-di-tert-butyl-4-methylphenol (butylated hydroxytoluene, BHT) used as a preservative Reactions of Benzene ± The most characteristic reaction of aromatic compounds is substitution at a ring carbon Reactions of Benzene Reactions of Benzene Reactions of Benzene Electrophilic Electrophilic Aromatic Substitution Aromatic Substitution ± Electrophilic Electrophilic aromatic substitution: aromatic substitution: a reaction in which a hydrogen atom of an aromatic ring is replaced by an electrophile
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Electrophilic Electrophilic Aromatic Substitution Aromatic Substitution ± We study several common types of electrophiles how each is generated the mechanism by which each replaces hydrogen EAS: General Mechanism EAS: General Mechanism ± A general mechanism ± General question: what is the electrophile and how is it generated? + E + H E H + slow, rate determining Step 1: Step 2: E H + fast + H + E Electro- phile Resonance-stabilized cation intermediate Chlorination and Bromination Bromination Step 1: formation of a chloronium or bromonium ion (very strong electrophiles ). Use a Lewis acid catalyst (FeCl 3 or AlCl 3 ) Chlorination Chlorination Step 1: formation of a chloronium ion (a very strong electrophile ). Cl Fe Fe FeCl 4 - + - A molecular complex with a positive charge on chlorine Ferric chloride (a Lewis acid) Chlorine (a Lewis base) + + An ion pair containing a chloronium ion Chlorination Step 2: attack of the chloronium ion on the ring + + + Resonance-stabilized cation intermediate; the positive charge is delocalized onto three atoms of the ring + slow, rate H H H Chlorination Chlorination Step 3: proton transfer regenerates the aromatic character of the ring H Cl-FeCl 3 HCl 3 Chlorobenzene fast Cation intermediate + + + - + Note: The regeneration of the Lewis acid catalyst
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Nitration (formation of the Nitration (formation of the electrophile electrophile ) ± Generation of the nitronium ion, NO 2 + Step 1: proton transfer to nitric acid Nitration Nitration ± Generation of the nitronium ion, NO 2 + Step 2: loss of H 2 O gives the nitronium ion, a very strong electrophile Nitration of Benzene Step 1: attack of the nitronium ion (an electrophile) on the aromatic ring (a nucleophile) Nitration of Benzene Nitration of Benzene Step 2: proton transfer regenerates the aromatic ring Nitration ± A particular value of nitration is that the nitro group can be reduced to a 1° amino group under mild conditions Note: The –COOH group is not reduced under these conditions A nitro group can also be reduced to a primary amino group by a metal (Fe, Zn, or Sn) in (aq) acid (HCl). The amine is obtained as a salt then treated with a strong base
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Sulfonation Sulfonation ± Carried out using concentrated sulfuric acid containing dissolved sulfur trioxide (fuming sulfuric acid) ± The electrophile is either SO 3 or HSO 3 + Sulfonation Sulfonation Friedel -Crafts Crafts Alkylation Alkylation ± Friedel-Crafts alkylation forms a new C-C bond
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This note was uploaded on 09/06/2009 for the course CHEM chem 12 AB taught by Professor Valentin during the Spring '09 term at Evergreen Valley.

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lecture 22 - Reactions of Benzene and its Derivatives...

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