ch15 - Chapter 15 Reactions of Aromatic Compounds...

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Chapter 15 Reactions of Aromatic Compounds
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Chapter 15 2 Electrophilic Aromatic Substitution Arene (Ar-H) is the generic term for an aromatic hydrocarbon The aryl group (Ar) is derived by removal of a hydrogen atom from an arene Aromatic compounds undergo electrophilic aromatic substitution (EAS) The electrophile has a full or partial positive charge
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Chapter 15 3 A General Mechanism for Electrophilic Aromatic Substitution: Arenium Ion Intermediates Benzene reacts with an electrophile using two of its π electrons This first step is like an addition to an ordinary double bond Unlike an addition reaction, the benzene ring reacts further so that it may regenerate the very stable aromatic system In step 1 of the mechanism, the electrophile reacts with two π electrons from the aromatic ring to form an arenium ion The arenium ion is stabilized by resonance which delocalizes the charge In step 2, a proton is removed and the aromatic system is regenerated
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Chapter 15 4 The energy diagram of this reaction shows that the first step is highly endothermic and has a large G (1) The first step requires the loss of aromaticity of the very stable benzene ring, which is highly unfavorable The first step is rate-determining The second step is highly exothermic and has a small G (2) The ring regains its aromatic stabilization, which is a highly favorable process
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Chapter 15 5 Halogenation of Benzene Halogenation of benzene requires the presence of a Lewis acid Fluorination occurs so rapidly it is hard to stop at monofluorination of the ring A special apparatus is used to perform this reaction Iodine is so unreactive that an alternative method must be used
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Chapter 15 6 In the step 1 of the mechanism, bromine reacts with ferric bromide to generate an electrophilic bromine species In step 2, the highly electrophilic bromine reacts with π electrons of the benzene ring, forming an arenium ion In step 3, a proton is removed from the arenium ion and aromaticity is regenerated The FeBr 3 catalyst is regenerated
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Chapter 15 7 Nitration of Benzene Nitration of benzene occurs with a mixture of concentrated nitric and sulfuric acids The electrophile for the reaction is the nitronium ion (NO 2 + )
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Chapter 15 8 Sulfonation of Benzene Sulfonation occurs most rapidly using fuming sulfuric acid (concentrated sulfuric acid that contains SO 3 ) The reaction also occurs in conc. sulfuric acid, which generates small quantities of SO 3 , as shown in step 1 below
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Chapter 15 9 Sulfonation is an equilibrium reaction; all steps involved are equilibria The sulfonation product is favored by use of concentrated or fuming sulfuric acid Desulfonation can be accomplished using dilute sulfuric acid ( i.e . with a high concentration of water), or by passing steam through the reaction and collecting the volatile desulfonated compound as it distils with the steam
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ch15 - Chapter 15 Reactions of Aromatic Compounds...

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