Chapter16 - 16. Chemistry of Benzene: Electrophilic...

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16. Chemistry of Benzene: Electrophilic Aromatic Substitution Based on McMurry’s Organic Chemistry , 6 th edition, Chapter 16 ©2003 Ronald Kluger Department of Chemistry University of Toronto
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McMurry Organic Chemistry 6th edition Chap 2 Substitution Reactions of Benzene and Its Derivatives Benzene is aromatic: a cyclic conjugated compound with 6 π electrons Reactions of benzene lead to the retention of the aromatic core Electrophilic aromatic substitution replaces a proton on benzene with another electrophile
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McMurry Organic Chemistry 6th edition Chap 3 16.1 Bromination of Aromatic Rings Benzene’s π electrons participate as a Lewis base in reactions with Lewis acids The product is formed by loss of a proton, which is replaced by bromine FeBr 3 is added as a catalyst to polarize the bromine reagent
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McMurry Organic Chemistry 6th edition Chap 4 Addition Intermediate in Bromination The addition of bromine occurs in two steps In the first step the π electrons act as a nucleophile toward Br 2 (in a complex with FeBr 3 ) This forms a cationic addition intermediate from benzene and a bromine cation The intermediate is not aromatic and therefore high in energy (see Figure 16.2)
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McMurry Organic Chemistry 6th edition Chap 5 Formation of Product from Intermediate The cationic addition intermediate transfers a proton to FeBr 4 - (from Br - and FeBr 3 ) This restores aromaticity (in contrast with addition in alkenes)
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McMurry Organic Chemistry 6th edition Chap 6 16.2 Other Aromatic Substitutions The reaction with bromine involves a mechanism that is similar to many other reactions of benzene with electrophiles The cationic intermediate was first proposed by G. W. Wheland of the University of Chicago and is often called the Wheland intermediate George Willard Wheland 1907-1974
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McMurry Organic Chemistry 6th edition Chap 7 Aromatic Chlorination and Iodination Chlorine and iodine (but not fluorine, which is too reactive) can produce aromatic substitution with the addition of other reagents to promote the reaction Chlorination requires FeCl 3 Iodine must be oxidized to form a more powerful I + species (with Cu + or peroxide)
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McMurry Organic Chemistry 6th edition Chap 8 Aromatic Nitration The combination of nitric acid and sulfuric acid produces NO 2 + (nitronium ion) The reaction with benzene produces nitrobenzene
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McMurry Organic Chemistry 6th edition Chap 9 Aromatic Sulfonation Substitution of H by SO 3 (sulfonation) Reaction with a mixture of sulfuric acid and SO 3 Reactive species is sulfur trioxide or its conjugate acid Reaction occurs via Wheland intermediate and is reversible
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McMurry Organic Chemistry 6th edition Chap 10 Alkali Fusion of Aromatic Sulfonic
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Chapter16 - 16. Chemistry of Benzene: Electrophilic...

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