pp232_3_ch16 - 16. Chemistry of Benzene: Electrophilic...

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Unformatted text preview: 16. Chemistry of Benzene: Electrophilic Aromatic Substitution Based on McMurrys Organic Chemistry , Chapter 16 Substitution Reactions of Benzene and Its Derivatives Benzene is aromatic: a cyclic conjugated compound with 6 electrons Reactions of benzene lead to the retention of the aromatic core Electrophilic aromatic substitution replaces a proton on benzene with another electrophile 16.1 Bromination of Aromatic Rings Benzenes 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 Addition Intermediate in Bromination The addition of bromine occurs in two steps In the first step the 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) 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) 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) Aromatic Nitration The combination of nitric acid and sulfuric acid produces NO 2 + (nitronium ion) The reaction with benzene produces nitrobenzene 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 Alkali Fusion of Aromatic Sulfonic Acids Sulfonic acids are useful as intermediates Heating with NaOH at 300 C followed by neutralization with acid replaces the SO 3 H group with an OH Example is the synthesis of p-cresol 16.3 Alkylation of Aromatic Rings: The FriedelCrafts Reaction Aromatic substitution of a R + for H Aluminum chloride promotes the formation of the carbocation Wheland intermediate forms Limitations of the Friedel-Crafts...
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pp232_3_ch16 - 16. Chemistry of Benzene: Electrophilic...

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