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16. Chemistry of Benzene: Electrophilic Aromatic Substitution Part 1

16. Chemistry of Benzene: Electrophilic Aromatic Substitution Part 1. Based on McMurry’s Organic Chemistry , 6 th edition, Chapter 16. Benzene is aromatic: a cyclic conjugated compound with 6  electrons. Reactions of benzene retain the aromatic core.

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16. Chemistry of Benzene: Electrophilic Aromatic Substitution Part 1

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  1. 16. Chemistry of Benzene: Electrophilic Aromatic Substitution Part 1 Based on McMurry’s Organic Chemistry, 6th edition, Chapter 16

  2. Benzene is aromatic: a cyclic conjugated compound with 6  electrons. Reactions of benzene retain the aromatic core. Electrophilic aromatic substitution replaces a proton on benzene with another electrophile. Substitution Rxns of Benzene Derivatives Electrophile (E+) is looking for electron density.

  3. Bromination of Double Bonds Alkenes react readily with Br2 to give an addition product: Under these conditions, benzene + Br2 give no reaction: Using a Lewis acid catalyst results in a substitution reaction:

  4. Bromination of Aromatic Rings In reactions with Lewis acids, Benzene’s  electrons participate as a Lewis base (or as a nucleophile). FeBr3 is added as a catalyst to polarize the bromine reagent and make Br2 more electrophilic – forms “Br+” Role of catalyst:

  5. Mechanism of Bromination – 2 Steps STEP ONE:The pi electrons act as a nucleophile, attack Br+ (complexed with FeBr4-).The cationic intermediate formed (not aromatic) is high in energy. Step 1 E+ FeBr3 Benzene - acts as nucleophile Delocalized cationic intermediate3 resonance structures, all ~equal energy (Know how to draw!!)

  6. Formation of Product from Intermediate stable aromatic system Step 1 STEP TWO:The cationic addition intermediate transfers a proton to FeBr4- (from Br- and FeBr3). This restores aromaticity and gives a substitution product (in contrast with addition seen in alkenes). Step 2

  7. Reaction with other Halogens Cl2: Like the bromination reaction, chlorine will react with benzene using FeCl3 as catalyzt to form chlorobenzene. F2: Fluorine is too reactive and cannot be used in standard EAS halogenation reactions. I2: Iodine has very low reactivity, requiring special reagents (Cu+ or peroxide). We will not cover this reaction. skip:

  8. Aromatic Nitration Use nitric acid (HNO3) plus catalytic sulfuric acid to add a nitro group to the aromatic ring. Overall Reaction: With isopropyl, nitro group adds at the para position.

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