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

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

Substitution Rxns of Benzene Derivatives 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. Electrophile (E + ) is looking for electron density.

Bromination of Double Bonds Alkenes react readily with Br 2 to give an addition product: Under these conditions, benzene + Br 2 give no reaction: Using a Lewis acid catalyst results in a substitution reaction:

Bromination of Aromatic Rings In reactions with Lewis acids, Benzene’s  electrons participate as a Lewis base (or as a nucleophile). FeBr 3 is added as a catalyst to polarize the bromine reagent and make Br 2 more electrophilic – forms “ Br + ” Role of catalyst:

Mechanism of Bromination – 2 Steps STEP ONE: The pi electrons act as a nucleophile, attack Br + (complexed with FeBr 4 - ). The cationic intermediate formed (not aromatic) is high in energy. Delocalized cationic intermediate 3 resonance structures, all ~equal energy (Know how to draw!!) Benzene - acts as nucleophile Step 1 FeBr 3 E+E+

Formation of Product from Intermediate STEP TWO: The cationic addition intermediate transfers a proton to FeBr 4 - (from Br - and FeBr 3 ). This restores aromaticity and gives a substitution product (in contrast with addition seen in alkenes). stable aromatic system Step 1 Step 2

Reaction with other Halogens Cl 2 : Like the bromination reaction, chlorine will react with benzene using FeCl 3 as catalyzt to form chlorobenzene. F 2 : Fluorine is too reactive and cannot be used in standard EAS halogenation reactions. I 2 : Iodine has very low reactivity, requiring special reagents (Cu + or peroxide). We will not cover this reaction. skip:

Aromatic Nitration Use nitric acid (HNO 3 ) plus catalytic sulfuric acid to add a nitro group to the aromatic ring. With isopropyl, nitro group adds at the para position. Overall Reaction: