Ch. 18 Aromatic Substitution

Chapter 18: Aromatic Substitution Reactions

18.1 Electrophilic Aromatic Substitution (EAS)

• Introduction to EAS

  • Aromatic π bonds are less reactive than regular alkenes (discussed in Chapter 17).

  • EAS Overview: When an electrophile (e.g., Fe) is introduced, a substitution reaction where an aromatic proton is replaced occurs.

• Mechanism of EAS

  • The aromatic ring acts as the nucleophile and maintains its aromaticity during the reaction.

18.2 Halogenation

• Bromination

  • In benzene, Br2 acts as an electrophile but needs a Lewis acid catalyst (e.g., FeBr3) to be sufficiently reactive.

  • Mechanism of Bromination:

    • Step 1: The aromatic ring attacks Br2 to form a sigma complex intermediate.

    • Step 2: Deprotonation occurs leading to rearomatization.

• Alternative Catalysts

  • Aluminum tribromide (AlBr3) can also serve as a catalyst, identical mechanism as FeBr3.

• Chlorination

  • Can replace bromination, Cl2 used, with similar EAS mechanism.

  • Fluorination (F2) is not practical due to violent reactions and iodination is generally slow with low yield.

18.3 Sulfonation

• Reaction Details

  • Utilizes SO3 as the electrophile and H2SO4 as the catalyst, particularly fuming H2SO4 that contains SO3.

• Mechanism:

  • Benzene reacts with SO3 to form a sulfonic acid intermediate.

18.4 Nitration

• Electrophile in Nitration

  • HNO3 is the source of the electrophile, with H2SO4 as the catalyst, producing a nitronium ion (NO2+) as the active electrophile.

• Nitration Mechanism:

  • Similar to other EAS reactions, benzene forms a sigma complex, followed by deprotonation.

18.5 Friedel-Crafts Alkylation

• Reaction Overview

  • Utilizes an alkyl halide as the electrophile and AlCl3 as the catalyst.

• Mechanism:

  • Involves the formation of a carbocation or its equivalent, adding to the benzene ring.

  • Note: Simple primary (1°) halides are most effective.

18.6 Friedel-Crafts Acylation

• Comparison to Alkylation

  • Similar to alkylation but forms new carbon-carbon bond via acylium ion as the active electrophile.

• Mechanism:

  • Acylium ions are resonance-stabilized, reducing the need for rearrangement during the reaction.

18.7 Activating and Deactivating Groups

• Activating Groups

  • Increase the reactivity of the aromatic ring towards electrophiles and can be ortho/para or meta directors.

  • Example: Methyl (−CH3) group is an electron-donating group that enhances reactivity.

• Deactivating Groups

  • Nitro group (−NO2) is an example of a meta director and deactivates the ring, making it less reactive than benzene.

  • Mechanism inhibits sigma complex stability for ortho/para products.

• Halogens as Exceptions

  • Halogens are unique as they are deactivating but still ortho/para directing due to resonance effects.

18.13 Nucleophilic Aromatic Substitution (SNAr)

• Introduction

  • Involves benzene being attacked by nucleophiles (e.g., OH-) and halides acting as leaving groups.

• Requirements for Reaction:

  • Must have a strong electron-withdrawing group and a good leaving group.

18.14 Elimination-Addition Mechanism

• Context

  • Occurs under specific conditions, usually requiring high temperature to drive the reaction.

  • This reaction can involve a benzyne intermediate, illustrating a two-step mechanism.

18.15 Identifying the Mechanism

• A flow chart is useful for determining the proper substitution mechanism depending on reaction conditions and starting materials.

Review of Reactions

• Overview of various aromatic substitution reactions including both electrophilic and nucleophilic mechanisms.

• Importance of understanding activating and deactivating effects in directing substitution patterns on aromatic compounds.