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Organic Chemistry Reviews Chapter 15

Organic Chemistry Reviews Chapter 15. Cindy Boulton March 29, 2009. Electrophilic Aromatic Substitution. Hydrogen on Benzene ring is replaced or substituted by an elctrophile General Mechanism: Benzene ring acts as nucleophile or Lewis base

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Organic Chemistry Reviews Chapter 15

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  1. Organic Chemistry ReviewsChapter 15 Cindy Boulton March 29, 2009

  2. Electrophilic Aromatic Substitution • Hydrogen on Benzene ring is replaced or substituted by an elctrophile • General Mechanism: • Benzene ring acts as nucleophile or Lewis base • Electrophile has a positive charge or a Lewis acid • Pair of electrons from double bond in benzene ring reacts with the electrophile • Forms a AreniumCation Intermediate, not very stable, with both the electrophile and a Hydrogen attached to a Carbon in the benzene ring • Base already present in solution with a pair of electrons is used to remove the Hydrogen and reform the benzene ring

  3. Electrophilic Aromatic Substitution • Reaction Coordinate • Two humps • Large initial Activation Energy • Intermediate is formed • Small second Activation Energy

  4. Field Crafts Alkylation • Alkyl group is added to a benzene ring • Reactants: • Alkyl halide (R-Cl) • Chloride is the best leaving group • Lewis Acid Catalyst • M-Cl3 (M: Fe, Al, B) • Electorphile Generation • The Lewis acid is electron deficient and is used to remove the Chloride and generate the carbocation/electrophile (R+) and the base (M-Cl4) • The carbocation can undergo skeletal rearrangement • Attack • The carbocation/electrophile attacks the benzene ring and the base removes the Hydrogen forming H-Cl and regenerating the Lewis Acid catalyst

  5. Field Crafts Acylation • Acyl group (aldehyde or ketone) is added to a benzene ring • Reactants: • Acyl halide (R-(C=O)-Cl) • Chloride is the best leaving group • Lewis Acid Catalyst • M-Cl3 (M: Fe, Al, B) • Electorphile Generation • The Lewis acid is electron deficient and is used to remove the Chloride and generate the electrophile (R-(C=O)+) and the base (M-Cl4) • Attack • The electrophile attacks the benzene ring and the base removes the Hydrogen forming H-Cl and regenerating the Lewis Acid catalyst

  6. Field Crafts Halogenation • Halogen is added to a benzene ring • Reactants: • Halogen (X2) • Lewis Acid Catalyst • M-X3 (M: Fe, Al, B) (X: Cl, Br, I, difficult with F) • Lewis Acid has to have the same Halogen • Electorphile Generation • The Lewis acid is electron deficient and is used to remove the Chloride and generate the electrophile (X+) and the base (M-X4) • Attack • The electrophile attacks the benzene ring and the base removes the Hydrogen forming H-Cl and regenerating the Lewis Acid catalyst

  7. Field Crafts Sulfonation • Sulfate is added to a benzene ring • Reactants: • Sulfuric Acid • No Lewis Acid Catalyst • Electorphile Generation • Sulfuric Acid generates water and sulfate which acts as the electrophile and the intramolecular base • Attack • The electrophile attacks the benzene ring and then acts as an intermolecular base by removing the Hydrogen and adding it to the Oxygen bonded to the Sulfur with a single bond

  8. Field Crafts Nitration • Nitrite is added to a benzene ring • Reactants: • Nitric Acid and Sulfuric Acid • No Lewis Acid Catalyst • Electorphile Generation • Sulfuric Acid reacts with Nitric Acid to generate nitrite which acts as the electrophile and water which acts as the base • Attack • The electrophile attacks the benzene ring forming Zwitterion and then water removes the hydrogen forming nitrobenzene

  9. Electrophilic Aromatic Substitution of Substituted Benzene • Electron Donating Groups • Activators • Ortho/Para Directors • Increase the rate of the reaction by donating electron density • Electron Withdrawing Groups • Deactivators • Meta Director • Decrease the rate of the reaction by removing electron density

  10. Electrophilic Aromatic Substitution of Substituted Benzene • Electron Groups with Lone Pairs • Activators • Ortho/Para Directors • Increase the rate of the reaction by removing electron density • Halogens • Deactivators • Ortho/Para Directors • Decrease the rate of the reaction by removing electron density

  11. Electrophilic Aromatic Substitution of Substituted Benzene • Mechanism NOT dependent on Electrophile but on group already present • Electron Donating Group • Ortho: • The AreniumCation Intermediate is formed and is stable • Tertiary Carbocation can be formed with electron density donated – Inductive Effect • Meta: • The AreniumCation Intermediate cannot form a tertiary carbocation, the positive charge is never on the ipso Carbon • High Activation Energy • Para: • Carbocation on tertiary, ipso Carbon • Ortho and Para have same electronics by differ in sterics. They have the same activation energy, but lower than meta. • AreniumCation Intermediate forms a resonance hybrid with partial positive charges in a horseshoe shape

  12. Electrophilic Aromatic Substitution of Substituted Benzene • Electron Withdrawing Group • Inductive Effect • Ortho/Para resonance hybrids are destablized with the electron withdrawing group, raising the activation energy • Meta does not lower activation energy, but is the best by default • Group with Lone Pairs • Ortho/Para resonance hybrids can form a 4th resonance structure • Halogen • Ortho/Para resonance hybrids can form a 4th resonance structure • Deactivator by inductive effect because it is an electron withdrawing group

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