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Chapter 5-2. Chemistry of Benzene: Electrophilic Aromatic Substitution

Chapter 5-2. Chemistry of Benzene: Electrophilic Aromatic Substitution. Benzene is aromatic: a cyclic conjugated compound with 6  electrons Reactions of benzene lead to the retention of the aromatic p -system

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Chapter 5-2. Chemistry of Benzene: Electrophilic Aromatic Substitution

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  1. Chapter 5-2. Chemistry of Benzene: Electrophilic Aromatic Substitution

  2. Benzene is aromatic: a cyclic conjugated compound with 6  electrons Reactions of benzene lead to the retention of the aromatic p-system Electrophilic aromatic substitution replaces a proton on benzene with another electrophile Substitution Reactions of Benzene and Its Derivatives

  3. Bromination of Aromatic Rings • Benzene’s  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

  4. Bromination of Aromatic Rings • FeBr3 is added as a catalyst to polarize the bromine reagent

  5. Cationic Intermediate in Bromination • The addition of bromine occurs in two steps • In the first step the  electrons act as a nucleophile toward Br2 (in a complex with FeBr3)

  6. This forms a cationic addition intermediate The intermediate is not aromatic and therefore high in energy

  7. Formation of Product from Intermediate • The cationic addition intermediate transfers a proton to FeBr4- (from Br- and FeBr3) • This restores aromaticity (in contrast with addition in alkenes)

  8. Other Aromatic Substitutions • The reaction with bromine involves a mechanism that is similar to many other reactions of benzene with electrophiles • The cationic intermediate was first proposed by G. W. Wheland of the University of Chicago and is often called the Wheland intermediate

  9. Aromatic Chlorination and Iodination • Chlorine and iodine (but not fluorine, which is too reactive) can produce aromatic substitution in the presence of Lewis acids. • Chlorination requires FeCl3

  10. Aromatic Chlorination and Iodination • Iodine must be oxidized to form a more powerful I+ species (with Cu+ or peroxide)

  11. Aromatic Nitration • The combination of nitric acid and sulfuric acid produces NO2+ (nitronium ion), which is isoelectronic with CO2

  12. Aromatic Nitration • The reaction with benzene produces nitrobenzene

  13. Reduction of nitro compounds to amines

  14. Aromatic Sulfonation • Substitution of H by SO3 (sulfonation) • Reaction with a mixture of sulfuric acid and SO3 (fuming sulfuric acid) • Reactive species is sulfur trioxide or its conjugate acid

  15. Aromatic Sulfonation • Sulfur trioxide, or its conjugate acid, react by the usual mechanism:

  16. Useful reactions of sulfonic acids • Sulfonic acids are useful as intermediates in the synthesis of sulfa drugs and phenols:

  17. Alkylation of Aromatic Rings: The Friedel–Crafts Reaction • Aromatic substitution of “R+” for H, alkylating the ring

  18. Alkylation of Aromatic Rings: The Friedel–Crafts Reaction • Aromatic substitution of a R+ for H • Aluminum chloride promotes the formation of the carbocation • Wheland intermediate forms

  19. Limitations of the Friedel-Crafts Alkylation • Only alkyl halides can be used (F, Cl, I, Br) • Aryl halides and vinylic halides do not react (their carbocations are too hard to form)

  20. Control Problems • Multiple alkylations can occur because the first alkylation is activating

  21. Carbocation Rearrangements During Alkylation • Similar to those that occur during electrophilic additions to alkenes

  22. Similar reactions: Mechanism?

  23. Solution:

  24. Another variation:

  25. Acylation of Aromatic Rings • Reaction of an acid chloride (RCOCl) and an aromatic ring in the presence of AlCl3 introduces acyl group, COR

  26. Mechanism of Friedel-Crafts Acylation • Similar to alkylation; reactive electrophile is a resonance-stabilized acyl cation, which does not rearrange

  27. Problem: acid chloride reactant?

  28. Substituent Effects in Aromatic Rings • Substituents can cause a compound to be (much) more or (much) less reactive than benzene

  29. Substituent Effects in Aromatic Rings • Substituents affect the orientation of the reaction – the positional relationship is controlled • ortho- and para-directing activators, ortho- and para-directing deactivators, and meta-directing deactivators

  30. Origins of Substituent Effects • An interplay of inductive effects and resonance effects • Inductive effect - withdrawal or donation of electrons through a s bond • Resonance effect - withdrawal or donation of electrons through a  bond due to the overlap of a p orbital on the substituent with a p orbital on the aromatic ring

  31. Inductive Effects • Controlled by electronegativity and the polarity of bonds in functional groups • Halogens, C=O, CN, and NO2withdraw electrons through s bond connected to ring • Alkyl groups donate electrons

  32. Resonance Effects – Electron Withdrawal • C=O, CN, NO2 substituents withdraw electrons from the aromatic ring by resonance

  33. Resonance Effects – Electron Withdrawal

  34. Resonance Effects – Electron Donation • Halogen, OH, alkoxyl (OR), and amino substituents donate electrons • Effect is greatest at ortho and para

  35. Resonance Effects – Electron Donation

  36. Contrasting Effects • Halogen, OH, OR, withdraw electrons inductively so that they deactivate the ring • Resonance interactions are generally weaker, affecting orientation • The strongest effects dominate

  37. An Explanation of Substituent Effects • Activating groups donate electrons to the ring, stabilizing the Wheland intermediate (carbocation) • Deactivating groups withdraw electrons from the ring, destabilizing the Wheland intermediate

  38. Electron Donation & Withdrawal from Benzene Rings

  39. Ortho- and Para-Directing Activators: Alkyl Groups • Alkyl groups activate: direct further substitution to positions ortho and para to themselves • Alkyl group is most effective in the ortho and para positions

  40. Ortho- and Para-Directing Activators: OH and NH2 • Alkoxyl, and amino groups have a strong, electron-donating resonance effect • Most pronounced at the ortho and para positions

  41. Ortho- and Para-Directing Deactivators: Halogens • Electron-withdrawing inductive effect outweighs weaker electron-donating resonance effect • Resonance effect is only at the ortho and para positions, stabilizing carbocation intermediate

  42. Meta-Directing Deactivators • Inductive and resonance effects reinforce each other • Ortho and para intermediates destabilized by deactivation from carbocation intermediate • Resonance cannot produce stabilization

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