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Electrophilic Attack

Electrophilic Attack. Electrophile substitutes for a hydrogen on the benzene ring. Electrophilic Aromatic Substitution. Mechanism. =>. Requires a stronger electrophile than Br 2 . Use a strong Lewis acid catalyst, FeBr 3 . Bromination of Benzene. Energy Diagram for Bromination. =>.

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Electrophilic Attack

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  1. Electrophilic Attack

  2. Electrophile substitutes for a hydrogen on the benzene ring. Electrophilic Aromatic Substitution

  3. Mechanism =>

  4. Requires a stronger electrophile than Br2. Use a strong Lewis acid catalyst, FeBr3. Bromination of Benzene

  5. Energy Diagram for Bromination =>

  6. Chlorination is similar to bromination. Use AlCl3 as the Lewis acid catalyst. Iodination requires an acidic oxidizing agent, like nitric acid, which oxidizes the iodine to an iodonium ion. Chlorination and Iodination

  7. Use sulfuric acid with nitric acid to form the nitronium ion electrophile. Nitration of Benzene NO2+ then forms a sigma complex with benzene, loses H+ to form nitrobenzene. =>

  8. Sulfur trioxide, SO3, in fuming sulfuric acid is the electrophile. Sulfonation

  9. Toluene reacts 25 times faster than benzene. The methyl group is an activator. The product mix contains mostly ortho and para substituted molecules. Nitration of Toluene

  10. Intermediate is more stable if nitration occurs at the orthoor para position. Sigma Complex

  11. Energy Diagram =>

  12. Synthesis of alkyl benzenes from alkyl halides and a Lewis acid, usually AlCl3. Reactions of alkyl halide with Lewis acid produces a carbocation which is the electrophile. Other sources of carbocations: alkenes + HF or alcohols + BF3. Friedel-Crafts Alkylation

  13. => Examples of Carbocation Formation

  14. Formation of Alkyl Benzene - +

  15. Reaction fails if benzene has a substituent that is more deactivating than halogen. Carbocations rearrange. Reaction of benzene with n-propyl chloride and AlCl3 produces isopropylbenzene. The alkylbenzene product is more reactive than benzene, so polyalkylation occurs. Limitations of Friedel-Crafts

  16. Acyl chloride is used in place of alkyl chloride. The acylium ion intermediate is resonance stabilized and does not rearrange like a carbocation. The product is a phenyl ketone that is less reactive than benzene. Friedel-Crafts Acylation

  17. Mechanism of Acylation

  18. Acylbenzenes can be converted to alkylbenzenes by treatment with aqueous HCl and amalgamated zinc. Clemmensen Reduction

  19. Formyl chloride is unstable. Use a high pressure mixture of CO, HCl, and catalyst. Product is benzaldehyde. Gatterman-Koch Formylation

  20. Alkyl groups stabilize the sigma complex by induction, donating electron density through the sigma bond. Substituents with a lone pair of electrons stabilize the sigma complex by resonance. Activating, O-, P-Directing Substituents

  21. Aniline reacts with bromine water (without a catalyst) to yield the tribromide. Sodium bicarbonate is added to neutralize the HBr that’s also formed. => The Amino Group

  22. Summary of Activators

  23. Electrophilic substitution reactions for nitrobenzene are 100,000 times slower than for benzene. The product mix contains mostly the meta isomer, only small amounts of the orthoand para isomers. Meta-directors deactivate all positions on the ring, but the meta position is less deactivated. Deactivating Meta-Directing Substituents

  24. Ortho Substitution on Nitrobenzene

  25. Para Substitution on Nitrobenzene =>

  26. Meta Substitution on Nitrobenzene

  27. Energy Diagram

  28. The atom attached to the aromatic ring will have a partial positive charge. Electron density is withdrawn inductively along the sigma bond, so the ring is less electron-rich than benzene. Structure of Meta-Directing Deactivators

  29. Summary of Deactivators

  30. More Deactivators

  31. Halogens are deactivating toward electrophilic substitution, but are ortho, para-directing! Since halogens are very electronegative, they withdraw electron density from the ring inductively along the sigma bond. But halogens have lone pairs of electrons that can stabilize the sigma complex by resonance. Halobenzenes

  32. Sigma Complex for Bromobenzene Ortho and para attacks produce a bromonium ionand other resonance structures. No bromonium ion possible with meta attack.

  33. Energy Diagram

  34. Summary of Directing Effects

  35. The most strongly activating substituent will determine the position of the next substitution. May have mixtures. Multiple Substituents

  36. II. Electrophilic Addition “Loose” p electrons are nucleophilic (Lewis bases), react with electrophiles (Lewis acids).

  37. II. Electrophilic Addition A. Addition of hydrogen halides (X = Cl, Br, I) Reactivity: HI > HBr > HCl >> HF (stronger acid = better electrophile)

  38. CheckAnswer II. Electrophilic Addition A. Addition of hydrogen halides 1. Markovnikov’s rule In the addition of HX to an alkene, the H goes to the carbon with more H’s. Question 6-2. Draw the products. Click on the arrow to check answers.

  39. II. Electrophilic Addition A. Addition of hydrogen halides 1. Markovnikov’s rule In the addition of HX to an alkene, the H goes to the carbon with more H’s. Answer 6-2.

  40. II. Electrophilic Addition A. Addition of hydrogen halides 2. mechanism Mechanistic interpretation of Markovnikov’s rule: The reaction proceeds through the more stable carbocation intermediate.

  41. II. Electrophilic Addition A. Addition of hydrogen halides 2. mechanism lower Ea faster rate of formation

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