Classify cyclononatetrene and it’s various ions as either aromatic, antiaromatic or nonaromatic.

1. Treatment of cyclooctatetrene with potassium gives you a dianion. Classify the starting material and product as aromatic, antiaromatic or nonaromatic?

3. Classify cyclononatetrene and it’s various ions as either aromatic, antiaromatic or nonaromatic.

5. Electrophilic Aromatic Substitution Background • The characteristic reaction of benzene is electrophilic aromatic substitution—a hydrogen atom is replaced by an electrophile.

6. Benzene does not undergo addition reactions like other unsaturated hydrocarbons, because addition would yield a product that is not aromatic. • Substitution of a hydrogen keeps the aromatic ring intact. • There are five main examples of electrophilic aromatic substitution.

8. Regardless of the electrophile used, all electrophilic aromatic substitution reactions occur by the same two-step mechanism—addition of the electrophile E+ to form a resonance-stabilized carbocation, followed by deprotonation with base, as shown below:

9. The first step in electrophilic aromatic substitution forms a carbocation, for which three resonance structures can be drawn. To help keep track of the location of the positive charge:

10. The energy changes in electrophilic aromatic substitution are shown below:

11. Halogenation • In halogenation, benzene reacts with Cl2 or Br2 in the presence of a Lewis acid catalyst, such as FeCl3 or FeBr3, to give the aryl halides chlorobenzene or bromobenzene respectively. • Analogous reactions with I2 and F2 are not synthetically useful because I2 is too unreactive and F2 reacts too violently.

12. Chlorination proceeds by a similar mechanism.

13. Nitration and Sulfonation • Nitration and sulfonation introduce two different functional groups into the aromatic ring. • Nitration is especially useful because the nitro group can be reduced to an NH2 group.

14. Generation of the electrophile in nitration requires strong acid.

15. Generation of the electrophile in sulfonation requires strong acid.

16. Friedel-Crafts Alkylation and Friedel-Crafts Acylation • In Friedel-Crafts alkylation, treatment of benzene with an alkyl halide and a Lewis acid (AlCl3) forms an alkyl benzene.

17. In Friedel-Crafts acylation, a benzene ring is treated with an acid chloride (RCOCl) and AlCl3 to form a ketone. • Because the new group bonded to the benzene ring is called an acyl group, the transfer of an acyl group from one atom to another is an acylation.

18. Friedel-Crafts Alkylation and Friedel-Crafts Acylation

20. In Friedel-Crafts acylation, the Lewis acid AlCl3 ionizes the carbon-halogen bond of the acid chloride, thus forming a positively charged carbon electrophile called an acylium ion, which is resonance stabilized. • The positively charged carbon atom of the acylium ion then goes on to react with benzene in the two step mechanism of electrophilic aromatic substitution.

21. Three additional facts about Friedel-Crafts alkylation should be kept in mind. [1] Vinyl halides and aryl halides do not react in Friedel- Crafts alkylation.

22. [2] Rearrangements can occur. These results can be explained by carbocation rearrangements.

24. Rearrangements can occur even when no free carbocation is formed initially.

25. [3] Other functional groups that form carbocations can also be used as starting materials.

26. Each carbocation can then go on to react with benzene to form a product of electrophilic aromatic substitution. For example:

27. Starting materials that contain both a benzene ring and an electrophile are capable of intramolecular Friedel-Crafts reactions.

28. For Monday, do problems 18.1-18.11.

29. 1) Why is benzene less reactive than an alkene? The pi electrons of benzene are delocalized over 6 atoms, thus making benzene more stable and less available for electron donation. While an alkene’s electrons are localized between two atoms, thus making it more nucleophillc and more reactive toward electrophiles.

30. 2) Show how the other two resonance structures can be deprotonated in step two of electrophillic aromatic substitution.

32. 3) Draw a detailed mechanism of the chlorination of benzene. Formation of Electrophile Electrophillic Additon

33. Deprotonation

34. 4) Draw stepwise mechanism for the sulfonation of A. Formation of Electrophile

35. Electrophillic Addition Deprotonation

36. 5) What product is formed when benzene is reacted with each of the following alkyl halides? a) b)

37. c)

38. 6) What acid chloride is necessary to produce each product from benzene using a Friedal-Crafts acylation? a) b)

39. c)

40. 7) Draw a stepwise mechanism for the following friedal-Crafts alkylation? Formation of Electrophile

41. Electrophillic Additoon Protonation

42. 8) Which of these halides are reactive in a Friedal-Crafts alkylation? Look at the carbon to which the halogen is attached and determine its hybridization. If sp2 its unreactive, while sp3 is reactive.

43. 9) Draw a stepwise mechanism for the following reaction. Formation of Electrophile 1,2 H shift

44. Electrophillic Additon Deprotonation

45. 10) Draw the product of each reaction a) b)

46. c) d)

47. 11) Draw a stepwise mechanism for the intermolecular Friedal-Crafts acylation below

48. Formation of Electrophile

49. Electrophillic addition

50. Deprotonation