Aromaticity

1. Aromaticity 4/19/06

2. Cyclobutadiene & Cyclooctatetraene

3. H�ckel�s Rule - Annulenes

4. Constructing the p-Orbital MO Diagrams

5. Interpreting the MO Figure

6. H�ckel�s 4n+2 Rule Monocyclic planar, fully conjugated polyenes are called annulenes. Among annulenes, only those possessing 4n+2 p electrons, where n is an integer, will have special aromatic stability. Planarity and complete conjugation are important criteria. The 4n+2 rule may be fulfilled for neutral or ionic moieties.

7. Cycloheptatriene & the Cycloheptatrienyl Cation

8. The Cycloheptatrienyl Cation p-MO Diagram

9. Additional Aromatic Ions

10. A Couple More Examples

11. Heterocyclic Aromatic Compounds

12. Where are Those Lone Pairs?

13. Some Interesting Heterocycles

14. Polycyclic Aromatic Heterocycles

15. DNA & RNA Bases

16. Electrophilic Aromatic Substitution

17. Substitution Directly on the Benzene Ring

18. What do Reactants See?

19. Electrophilic Attack; Aromatics vs. Alkenes

20. Nitration of Benzene

21. Mechanism for the Nitration of Benzene

22. Sulfonation of Benzene

23. Mechanism for the Sulfonation of Benzene

24. Bromination and Chlorination of Benzene

25. Bromination Mechanism

26. Friedel-Crafts Alkylation of Benzene

27. The Mechanism

28. The Limitation!

29. Friedel-Crafts Acylation of Benzene

30. Activation of Propanoyl Chloride

31. To Rearrange or Not to Rearrange

32. Two Step Synthesis of Alkyl Benzenes

33. Rate and Regioselectivity in Aromatic Substitution Does the presence of a substituent on a benzene ring influence the addition of other substituents? If so, what are the effects and can they be useful? Possible effects: Rate of substitution and position of the added substituent. Rates of electrophilic aromatic substitution are generally compared with benzene. An activating substituent causes subsequent substitution to be faster than that for benzene. A deactivating substituent causes subsequent substitution to be slower.

34. Rates of Substitution and Activation Energy

35. An Estimate of Eact Using ab initio calculations, a value for EHOMO can be estimated. The program SPARTAN� was used in this case employing a 631G** basis set. The activation for the formation of the respective cations can be roughly approximated with the energy necessary to remove an electron from the benzene derivative p-system. Because of the nature of ab initio calculations comparisons of relative energies is appropriate. In this case we will use EHOMO for benzene as the reference. EHOMO for toluene is 19.05 kJ/mol higher (less negative) than that for benzene. For trifluoromethylbenzene the value is 54.72 kJ/mol lower (more negative) than that for benzene.

37. A Visual Comparison The same theoretical calculations used to estimated the energy of the HOMO also can be used to produce some visible aids to compare the three molecules.

38. Regioselectivity in Electrophilic Aromatic Substitution

39. Theory of Directing Effects For ortho-para directors, ortho-para attack forms a more stable cation than meta attack ortho-para products are formed faster than meta products For meta directors, meta attack forms a more stable cation than ortho-para attack meta products are formed faster than ortho-para products

40. Theory of Directing Effects -NO2: assume ortho-para attack

41. Theory of Directing Effects -OCH3: assume ortho-para attack

42. Di- and Polysubstitution the order of steps is important

43. Di- and Polysubstitution From the information, we can make these generalizations alkyl, phenyl, and all other groups in which the atom bonded to the ring has an unshared pair of electrons are ortho-para directing. All other groups are meta directing all ortho-para directing groups except the halogens are activating toward further substitution. The halogens are weakly deactivating

44. Di- and Polysubstitution