Chapter 21, Benzene and and the Concept of Aromaticity

1. Chapter 21, Benzene and and the Concept of Aromaticity

2. Benzene - Kekulé • In 1872, August Kekulé proposed the following structure for benzene. • This structure, however, did not account for the unusual chemical reactivity of benzene.

3. Benzene - Resonance • We often represent benzene as a hybrid of two equivalent Kekulé structures. • Each makes an equal contribution to the hybrid and thus the C-C bonds are neither double nor single, but something in between.

4. Benzene - Resonance Model • The concepts of hybridization of atomic orbitals and the theory of resonance, developed in the 1930s, provided the first adequate description of benzene’s structure. • The carbon skeleton is a planar regular hexagon. • All C-C-C and H-C-C bond angles 120°.

5. The Pi System of Benzene • (a) The carbon framework with the six 2p orbitals. • (b) Overlap of the parallel 2p orbitals forms one torus above the plane of the ring and another below it • this orbital represents the lowest-lying pi-bonding molecular orbital.

6. Benzene-Molecular Orbital Model • The molecular orbital representation of the pi bonding in benzene.

7. Orbitals of the pi System of Benzene Number of nodal surfaces 3 2 1 0

8. Benzene - Resonance • Resonance energy:The difference in energy between a resonance hybrid in which the electrons are delocalized and • the most stable one of its hypothetical contributing structures in which electrons are localized on particular atoms and in particular bonds. • One way to estimate the resonance energy of benzene is to compare the heats of hydrogenation of benzene and cyclohexene.

9. Benzene- Resonance Energy Model Experimental data

10. Concept of Aromaticity • The underlying criteria for aromaticity were recognized in the early 1930s by Erich Hückel, based on molecular orbital (MO) calculations. • To be aromatic, a compound must • Be cyclic. • Have one p orbital on each atom of the ring. • Be planar or nearly planar so that there is continuous or nearly continuous overlap of all p orbitals of the ring. • Have a closed loop of (4n + 2) pi electrons in the cyclic arrangement of p orbitals.

11. Frost Circles • Frost circle: A graphic method for determining the relative order of pi MOs in planar, fully conjugated monocyclic compounds. • Inscribe in a circle a polygon of the same number of sides as the ring to be examined such that one of the vertices of the polygon is at the bottom of the circle. • The relative energies of the MOs in the ring are given by where the vertices of the polygon touch the circle. • Those MOs • Below the horizontal line through the center of the ring are bonding MOs. • on the horizontal line are nonbonding MOs. • above the horizontal line are antibondingMOs.

12. Frost Circles • Frost circles describing the MOs for monocyclic, planar, fully conjugated four-, five-, and six-membered rings.

13. Relationship of hexa-1,3,5-triene to benzene How does the linear triene relate to benzene?

14. Relationship of hexa-1,3,5-triene to benzene ?

15. Relationship of hexa-1,3,5-triene to benzene Look at orbitals 2 and 3. p3 ? p2 Bonding, stabilizing Curve around Antibonding, destabilizing

16. Aromatic Hydrocarbons • Annulene:A cyclic hydrocarbon with a continuous alternation of single and double bonds. • [14]Annulene is aromatic according to Hückel’s criteria. n = 3

17. Aromatic Hydrocarbons • [18]Annulene is also aromatic. n = 4

18. Aromatic Hydrocarbons • According to Hückel’s criteria, [10]annulene should be aromatic; it has been found, however, that it is not. • Nonbonded interactions between the two hydrogens that point inward toward the center of the ring force the ring into a nonplanar conformation in which overlap of the ten 2p orbitals is no longer continuous.

19. Aromatic Hydrocarbons • What is remarkable relative to [10]annulene is that if the two hydrogens facing inward toward the center of the ring are replaced by a methylene (CH2) group, the ring is able to assume a conformation close enough to planar that it becomes aromatic.

20. Antiaromatic Hydrocarbons • Antiaromatic hydrocarbon:A monocyclic, planar, fully conjugated hydrocarbon with 4n pi electrons (4, 8, 12, 16, 20...). • An antiaromatic hydrocarbon is especially unstable relative to an open-chain fully conjugated hydrocarbon of the same number of carbon atoms. • Cyclobutadiene is antiaromatic. • In the ground-state electron configuration of this molecule, two electrons fill the 1 bonding MO. • The remaining two electrons lie in the 2 and 3 nonbonding MOs.

21. Cyclobutadiene • The ground state of planar cyclobutadiene has two unpaired electrons, which make it highly unstable and reactive.

22. Cyclooctatetraene • Cyclooctatetraene, with 8 pi electrons is not aromatic; it shows reactions typical of alkenes. • X-ray studies show that the most stable conformation is a nonplanar “tub” conformation. • Although overlap of 2p orbitals occurs to form pi bonds, there is only minimal overlap between sets of 2p orbitals because they are not parallel.

23. Cyclooctatetraene • MO energy diagram for a planar conformation of cyclooctatetraene.

24. Heterocyclic Aromatics • Heterocyclic compound: A compound that contains more than one kind of atom in a ring. • In organic chemistry, the term refers to a ring with one or more atoms that differ from carbon. • Pyridine and pyrimidine are heterocyclic analogs of benzene; each is aromatic.

25. Pyridine • The nitrogen atom of pyridine is sp2 hybridized. • The unshared pair of electrons lies in an sp2 hybrid orbital and is not a part of the six pi electrons of the aromatic system (the aromatic sextet). • Resonance energy of pyridine is134 kJ (32 kcal)/mol.

26. Furan and Pyrrole • The oxygen atom of furan is sp2 hybridized. • one unshared pairs of electrons on oxygen lies in an unhybridized 2p orbital and is a part of the aromatic sextet. • The other unshared pair lies in an sp2 hybrid orbital and is not a part of the aromatic system. • The resonance energy of furan is 67 kJ (16 kcal)/mol.

27. Other Heterocyclics

28. Aromatic Hydrocarbon Ions • Any neutral, monocyclic, unsaturated hydrocarbon with an odd number of carbons must have at least one CH2 group and, therefore, cannot be aromatic. • Cyclopropene, for example, has the correct number of pi electrons to be aromatic, 4(0) + 2 = 2, but does not have a closed loop of 2p orbitals.

29. Cyclopropenyl Cation • If, however, the CH2 group of cyclopropene is transformed into a CH+ group in which carbon is sp2 hybridized and has a vacant 2p orbital, the overlap of orbitals is continuous and the cation is aromatic.

30. Cyclopropenyl Cation • When 3-chlorocyclopropene is treated with SbCl5, it forms a stable salt. • This chemical behavior is to be contrasted with that of 5-chloro-1,3-cyclopentadiene, which cannot be made to form a stable salt.

31. Cyclopentadienyl Cation • If planar cyclopentadienyl cation were to exist, it would have 4 pi electrons and be antiaromatic. • Note that we can draw five equivalent contributing structures for the cyclopentadienyl cation. Yet this cation is not aromatic because it has only 4 pi electrons.

32. Cyclopentadienyl Anion, C5H5- • To convert cyclopentadiene to an aromatic ion, it is necessary to convert the CH2 group to a CH group in which carbon becomes sp2 hybridized and has 2 electrons in its unhybridized 2p orbital. n = 1

33. Cyclopentadienyl Anion, C5H5- • As seen in the Frost circle, the six pi electrons of cyclopentadienyl anion occupy the p1, p2, and p3 molecular orbitals, all of which are bonding.

34. Cyclopentadienyl Anion, C5H5- • The pKa of cyclopentadiene is 16. • In aqueous NaOH, it is in equilibrium with its sodium salt. • It is converted completely to its anion by very strong bases such as NaNH2 , NaH, and LDA.

35. CycloheptatrienylCation, C7H7+ • Cycloheptatriene forms an aromatic cation by conversion of its CH2 group to a CH+ group with its sp2 carbon having a vacant 2p orbital.

36. Nomenclature • Monosubstituted alkylbenzenes are named as derivatives of benzene. • Many common names are retained.

37. Nomenclature • Benzyl and phenyl groups

38. Disubstituted Benzenes • Locate two groups by numbers or by the locators ortho (1,2-), meta (1,3-), and para (1,4-). • Where one group imparts a special name, name the compound as a derivative of that molecule.

39. Disubstituted Benzenes • Where neither group imparts a special name, locate the groups and list them in alphabetical order.

40. Polysubstituted Derivatives • If one group imparts a special name, name the molecule as a derivative of that compound. • If no group imparts a special name, list them in alphabetical order, giving them the lowest set of numbers.

41. Phenols • The functional group of a phenol is an -OH group bonded to a benzene ring.

42. Acidity of Phenols • Phenols are significantly more acidic than alcohols.

43. Acidity of Phenols • Separation of water-insoluble phenols from water-insoluble alcohols.

44. Acidity of Phenols (Resonance) • The greater acidity of phenols compared with alcohols is due to the greater stability of the phenoxide ion relative to an alkoxide ion.

45. Phenol Subsitituents (Inductive Effect) • Alkyl and halogen substituents effect acidities by inductive effects: • Alkyl groups are electron-releasing. • Halogens are electron-withdrawing.

46. Phenol Subsitituents(Resonance, Inductiion) • Nitro groups increase the acidity of phenols by both an electron-withdrawing inductive effect and a resonance effect.

47. Acidity of Phenols • Part of the acid-strengthening effect of -NO2 is due to its electron-withdrawing inductive effect. • In addition, -NO2 substituents in the ortho and para positions help to delocalize the negative charge.

48. Synthesis: Alkyl-Aryl Ethers • Alkyl-aryl ethers can be prepared by the Williamson ether synthesis: • but only using phenoxide salts and haloalkanes. • haloarenes cannot be used because they are unreactive to SN2 reactions.

49. Synthesis: Alkyl-Aryl Ethers

50. Synthesis: Kolbe Carboxylation • Phenoxide ions react with carbon dioxide to give a carboxylate salt.