Benzene and and the Concept of Aromaticity

1. Benzene and and the Concept of Aromaticity Chapter 21

2. 21.1 A.Benzene - Kekulé • Discovered by Michael Faraday in 1825. (#C = #H) • The first structure for benzene was proposed by August Kekulé in 1872. • This structure, however, did not account for the unusual chemical reactivity of benzene.

3. Benzene • 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 regular hexagon • all C-C-C and H-C-C bond angles 120°.

4. B. Benzene - Molecular Orbital Model • The linear combination of six overlapping p orbitals must form six molecular orbitals. • Three will be bonding, three antibonding. • Lowest energy MO will have all bonding interactions, no nodes. • As energy of MO increases, the number of nodes increases.

5. Benzene - Molecular Orbital Model Orbitals of equal energy are degenerate. Antibonding orbitals are starred (*). Electrons in the orbitals of lowest energy are in the “ground state”.

6. Benzene - Molecular Orbital Model • 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. Figure 21.2

7. Benzene - Molecular Orbitals Viewed from the top of each carbon atom

8. C. Benzene - Resonance Model • 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.

9. Benzene - Resonance • Resonance energy:the difference in energy between a resonance hybrid and the most stable 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.

10. Benzene, Fig 21.3

11. 21.2 A.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: 1. be cyclic. 2. have one p orbital on each atom of the ring (sp2). 3. be planar or nearly planar so that there is continuous or nearly continuous overlap of all p orbitals of the ring. 4. have a closed loop of (4n + 2) pi electrons in the cyclic arrangement of p orbitals.

12. Frost Circles (Polygon Rule) • Frost circle: a graphic method for determining the relative order of pi MOs in planar, fully conjugated monocyclic compounds. • inscribe a polygon of the same number of sides as the ring to be examined such that one of the vertices is at the bottom of the ring. • the relative energies of the MOs in the ring are given by where the vertices 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 antibonding MOs.

13. Frost Circles, Fig 21.4 • following are Frost circles describing the MOs for monocyclic, planar, fully conjugated four-, five-, and six-membered rings.

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

15. Aromatic Hydrocarbons • [18]annulene is also aromatic.

16. 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.

17. 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.

18. C. Antiaromatic Hydrocarbons • Antiaromatic hydrocarbon:a monocyclic, planar, fully conjugated hydrocarbon with 4n pi electrons (4, 8, 12, 16, 20...), it does not obey Huckel’s rule. • 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.

19. Cyclobutadiene, Fig. 21.5 • planar cyclobutadiene has two unpaired electrons, which make it highly unstable and reactive.

20. 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.

21. Cyclooctatetraene 2p orbital overlap forms each pi bond, there is essentially no overlap between adjacent alkenes.

22. Cyclooctatetraene, Fig. 21.6 • planar cyclooctatetraene, if it existed, would be antiaromatic. • it would have unpaired electrons in the 4 and 5 nonbonding MOs.

23. D. 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 are other than carbon. • Pyridine and pyrimidine are heterocyclic analogs of benzene; each is aromatic.

24. 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. • pyridine has a resonance energy of 134 kJ (32 kcal)/mol, slightly less than that of benzene.

25. Furan, Fig. 21.7 • 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.

26. Other Heterocyclics

27. C H C H N H 2 2 2 H O N N H H Serotonin Indole (a neurotransmitter) O C H 3 N H C N N 3 N N N O N N H C H 3 Purine Caffeine Other Heterocyclics

28. E. 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 existed, 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 • 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. pKa = ~16

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

34. Cyclopentadienyl Anion • 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. MOs of Aromatic Ions • Cyclopropenyl cation and cyclopentadienyl anion.

36. pKa's of some hydrogens

37. Cycloheptatrienyl Cation • Cycloheptatriene forms an aromatic cation by conversion of its CH2 group to a CH+ group with its sp2 carbon having a vacant 2p orbital.

38. T oluene E thylbenzene C umene S tyrene O H N H C H O C O O H O C H 2 3 21.3 A.Nomenclature • Monosubstituted alkylbenzenes are named as derivatives of benzene. • many common names are retained. Phenol Aniline Benzaldehyde Benzoic acid Anisole

39. C H C H - 3 2 P henyl group, Ph- B enzyl group, Bn- H C O 3 P h 4-(3-Methoxyphenyl)- (Z)-2-Phenyl- 2-butanone 2-butene Nomenclature • Benzyl and phenyl groups. Benzene Toluene O O 1-Phenyl-1pentanone

40. B. 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.

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

42. N O O H C H 2 3 B r N O B r 2 B r C l B r C H C H 2 3 2,4,6-Tribromo- 4-Chloro-2-nitro- 2-Bromo-1-ethyl-4- phenol toluene nitrobenzene C. 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. 1 1 4 2 6 2 2 4 4 1

43. O H O H O H O H O H C H 3 O H 3-Methylphenol 1,2-Benzenediol 1,4-Benzenediol ( m- Cresol) (Catechol) (Hydroquinone) 21.4 A.Phenols • The functional group of a phenol is an -OH group bonded to a benzene ring. Phenol 1,3-Benzenediol is resorcinol

44. Phenols • hexylresorcinol is a mild antiseptic and disinfectant. • eugenol is used as a dental antiseptic and analgesic. • urushiol is the main component of the oil of poison ivy.

45. B. Acidity of Phenols • Phenols are significantly more acidic than alcohols, compounds that also contain the OH group.

46. Acidity of Phenols • the greater acidity of phenols compared with alcohols is due to the greater stability of the phenoxide ion relative to an alkoxide ion.

47. Acidity of Phenols • Alkyl and halogen substituents effect acidities by inductive effects. • alkyl groups are electron-releasing. • halogens are electron-withdrawing.

48. Acidity of Phenols • nitro groups increase the acidity of phenols by both an electron-withdrawing inductive effect and a resonance effect.

49. 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.

50. C. Acid-Base Reactions of Phenols • Phenols are weak acids and react with strong bases to form water-soluble salts. • water-insoluble phenols dissolve in NaOH(aq).