1. Chapter 14
Aromatic Compounds

2. About The Authors
These PowerPoint Lecture Slides were created and prepared by Professor William Tam and his wife, Dr. Phillis Chang.
Professor William Tam received his B.Sc. at the University of Hong Kong in 1990 and his Ph.D. at the University of Toronto (Canada) in He was an NSERC postdoctoral fellow at the Imperial College (UK) and at Harvard University (USA). He joined the Department of Chemistry at the University of Guelph (Ontario, Canada) in 1998 and is currently a Full Professor and Associate Chair in the department. Professor Tam has received several awards in research and teaching, and according to Essential Science Indicators, he is currently ranked as the Top 1% most cited Chemists worldwide. He has published four books and over 80 scientific papers in top international journals such as J. Am. Chem. Soc., Angew. Chem., Org. Lett., and J. Org. Chem.
Dr. Phillis Chang received her B.Sc. at New York University (USA) in 1994, her M.Sc. and Ph.D. in 1997 and 2001 at the University of Guelph (Canada). She lives in Guelph with her husband, William, and their son, Matthew.

3. The Discovery of Benzene
In 1825, Faraday isolated benzene from a compressed illuminating gas that had been made by pyrolyzing whale oil

4. In 1834, a German chemist, Eilhardt Mitscherlich, synthesized benzene by heating benzoic acid with calcium oxide

5. In 19th century, organic compounds were classified as being either aliphatic or aromatic
The chemical behavior of a compound was “fatlike”
Aromatic
The compound had a low hydrogen-to-carbon ratio and it was “fragrant”

6. Nomenclature of Benzene Derivatives
Naming monosubstituted benzenes
In many simple compounds, benzene is the parent name and the substituent is simply indicated by a prefix

7. For other simple and common compounds, the substituent and the benzene ring taken together may form a commonly accepted parent name

8. Naming disubstituted benzenes
When two substituents are present, their relative positions are indicated by the prefixes ortho-, meta-, and para- (abbreviated o-, m-, and p-) or by the use of numbers

9. Other examples

10. The dimethylbenzenes are often called xylenes

11. Naming benzene rings with more than two groups
If more than two groups are present on the benzene ring, their positions must be indicated by the use of numbers
The benzene ring is numbered so as to give the lowest possible numbers to the substituents

12. When more than two substituents are present and the substituents are different, they are listed in alphabetical order

13. When a substituent is one that, together with the benzene ring gives a new base name, that substituent is assumed to be in position 1 and the new parent name is used

14. When the C6H5 group is named as a substituent, it is called a phenyl group
A hydrocarbon composed of one saturated chain and one benzene ring is usually named as a derivative of the larger structural unit. However, if the chain is unsaturated, the compound may be named as a derivative of that chain, regardless of ring size

15. Examples

16. Benzyl is an alternative name for the phenylmethyl group
Benzyl is an alternative name for the phenylmethyl group. It is sometimes abbreviated Bn

17. Reactions of Benzene

21. Benzene undergoes substitution but not addition

22. The Kekulé Structure for
Benzene

23. X These 1,2-dibromobenzenes do not exist as isomers
There is no such equilibrium between benzene ring bond isomers

25. The Thermodynamic Stability of Benzene
Since p bonds are formed from side-way overlap of p orbitals, p electron clouds are above & below the plane of the double bond
p-electrons above and below ring

27. Modern Theories of the Structure of Benzene
6A. The Resonance Explanation of the Structure of Benzene
All bond lengths the same (1.39 Å) (compare with C–C single bond 1.54 Å, C=C double bond 1.34 Å)
Extra stabilization due to resonance  aromatic

28. 3-D structure
p-electrons above and below ring
Planar structure
All carbons sp2 hybridized

29. 6B. The Molecular Orbital Explanation of the Structure of Benzene

31. Hückel’s Rule: The 4n + 2 p Electron Rule
Hückel’s rule is concerned with compounds containing one planar ring in which each atom has a p orbital as in benzene
Planar monocyclic rings containing 4n + 2 p electrons, where n = 0, 1, 2, 3, and so on (i.e., rings containing 2, 6, 10, etc. p electrons), have closed shells of delocalized electrons like benzene and have substantial resonance energies

32. Hückel’s rule states that planar monocyclic rings with 2, 6, 10, 14
Hückel’s rule states that planar monocyclic rings with 2, 6, 10, delocalized electrons should be aromatic

33. 7A. How To Diagram the Relative
Energies of p Molecular Orbitals in
Monocyclic Systems Based on
Hückel’s Rule

34. The p molecular orbitals that cyclooctatetraene would have if it were planar. Notice that, unlike benzene, this molecule is predicted to have two nonbonding orbitals, and because it has eight p electrons, it would have an unpaired electron in each of the two nonbonding orbitals. Such a system would not be expected to be aromatic.

35. The bonds of cyclooctatetraene are known to be alternately long and short; X-ray studies indicate that they are 1.48 and 1.34 Å, respectively

36. 7B. The Annulenes
Hückel’s rule predicts that annulenes will be aromatic if their molecules have 4n + 2 p electrons and have a planar carbon skeleton

37. All these (4n + 2)p, planar annulenes are aromatic

38. Non-planar (4n + 2)p annulenes are antiaromatic

39. (4n)p non-planar annulenes are antiaromatic

40. 7C. NMR Spectroscopy: Evidence for Electron Delocalization in Aromatic Compounds
The 1H NMR spectrum of benzene consists of a single unsplit signal at d 7.27
The signal occurs at relatively high frequency, which is compelling evidence for the assertion that the p electrons of benzene are delocalized

41. The circulation of p electrons in benzene creates an induced magnetic field that, at the position of the protons, reinforces the applied magnetic field. This reinforcement causes the protons to be strongly deshielded and to have a relatively high frequency (d ~ 7) absorption

43. (d 9.3)
(d -3.0)

44. 7D. Aromatic Ions
pka = 36
pka = 16

45. 6 p electrons  aromatic
sp3
sp2

47. 7E. Aromatic, Antiaromatic, and Nonaromatic Compounds
An aromatic compound has its p electrons delocalized over the entire ring and it is stabilized by the p-electron delocalization

48. Based on sound calculations or experiments
One way to evaluate whether a cyclic compound is stabilized by delocalization of p electrons through its ring is to compare it with an open-chain compound having the same number of p electrons
Based on sound calculations or experiments
If the ring has lower p-electron energy, then the ring is aromatic
If the ring and the chain have the same p-electron energy, then the ring is nonaromatic
If the ring has greater p-electron energy than the open chain, then the ring is antiaromatic

49. Cyclobutadiene
Benzene

50. Other Aromatic Compounds
8A. Benzenoid Aromatic Compounds
Benzenoid polycyclic aromatic hydrocarbons consist of molecules having two or more benzene rings fused together

51. 8B. Nonbenzenoid Aromatic Compounds

52. 8C. Fullerenes

53. Heterocyclic Aromatic Compounds
Cyclic compounds that include an element other than carbon are called heterocyclic compounds

54. Examples of useful heterocyclic aromatic compounds

55. Aromaticity

56. Aromaticity
Evidence: 1H NMR shift

57. Basicity of nitrogen-containing heterocycles

58. Basicity of nitrogen-containing heterocycles

59. Non-basic nitrogen
basic nitrogen

60. Aromatic Compounds in Biochemistry
Two amino acids necessary for protein synthesis contain the benzene ring

61. Derivatives of purine and pyrimidine are essential parts of DNA and RNA

62. Nicotinamide adenine dinucleotide, one of the most important coenzymes in biological oxidations and reductions, includes both a pyridine derivative (nicotinamide) and a purine derivative (adenine) in its structure

63. Spectroscopy of Aromatic Compounds
11A. 1H NMR Spectra
The ring hydrogens of benzene derivatives absorb downfield in the region between d 6.0 and d 9.5 ppm
11B. 13C NMR Spectra
The carbon atoms of benzene rings generally absorb in the d 100–170 ppm region of 13C NMR spectra

67. 11C. Infrared Spectra of Substituted Benzenes

68. 11D. Ultraviolet–Visible Spectra of Aromatic Compounds

70. 11E. Mass Spectra of Aromatic Compounds

71.  END OF CHAPTER 14 