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Chapter 14

Chapter 14. Aromatic Compounds. About The Authors. These PowerPoint Lecture Slides were created and prepared by Professor William Tam and his wife, Dr. Phillis Chang.

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Chapter 14

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  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 1995. 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 • 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 aliphaticor aromatic • Aliphatic • 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 BenzeneDerivatives • 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. It is sometimes abbreviated Bn

  17. Reactions of Benzene

  18. Benzene undergoes substitution but not addition

  19. The Kekulé Structure for Benzene

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

  21. p-electrons above and below ring The Thermodynamic Stabilityof 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

  22. Modern Theories of the Structureof Benzene 6A. The Resonance Explanation of theStructure 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

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

  24. 6B. The Molecular Orbital Explanationof the Structure of Benzene

  25. Hückel’s Rule: The 4n + 2 pElectron Rule • Hückel’s rule is concerned with compounds containing one planar ring in which each atom has a porbital 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, 14 . . . etc. p electrons), have closed shells of delocalized electrons like benzene and have substantial resonance energies

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

  27. 7A. How To Diagram the Relative Energies of p Molecular Orbitalsin Monocyclic Systems Based on Hückel’sRule

  28. 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.

  29. 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

  30. 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

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

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

  33. (4n)p non-planar annulenes are antiaromatic

  34. 7C. NMR Spectroscopy: Evidence forElectron Delocalization inAromatic 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

  35. 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

  36. (d 9.3) (d -3.0)

  37. 7D. Aromatic Ions pka = 36 pka = 16

  38. 6 pelectrons  aromatic sp3 sp2

  39. 7E. Aromatic, Antiaromatic, andNonaromatic Compounds • An aromatic compound has its p electrons delocalized over the entire ring and it is stabilized by the p-electron delocalization

  40. 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

  41. Cyclobutadiene • Benzene

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

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