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Chapter 14

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  1. Chapter 14 Aromatic Compounds Modified from sides of William Tam & Phillis Chang Ch. 14 - 1

  2. Nomenclature benzene Naming monosubstituted benzenes Most: benzene is the parent name and the substituent is a prefix

  3. Other simple, common benzenes, have accepted parent name (for substituent and ring)

  4. Disubstituted benzenes With two substituents: Their relative positions are indicated by prefixes or numbers : ortho- (abbreviated o-, or 1,2-) meta-, (m- , 1,3-) para-, (p- , 1,4-)

  5. examples

  6. Dimethylbenzenes = xylenes

  7. More than two groups 1. Positions must be indicated by numbers 2. Number the benzene ring to give Substituentsthe lowest possible numbers

  8. More than two different substituents list in alphabetical order

  9. A substituent gives “special” base name (aniline, anisole, etc.) that substituent is position 1

  10. Benzene as a substituent = phenyl group (C6H5) hydrocarbon with saturated chain and 1 benzene ring Base/parent is the larger structural unit. (S)-2-phenylheptane t-butylbenzene butylbenzene

  11. Unsaturated chains, the parent/base name is of that chain, (regardless of ring size) trans-1-phenyl-2-butene

  12. Benzyl (Bn) is a common name for the phenylmethyl group

  13. recall

  14. Reactions of Benzene substitution not addition [+ HBr] substitution

  15. The Kekulé Structure for Benzene

  16. X X RESONANCE These 1,2-dibromobenzenes are not isomers or an equilibrium

  17. + other Br2 additons However aromatic character?

  18. π-electrons above and below ring 3-D structure Note: Planar structure All carbons sp2 hybridized

  19. Hückel’s Rule: The 4n + 2 π Electron Rule (1) Planarmonocyclic rings (2) containing 4n + 2 π electrons, where n = 0 or an integer (2, 6, 10, 14 . . .etc.) have substantial resonance energies,“aromatic” i.e. a planar ring containing 6 π electrons is “aromatic”

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

  21. antibondingorbitals nonbondingorbitals bondingorbitals orbital energy levels How To Diagram the Relative Energies of p Molecular Orbitals in Monocyclics Based on Hückel’s Rule circled polygon type of  orbitals

  22. π molecular orbitals of cyclooctatetraene, if planar Predicted to have 2 nonbonding orbitals and an unpaired electron in each nonbonding orbital Not be expected to be aromatic

  23. System not planar The bonds alternately long and short; (1.48 and 1.34 Å)

  24. The Annulenes Hückel’s rule predicts that annulenes will be aromatic if the molecule has 4n + 2 π electrons and have a planar carbon skeleton

  25. All these (4n + 2)π, planar annulenes are aromatic

  26. Non-planar (4n + 2)π annulenes are antiaromatic

  27. (4n)π non-planar annulenes are antiaromatic

  28. NMR Spectroscopy: Evidence for Electron Delocalization in Aromatic Compounds 1H NMR spectrum 1H occurs at relatively high frequency Is compelling evidence for aromaticity

  29. (δ 9.3) (δ -3.0)

  30. Aromatic Ions pKa unsaturated and saturated hydrocarbon 44-53 pka = 36 pka = 16

  31. 6 π electrons aromatic sp3 sp2

  32. strong base LA

  33. Aromatic, Antiaromatic, and Nonaromatic Compounds An aromatic compound has its π electrons delocalized over the entire ring and It is stabilized by the π-electron delocalization Evaluation: compare cyclic compound vs acyclic with same number of electrons.

  34. Based on sound calculations or experiments Ring is aromatic if the ring has lower -electron energy then the acyclic chain Nonaromatic if the ring and the chain have the same -electron energy [non-planar] Antiaromatic if the ring has greater π-electron energy than the open chain [4n e’s]

  35. Cyclobutadiene Benzene

  36. Other Aromatic Compounds Benzenoid Aromatic Compounds Benzenoid polycyclic aromatic hydrocarbons having two or more fused benzene rings.

  37. Nonbenzenoid Aromatic Compounds

  38. Fullerenes

  39. Heterocyclic Aromatic Compounds heterocyclic compounds cyclic compounds with an element(s) other than carbon, e.g. piperidine aromatic heterocyclic:

  40. Examples of useful heterocyclic aromatic compounds

  41. Aromaticity

  42. Basicity of nitrogen-containing heterocycles

  43. imidazole aromatic weak base Basicity of nitrogen-containing heterocycles poor base: loss of aromaticity still aromaticity

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

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

  46. O O Nicotinamide adenine dinucleotide important coenzymes in oxidations and reductions -pyridine derivative (nicotinamide) -purine derivative (adenine)