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Chapter 15 Benzene

Chapter 15 Benzene. Benzene Structure and Nomenclature Structure of Benzene Faraday in 1825 isolates a colorless liquid from whale oil Empirical formula = CH (C need 4 bonds?) Very inert Later, the molecular formula of C 6 H 6 was determined and named benzene Degrees of unsaturation = 4

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Chapter 15 Benzene

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  1. Chapter 15 Benzene • Benzene Structure and Nomenclature • Structure of Benzene • Faraday in 1825 isolates a colorless liquid from whale oil • Empirical formula = CH (C need 4 bonds?) • Very inert • Later, the molecular formula of C6H6 was determined and named benzene • Degrees of unsaturation = 4 • 1,3,5-cyclohexatriene structure is proposed • Not reactive as conjugated polyenes should be • Various possible structures are proposed: • Dewar Benzene • Ladenburg Prismane • Benzvalene • Claus Benzene • All but (d) go to benzene

  2. Reactivity of Benzene • Benzene is a relatively inert molecule: no reaction with Br2 • Reaction in the presence of a catalyst with Br2 does give product • Problem: If the ring is really alternating double and single bonds, we should have gotten 1,2 addition (Br on C=C) and 1,6 addition (C—C)

  3. The fact that we only have one 1,2-disubstituted product supports a resonance hybrid structure • Nomenclature • Aromatic Compounds = benzene and its substituted analogues • We draw them as a single resonance structure, but we always mean the resonance hybrid • Monosubstituted Benzenes are named with the substituent as prefix:

  4. Disubstituted benzene have three possible arrangements • 1,2 is also known as ortho (o-) • 1,3 is also known as meta (m-) • 1,4 is also known as para (p-) • Polysubstituted: number with the lowest set possible, label substituents as in cyclohexane nomenclature

  5. Special cases: common names a) Name with substituents before the common name b) The substituent giving the common name is #1

  6. A substituted benzene is called an arene • An arene as a substituent is called an aryl group (benzene itself is phenyl) • A phenylmethyl group is called benzyl • Aromaticity • Structure of benzene revisited • Ring of six sp2 hybridized carbons • Six p-orbitals give six MO’s with six p electrons • p-cloud above and below the plane of the molecule • Completely symmetric • Not a cyclohexatriene structure

  7. Orbital picture of benzene 2) Heats of Hydrogenation DHo = -28.6 kcal/mol DHo = -54.9 kcal/mol DHo = -49.3 kcal/mol DHocalculated = -78.9 kcal/mol

  8. Resonance Energy of Benzene = 30 kcal/mol • Also called: Delocalization Energy, Aromaticity

  9. MO Description of Benzene • 1,3,5-hexatriene • Similar to 1,3-butadiene MO picture b) 3 bonding MO’s are filled, so conjugation stabilizes the molecule c) 6 p-MO’s with 6 p-electrons 1,3,5-hexatriene

  10. Benzene is a cyclic system, which changes how the MO’s are arranged • 6 p-MO’s with 6 p-electrons • The nodes intersect • There are degenerate orbitals

  11. Energy comparison shows that the benzene structure is more stable • Benzene: 2 MO’s lowered in energy and 1 MO raised • Hexatriene: 1 MO lowered in energy and 2 MO’s raised

  12. Overlap of terminal p-orbitals in p1, p2, and p3 determine energies • Aromatic Transition states favor concerted reactions:

  13. Spectroscopy of Aromatic Systems • UV-Vis Spectroscopy • The energy gap between HOMO and LUMO is large for aromatics because of extra aromatic stabilization • More energetic absorption is needed for electronic transition • lmax will be smaller than for trienes • Sample spectra

  14. Substitution alters the energy levels and thus the spectrum and the color • Many dyes are aromatic compounds • Many sun-tan lotions contain PABA = p-aminobenzoic acid to block UV rays (lmax = 289 nm, e = 18,600) • IR Spectroscopy • Typical aromatic IR bands • Aromatic C—H stretch = 3030 cm-1 • Aromatic C—C = 1500-2000 cm-1 • Aromatic C—H bending = 650-1000 cm-1 • Can be used to determine substitution pattern • C—H bending for different substitutions: 690-710 730-770 735-770 690-710 750-810 790-840

  15. O-xylene

  16. 1H NMR Spectroscopy • Aromatic protons are highly deshielded due to ring current • Benzenes have C—H protons from 6.5-8.5 ppm • Alkenes are 4.6-5.7 ppm

  17. Benzylic groups are not as deshielded. Ring current fades quickly. • Substitution pattern dictates the spectrum pattern • Benzene itself has only one singlet at 7.27 ppm • Spectral Examples

  18. Coupling Constants • Ortho H’s = 9 Hz • Meta H’s = 3 Hz • Para H’s < 1 Hz

  19. 13C NMR Spectroscopy • Ring current does not have a large effect on the carbons • Carbon shifts are very similar to the alkenes: 120-135 ppm • Polycyclic Aromatic Hydrocarbons (PAH’s) • Structure and Nomenclature • Two or more benzene rings share 2 or more Carbons to be a PAH • The general name for the series is the acenes (pentacene = 5 rings) • Angular fusion gives different compounds

  20. Is Napthalene Aromatic? • White solid, mp = 80 oC, used as mothbolls • UV-Vis spectrum looks like a conjugated p-system, but with more delocalization than in benzene

  21. Structure = Symmetric like benzene • 1H NMR confirms that naphthalene is aromatic

  22. Large acenes are also aromatic. The more benzene rings the better. • Anthracene • Phenanthrene

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