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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
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
slide2
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)
slide3
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:
slide4
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
slide5
Special cases: common names

a) Name with substituents before the common name

b) The substituent giving the common name is #1

slide6
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
slide7
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

slide8
Resonance Energy of Benzene = 30 kcal/mol
  • Also called: Delocalization Energy, Aromaticity
slide9
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

slide10
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
slide11
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
slide12
Overlap of terminal p-orbitals in p1, p2, and p3 determine energies
  • Aromatic Transition states favor concerted reactions:
slide13
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
slide14
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

slide16
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
slide17
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
slide18
Coupling Constants
    • Ortho H’s = 9 Hz
    • Meta H’s = 3 Hz
    • Para H’s < 1 Hz
slide19
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
slide20
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
slide21
Structure = Symmetric like benzene
  • 1H NMR confirms that naphthalene is aromatic