Benzene Discovered in 1825 by Michael Faraday: He called it - - PowerPoint PPT Presentation

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Benzene Discovered in 1825 by Michael Faraday: He called it - - PowerPoint PPT Presentation

Dec 25, 2022 466 likes •681 views

Benzene Discovered in 1825 by Michael Faraday: He called it bicarburet of hydrogen. Named benzene by Eilhardt Mitscherlich in 1833. Molecular formula: C 6 H 6 Representative of the aromatic hydrocarbon family:

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SLIDE 1

C6H6 Br2 CCl4, dark no reaction C6H6 Br2

  • cat. FeBr3

C6H5Br + HBr C6H6

  • xs. H2
  • cat. Pd

no reaction

Benzene

  • Discovered in 1825 by Michael Faraday: He called it “bicarburet of hydrogen”.
  • Named “benzene” by Eilhardt Mitscherlich in 1833.
  • Molecular formula: C6H6
  • Representative of the aromatic hydrocarbon family:

benzene, reduction product of benzoic acid, found in gum benzoin, an aromatic balsam from Styrax benzoin, the Benjamin tree. toluene, distilled from gum tolu, an aromatic balsam from the Tolu tree.

  • C6H6 DU = (C) - ½(H + X) + ½(N) + 1 = 6 - ½(6) + 1 = 4
  • UV/Vis absorption spectrum:
  • max = 254 nm
  • Shouldn’t benzene react like alkenes or alkynes?
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SLIDE 2

H H H H H H H H H H H H

Some Possible Structures for Benzene

C6H6 DU = 4 Dewar benzene: Dewar, James, Proceedings of the Royal Society

  • f Edinburgh 1866/1867, 6, 82.

Ladenburg benzene: Ladenburg, Albert, Chemische Berichte 1869, 2, 140. Loschmidt’s C6 = one hexavalent C concept: Loschmidt, Joseph Konstitutions Formeln der organischen Chemie in graphischer Darstellung, Chemische Studien I, Vienna, 1861.

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SLIDE 3

H H H H H H H H H H H H H H H H H H Kekulé Benzene C6H6 DU = 4

“ I was sitting writing at my textbook but the work did not progress; my thoughts were

  • elsewhere. I turned by chair to the fire and dozed. Again the atoms were gamboling before my
  • eyes. . .My mental eye. . .could now distinguish larger structures of manifold conformation:

long rows. . .all twining and twisting in snake-line motion. But look! What was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes. . .I awoke; and this time also I spent the rest of the night in working out the consequences of the hypothesis. Let us learn to dream, gentlemen, then perhaps we shall find the truth. But let us beware

  • f publishing our dreams till they have been tested by the waking understanding.”

(Rapp, J. “Kekulé Memorial Lecture,” Journal of the Chemical Society 1898, 73, 100.)

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SLIDE 4

H H H H H H H H H H H H

C6H6 Benzene X-ray crystallographic studies show that:

  • all C–C bonds are the same length, 1.397
  • all CCC are equivalent, 120 (therefore, all carbons are sp2-hybridized)
  • recall:

Csp2–Csp2 σ bond = 1.46 Csp2=Csp2 σ + π bond = 1.34

  • so, benzene’s carbon-carbon bonds are like the bonds of conjugated alkadienes,

with partial double bond character.

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SLIDE 5

LCAO diagrams of benzene sigma bond system pi bond system

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SLIDE 6

( H) = 4 kcal / mol 58 54 28 28 E kcal / mol H2 Pt /C H2 Pt /C H2 Pt /C H2 Pt /C 28 H2 Pt /C 54 H2 Pt /C H2 Pt /C 86 50 3 H2 Pt /C ( H) = 36 kcal / mol

Tremendous resonance stabilization:

1,3,5-cylcohexatriene, a hypothetical molecule, vs. benzene, a real molecule:

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SLIDE 7

Nomenclature of Arenes name as a substituted benzene: fluorobenzene (S)-sec-butylbenzene

  • r

(S)-(1-methylpropyl)benzene nitrobenzene H

F NO2

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SLIDE 8

H O OH OH O NH2 OH O NH2

Common Arenes:

toluene styrene 1,2-dimethylbenzene

  • rtho-xylene
  • -xylene

phenol benzaldehyde 1,3-dimethylbenzene meta-xylene m-xylene aniline benzoic acid 1,4-dimethylbenzene para-xylene p-xylene

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SLIDE 9

Three examples of multiply substituted arenes

1-bromo-4-methylbenzene 4-bromotoluene p-bromotoluene (ortho, meta, and para are used only for disubstituted benzenes) 2-ethyl-1-isopropyl-4-methylbenzene 3-ethyl-4-isopropyltoluene 2,4,6-trinitrotoluene (TNT)

Br NO2 O2N NO2

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SLIDE 10

Common substituents containing aromatic rings

, C6H5 – “phenyl” trans-2-phenyl-1-cyclohexanol , C6H5CH2 – “benzyl” cis-4-benzyl-3-bromo-1-cyclopentene

OH Br

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SLIDE 11

Do all all-sp2 hybridized ring systems exhibit aromaticity? 1,3-cyclobutadiene benzene (1Z,3Z,5Z,7Z)1,3,5,7-cyclooctaetraene 4 electrons 6 electrons 8 electrons C2-C3 and C4-C1 planar nonplanar, so p orbtials don’t overlap all p orbtials overlap p orbtials don’t overlap not aromatic aromatic not aromatic

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SLIDE 12

H H H H H H H H H H

Is (1E, 3Z, 5E, 7Z, 9Z)-cyclodecapentaene an aromatic hydrocarbon?

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SLIDE 13

N H H H H H N H H H H

H

N H H H H H .. N H H H H H ..

Aromaticity of Heterocycles

pyrrole

pyridine 6 π electrons in 5 p orbitals 6 π electrons in 6 p orbitals

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SLIDE 14

O H H H H O N H H H

H H

O H H H H .. .. O N H .. .. ..

Aromaticity of Heterocycles

furan

  • xazoline

6 π electrons in 5 p orbitals not aromatic: one of the ring carbons is sp3-hybridized

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SLIDE 15

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 %Transmittance 500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)

Spectroscopy of aromatic compounds

  • IR

aromatic

  • vertones

~ 1600 Csp2=Csp2 stretch > 3000 Csp2–H stretch ~ 900 - 680 Csp2–H bend

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SLIDE 16

Spectroscopy of aromatic compounds

  • NMR

benzylic hydrogens: ~ 2.2 - 3.0 ppm aromatic hydrogens: ~ 6.5 - 8.0 ppm

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SLIDE 17

Spectroscopy of aromatic compounds

  • NMR

CH CH aromatic C region: ~ 110 - 170 ppm

CH3 C

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SLIDE 18

Spectroscopy of aromatic compounds

  • MS

allkylbenzenes often have a prominent m/z = 91, the benzyl or tropilium ion

+ +

  • r
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SLIDE 19

Spectroscopy of aromatic compounds

  • UV-Vis

arenes typically have”B band” λmax in the region 254 - 280 nm;

  • ccasionally may also see the more intense “E band” at shorter λ.

λmax = 265 nm