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C6H6
Br2
CCl4, darkno reaction
C6H6
Br2
cat. FeBr3
C6H5Br + HBr
C6H6
xs. H2
cat. Pdno 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 aromaticbalsam 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: 8888max = 254 nm
< Shouldn’t benzene react like alkenes or alkynes?
H
H
H
H
H
H
HH
H
HHH
Some Possible Structures for Benzene
C6H6 DU = 4
Dewar benzene:
Dewar, James, Proceedings of the Royal Society of 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 organischenChemie in graphischer Darstellung, Chemische Studien I,Vienna, 1861.
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 wereelsewhere. I turned by chair to the fire and dozed. Again the atoms were gamboling before myeyes. . .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 thesnakes had seized hold of its own tail, and the form whirled mockingly before my eyes. . .Iawoke; and this time also I spent the rest of the night in working out the consequences of thehypothesis. Let us learn to dream, gentlemen, then perhaps we shall find the truth. But let us bewareof 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.)
H
H
H
H
H
H
H
H
H
H
H
H
C6H6 Benzene
X-ray crystallographic studies show that:
C all C–C bonds are the same length, 1.397 D
C all pCCC are equivalent, 120 E (therefore, all carbons are sp2-hybridized)
C recall:Csp2–Csp2 σ bond = 1.46 D
Csp2=Csp2 σ + π bond = 1.34 D
C so, benzene’s carbon-carbon bonds are like the bonds of conjugated alkadienes,with partial double bond character.
) ( )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:
Nomenclature of Arenes
name as a substituted benzene:
fluorobenzene (S)-sec-butylbenzene
or
(S)-(1-methylpropyl)benzene
nitrobenzene
HF
NO2
H
OOH
OH
ONH2
OH
ONH2
Common Arenes:
toluene styrene 1,2-dimethylbenzeneortho-xylene o-xylene
phenol benzaldehyde 1,3-dimethylbenzenemeta-xylene m-xylene
aniline benzoic acid 1,4-dimethylbenzenepara-xylene p-xylene
Three examples of multiply substituted arenes
1-bromo-4-methylbenzene4-bromotoluenep-bromotoluene
(ortho, meta, and para are used only for disubstituted benzenes)
2-ethyl-1-isopropyl-4-methylbenzene3-ethyl-4-isopropyltoluene
2,4,6-trinitrotoluene (TNT)
Br
NO2O2N
NO2
Common substituents containing aromatic rings
, C6H5 – “phenyl”
trans-2-phenyl-1-cyclohexanol
, C6H5CH2 – “benzyl”
cis-4-benzyl-3-bromo-1-cyclopentene
OH
Br
Do all all-sp2 hybridized ring systems exhibit aromaticity?
1,3-cyclobutadiene benzene (1Z,3Z,5Z,7Z)1,3,5,7-cyclooctaetraene
4 B electrons 6 B electrons 8 B 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
N HHH
HH
N
HH
H HH
N
H
H
HH
H..
NH
H
H
H
H..
Aromaticity of Heterocycles
pyrrole pyridine
6 π electrons in 5 p orbitals 6 π electrons in 6 p orbitals
OHH
HH
ON
HH
HH
H
O H
HH
H.... O
NH
..
..
..
Aromaticity of Heterocycles
furan oxazoline
6 π electrons in 5 p orbitals not aromatic: one of the ring carbons is sp3-hybridized
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
%T
rans
mitt
ance
500 1000 1500 2000 2500 3000 3500 4000
Wavenumbers (cm-1)
Spectroscopy of aromatic compounds
• IR
aromatic overtones
~ 1600Csp2=Csp2
stretch
> 3000Csp2–H stretch
~ 900 - 680Csp2–H bend
Spectroscopy of aromatic compounds
• NMR
benzylic hydrogens:~ 2.2 - 3.0 ppm
aromatic hydrogens: ~ 6.5 - 8.0 ppm
Spectroscopy of aromatic compounds
• MS
allkylbenzenes often have a prominentm/z = 91, the benzyl or tropilium ion
+
+or