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.

LCAO diagrams of benzene

sigma bond system pi bond system

) ( )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

HH

H

HH

H

H

H

H

H

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

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

• NMR

CH

CH

aromatic C region:~ 110 - 170 ppm

CH3

C

Spectroscopy of aromatic compounds

• MS

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

+

+or

Spectroscopy of aromatic compounds

• UV-Vis

arenes typically have”B band” λmax in the region 254 - 280 nm;occasionally may also see the more intense “E band” at shorter λ.

λmax = 265 nm