Chapter 15 Benzene
I.
Benzene Structure and Nomenclature
A.
Structure of Benzene
1) Faraday in 1825 isolates a colorless liquid from whale oil
a) Empirical formula = CH (C need 4 bonds?)
b) Very inert
2)
Later, the molecular formula of C6H6 was determined and named benzene
a) Degrees of unsaturation = 4
b) 1,3,5-cyclohexatriene structure is proposed
c) Not reactive as conjugated polyenes should be
3)
Various possible structures are proposed:
a) Dewar Benzene
b) Ladenburg Prismane
c) Benzvalene
d) Claus Benzene
e) All but (d) go to benzene
a
b
c
d
4)
Reactivity of Benzene
a) Benzene is a relatively inert molecule: no reaction with Br2
b) Reaction in the presence of a catalyst with Br2 does give product
H
Br
H
H
H
H
Br2
FeBr3
H
H
H
Symmetric product
agrees with structure
H
H
H
Br
Br
H
H
H
Br
H
Br
Br2
FeBr3
H
H
H
H
H
+
H
H
H
c)
H
+
H
H
H
Br
Br
H
Br
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)
Br
Br
H
Br
H
Br
H
H
H
H
H
1,2 product
H
1,6 product
a)
The fact that we only have one 1,2-disubstituted product supports a
resonance hybrid structure
=
B.
Nomenclature
1) Aromatic Compounds = benzene and its substituted analogues
2) We draw them as a single resonance structure, but we always mean the
resonance hybrid
3)
Monosubstituted Benzenes are named with the substituent as prefix:
F
fluorobenzene
NO2
nitrobenzene
t-butylbenzene
4)
Disubstituted benzene have three possible arrangements
1) 1,2 is also known as ortho (o-)
2) 1,3 is also known as meta (m-)
3) 1,4 is also known as para (p-)
Br
Cl
Cl
NO2
1,2-dichlorobenzene
o-dichlorobenzene
5)
1-bromo-3-nitrobenzene
m-bromonitrobenzene
1-ethyl-4-(1-methylethyl)benzene
p-ethylisopropylbenzene
Polysubstituted: number with the lowest set possible, label substituents as
in cyclohexane nomenclature
NO2
NO2
Br
1-bromo-2,3-dimethyl benzene
NO2
1,2,4-trinitrobenzene
1-ethenyl-3-ethyl-5-ethynylbenzene
6)
Special cases: common names
OH
CHO
COOH
benzoic acid
benzaldehyde
phenol
CH3
CH2
CH3
COCH3
acetophenone
OCH3
CH3
toluene
NH2
CH3
H3C
o-xylene
CH3
styrene
mesitylene
a)
b)
anisole
Name with substituents before the common name
The substituent giving the common name is #1
OH
CH3
I
o-iodotoluene
Br
CH2
Br
Cl
Br
2,4,6-tribromophenol
m-chlorostyrene
aniline
7)
8)
9)
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
CH2OH
CH3
benzyl alcohol
1-phenyl-2-methylcyclohexane
II.
Aromaticity
A.
Structure of benzene revisited
1) Ring of six sp2 hybridized carbons
a) Six p-orbitals give six MO’s with six p electrons
b) p-cloud above and below the plane of the molecule
c) Completely symmetric
d) Not a cyclohexatriene structure
e)
2)
Orbital picture of benzene
Heats of Hydrogenation
DHo = -28.6 kcal/mol
DHo
H2
= -54.9 kcal/mol
DHo = -49.3 kcal/mol
Pt
DHocalculated = -78.9 kcal/mol
a)
b)
Resonance Energy of Benzene = 30 kcal/mol
Also called: Delocalization Energy, Aromaticity
C.
MO Description of Benzene
1) 1,3,5-hexatriene
a) 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
2)
Benzene is a cyclic system, which changes how the MO’s are arranged
a) 6 p-MO’s with 6 p-electrons
b) The nodes intersect
c) There are degenerate orbitals
3)
Energy comparison shows that the benzene structure is more stable
a) Benzene: 2 MO’s lowered in energy and 1 MO raised
b) Hexatriene: 1 MO lowered in energy and 2 MO’s raised
c)
Overlap of terminal p-orbitals in p1, p2, and p3 determine energies
d)
Aromatic Transition states favor concerted reactions:
O
O
Os
O
O
III. Spectroscopy of Aromatic Systems
A.
UV-Vis Spectroscopy
1) The energy gap between HOMO and LUMO is large for aromatics
because of extra aromatic stabilization
2) More energetic absorption is needed for electronic transition
a) lmax will be smaller than for trienes
b) Sample spectra
COOH
3)
NH2
B.
Substitution alters the energy levels and thus the spectrum and the color
1) Many dyes are aromatic compounds
2) Many sun-tan lotions contain PABA = p-aminobenzoic acid to block
UV rays (lmax = 289 nm, e = 18,600)
IR Spectroscopy
1) Typical aromatic IR bands
a) Aromatic C—H stretch = 3030 cm-1
b) Aromatic C—C = 1500-2000 cm-1
c) Aromatic C—H bending = 650-1000 cm-1
i. Can be used to determine substitution pattern
ii. C—H bending for different substitutions:
R
R
R
R
R
R
R
690-710
730-770
735-770
690-710
750-810
790-840
O-xylene
C.
1H
NMR Spectroscopy
1) Aromatic protons are highly deshielded due to ring current
a) Benzenes have C—H protons from 6.5-8.5 ppm
b) Alkenes are 4.6-5.7 ppm
2)
H2C
Benzylic groups are not as deshielded. Ring current fades quickly.
CH3 2.35 ppm
CH
CH3 1.68 ppm
CH2CH3 1.10 ppm
3)
Substitution pattern dictates the spectrum pattern
a) Benzene itself has only one singlet at 7.27 ppm
b) Spectral Examples
5)
Coupling Constants
1) Ortho H’s = 9 Hz
2) Meta H’s = 3 Hz
3) Para H’s < 1 Hz
D.
13C
NMR Spectroscopy
1) Ring current does not have a large effect on the carbons
2) Carbon shifts are very similar to the alkenes: 120-135 ppm
CH3 21.3
137.8
129.3
125.6
128.5
NO2
148.3
123.4
134.7
129.5
IV. Polycyclic Aromatic Hydrocarbons (PAH’s)
A.
Naphthalene
Structure and Nomenclature
1) Two or more benzene rings share 2 or more Carbons to be a PAH
2) The general name for the series is the acenes (pentacene = 5 rings)
3) Angular fusion gives different compounds
Anthracene
Phenanthrene
Tetracene
B.
Is Napthalene Aromatic?
1) White solid, mp = 80 oC, used as mothbolls
2) UV-Vis spectrum looks like a conjugated p-system, but with more
delocalization than in benzene
3)
Structure = Symmetric like benzene
4)
1H
NMR confirms that naphthalene is aromatic
C.
Large acenes are also aromatic. The more benzene rings the better.
1)
Anthracene
2)
Phenanthrene