Benzene Benzaldehyde O H Toluene CH 3 aromatic: usually fragrant ; benzaldehyde: from cherries, peaches, almonds ; benzene: coal distillate ; toluene: Tolu balsam Aromatic; benzene-like, six-membered ring with three double bonds Ch.15 Benzene and Aromaticity
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Benzene Benzaldehyde
O
H
Toluene
CH3
aromatic: usually fragrant ; benzaldehyde: from cherries, peaches, almonds; benzene: coal distillate; toluene: Tolu balsam
Aromatic; benzene-like, six-membered ring with three double bonds
Ch.15 Benzene and Aromaticity
O
HO
CH3
H
H H
Estrone
aromatic compounds; benzene-like structure
N
N
Cl
OH3C
Diazepam(tranquilizer)
O NCH3
HO
H HHO
H
Morphine
benzene: toxic, cause bone-marrow depression prolonged exposure → leukopenia (lowered white blood cell count)
- come from coal and petroleum- coal: complex mixture, arrays of benzene-like rings- thermal breakdown of coal at 1000oC in the absence of air boils off volatile products coal tar- fractional distillation of coal tar yields benzene, toluene, xylene, naphthalene, etc.
15.1 Sources of Aromatic Hydrocarbons
- petroleum: contains few aromatic compounds, consists largely of alkanesbut, during petroleum refining, aromatic molecules are formed (500oC, high pressure); eg) heptane is converted to toluene by dehydrogenation and cyclization
Benzene Toluene
CH3
Xylenes
CH3H3C
Indene
Naphthalene Biphenyl Anthracene Phenanthracene
Some aromatic hydrocarbons found in coal tar
CH3
OH
NH2
C
CHO
COOH
CN
CH3
CH3
O
CH3
Toluene
Phenol
Aniline
Acetophenone
Benzaldehyde
Benzoic acid
Benzonitrile
ortho-Xylene
Cumene Styrene
15.2 Nomenclature- a lot of common names are used
Br NO2
Bromobenzene Nitrobenzene
Et
Ethylbenzene
Monosubstituted benzenes:- benzene as parent name- as a substituent: C6H5- (phenyl), C6H5CH2- (benzyl),
A phenyl group 2-Phenylheptane
2
1
A benzyl group
Cl Me
ortho-Dichlorobenzene meta-Xylene
Cl
Me CHO
Cl
para-Chlorobenzaldehyde
Disubstituted benzenes:- ortho (o), meta (m), para (p)
X
ortho
meta
para
meta
ortho
Br CH3
CH31
24
4-Bromo-1,2-dimethylbenzene
Benzenes with more than two substitents- numbering: lowest numbers possible, alphabetically
O2N Cl
NO21
24
2-Chloro-1,4-dinitrobenzene
Common names as parent names: with the principal substituent assumed to be on carbon 1
O2N NO21
2
4
2,4,6-Trinitrotoluene
CH3
NO2
Br Br12
2,6-Dibromophenol
OH
Cl COOH
m-Chlorobenzoic acid
H
HH
HH
H Iron
Br2
Br
HH
HH
H
H
HH
BrH
Br
all six hydrogens are equivalent
Not formed
H
H
+ HBr
15.3 Structure and Stability of Benzene
- benzene is relatively unreactive toward most reagents that react with alkenes
- benzene reacts with Br2 in the presence of iron catalyst: substitution product but not addition product
OsO4
H3O+
HBr
OH
OH
OH
Br
Benzene does not undergo electrophilic addition reactions.
OsO4
H3O+
HBr
No reaction
No reaction
No reaction
Why is benzene unreactive compared with other alkenes?Why does benzene give a substitution product rather than an addition product on reaction with Br2?
-118 kJ/mol
-230 kJ/mol-356 kJ/mol(expected)
150 kJ/mol(difference)
-206 kJ/mol(actual)
Heat of hydrogenations
- benzene has 150 kJ/mol extra stability
H
HH
HH
H
H
HH
H H
H
1.5 bonds average
The Resonance Proposal
- resonance stabilization in benzene: more stable, less reactive than normal isolated alkenes
bond length: C-C (154 pm), C=C (134 pm)but benzene: 139 pm (all C-C bonds are equivalent)
15.4 Molecular Orbital Description of Benzene
structural properties of benzene: - flat, symmetrical, regular hexagon- bond angle: 120o, sp2
- six orbitals are equivalent
C
C
C C
C
H
H H
C H
HH
ψ2 ψ3
ψ1
ψ4* ψ5
*
ψ6*
Antibonding
Bonding
Nonbonding
six p- atomic orbitals
six benzene molecular orbitals
Benzene π molecular orbitals
Degenerated: orbitals with same energy level (ψ2 and ψ3, ψ4∗ and ψ5∗)
15.5 Aromaticity and the Hückel 4n + 2 Rule
review of benzene: - a cyclic conjugated molecule- unusually stable- planar, regular hexagon- substitution reaction (not addition reaction)- resonance hybrid
Hückel 4n + 2 Rule: needed to complete a description of aromaticity
For a planar, monocyclic system of conjugation;- aromatic: 4n + 2 electrons, n = 0, 1, 2, 3... (2, 6, 10, 14...), stabilized- antiaromatic: 4n electrons, (4, 8, 12, 16...), unstabilized
- complex motion of ring - 1500, 1600 bands are intense
1450-1600 cm-1
- weak1660-2000 cm-1C-H out of plane stretching
- low intensity- occurs left of typical saturated C-H band
3030 cm-1
C-H stretching
UV Spectroscopy
255-275 nmless intense205 nmintense
NMR Spectroscopy
110-140 ppm13C NMR6.5-8.0 ppm1H NMR
ring current: low field shifts (vinylic protons, 4.5-6.5 ppm)
Aromatic ring current: aromatic protons are deshielded by induced magnetic field caused by delocalized π electrons circulating in the molecular orbitals of the aromatic ring
π systems: C=C, C=O
induced magnetic field
proton deshielded by induced field
H0
Inside protons of aromatic ring: shielded, upfield shift
HH
HHH
H
HH
H
H
HH H
H
H
H
HH
[18]Annulene Inside H: δ -3.0
outside H: δ 9.3
O O
HH
δ 4.41
- ring current is characteristic of all Hückel aromatic molecules and is a good test of aromaticity
H 147 pm
134 pm
5.7 ppmH
HH
HH
H
7.37 ppm
H
HH
HH
CH2
2.3-3.0 ppm
R
- benzylic position: downfield shift
CH3 Cl
128.4
21.3137.7
129.3128.5
138.8127.6128.4
125.6 125.4
133.7 128.1
126.0
13C NMR
110-140 ppm13C NMR6.5-8.0 ppm1H NMR
- intense C-H out-of-plane bending600-900
- benzylic protons- aryl protons
2.3-3.06.5-8.0
1H NMR (δ)
- aromatic ring carbons110-14013C NMR (δ)
- weak255-275
- two absorptions due to ring motions1500, 1600
- intense205UV (nm)
- aryl C-H stretch3030
IR (cm-1)
Summary of Spectroscopic Information on Aromatic Compounds
CH2OH
OH
Salicyl alcohol
COOH
OH
Salicylic acid
COOH
O
Aspirin
CO
CH3
analgesic, anti-flammatorynonsteroidal anti-inflammatory drug (NSAID)
Aspirin, NSAID's, and COX-2 Inhibotors
• Aspirin: toxic side effects; stomach bleeding, long-term allergic reactionsReye's syndrome: fatal, often seen in children recovering from the flu