Download Aromatic Compounds

CHE 242
Unit VI
The Study of Conjugated
Systems, Aromaticity, and
Reactions of Aromatic
Compounds
CHAPTER SIXTEEN
Terrence P. Sherlock
Burlington County College
2004
Resonance Structure
Each sp2 hybridized C in the ring has an
unhybridized p orbital perpendicular to
the ring which overlaps around the ring.
=>
Chapter 16
2
Unusual Reactions
• Alkene + KMnO4  diol (addition)
Benzene + KMnO4  no reaction.
• Alkene + Br2/CCl4  dibromide (addition)
Benzene + Br2/CCl4  no reaction.
• With FeCl3 catalyst, Br2 reacts with
benzene to form bromobenzene + HBr
(substitution!). Double bonds remain.
=>
Chapter 16
3
Unusual Stability
Hydrogenation of just one double
bond in benzene is endothermic!
=>
Chapter 16
4
Annulenes
• All cyclic conjugated
hydrocarbons were
proposed to be aromatic.
• However, cyclobutadiene
is so reactive that it
dimerizes before it can
be isolated.
• And cyclooctatetraene
adds Br2 readily.
• Look at MO’s to explain
aromaticity.
=>
Chapter 16
5
MO Rules for Benzene
• Six overlapping p orbitals must form six
molecular orbitals.
• Three will be bonding, three antibonding.
• Lowest energy MO will have all bonding
interactions, no nodes.
• As energy of MO increases, the number of
nodes increases.
=>
Chapter 16
6
Aromatic Requirements
• Structure must be cyclic with conjugated
pi bonds.
• Each atom in the ring must have an
unhybridized p orbital.
• The p orbitals must overlap continuously
around the ring. (Usually planar structure)
• Compound is more stable than its openchain counterpart.
=>
Chapter 16
7
Anti- and Nonaromatic
• Antiaromatic compounds are cyclic,
conjugated, with overlapping p orbitals
around the ring, but the energy of the
compound is greater than its open-chain
counterpart.
• Nonaromatic compounds do not have a
continuous ring of overlapping p orbitals
and may be nonplanar.
=>
Chapter 16
8
Hückel’s Rule
• If the compound has a continuous ring
of overlapping p orbitals and has 4N + 2
electrons, it is aromatic.
• If the compound has a continuous ring
of overlapping p orbitals and has 4N
electrons, it is antiaromatic.
=>
Chapter 16
9
[N]Annulenes
• [4]Annulene is antiaromatic (4N e-’s)
• [8]Annulene would be antiaromatic, but
it’s not planar, so it’s nonaromatic.
• [10]Annulene is aromatic except for the
isomers that are not planar.
• Larger 4N annulenes are not
antiaromatic because they are flexible
enough to become nonplanar.
=>
Chapter 16
10
Cyclopentadienyl Ions
• The cation has an empty p orbital, 4 electrons,
so antiaromatic.
• The anion has a nonbonding pair of electrons
in a p orbital, 6 e-’s, aromatic.
=>
Chapter 16
11
Pyridine
• Heterocyclic aromatic compound.
• Nonbonding pair of electrons in sp2
orbital, so weak base, pKb = 8.8.
Chapter 16
12
=>
Other Heterocyclics
Chapter 16
13
=>
Fused Heterocyclic
Compounds
Common in nature, synthesized for drugs.
=>
Chapter 16
14
Allotropes of Carbon
• Amorphous: small particles of graphite;
charcoal, soot, coal, carbon black.
• Diamond: a lattice of tetrahedral C’s.
• Graphite: layers of fused aromatic rings.
=>
Chapter 16
15
Some New Allotropes
• Fullerenes: 5- and 6-membered rings
arranged to form a “soccer ball” structure.
• Nanotubes: half of a C60 sphere fused to a
cylinder of fused aromatic rings.
=>
Chapter 16
16
Common Names of
Benzene Derivatives
OH
CH3
phenol
toluene
H
C CH2
styrene
OCH3
NH2
aniline
anisole
O
O
O
C
C
C
acetophenone
CH3
benzaldehyde
Chapter 16
H
OH
benzoic acid
=>
17
Disubstituted Benzenes
The prefixes ortho-, meta-, and para- are
commonly used for the 1,2-, 1,3-, and 1,4positions, respectively.
Br
Br
o-dibromobenzene or
1,2-dibromobenzene
NO2
HO
p-nitrophenol or
4-nitrophenol
=>
Chapter 16
18
3 or More Substituents
Use the smallest possible numbers, but
the carbon with a functional group is #1.
OH
O2N
O2N
NO2
NO2
NO2
2,4,6-trinitrophenol
NO2
1,3,5-trinitrobenzene
=>
Chapter 16
19
Common Names for
Disubstituted Benzenes
CH3
O
CH3
OH
C
OH
CH3
CH3
m-xylene
H3C
CH3
mesitylene
o-toluic acid
H3C
p-cresol
=>
Chapter 16
20
Phenyl and Benzyl
Phenyl indicates the benzene ring
attachment. The benzyl group has
an additional carbon.
CH2Br
Br
phenyl bromide
benzyl bromide
=>
Chapter 16
21
Physical Properties
• Melting points: More symmetrical than
corresponding alkane, pack better into
crystals, so higher melting points.
• Boiling points: Dependent on dipole
moment, so ortho > meta > para, for
disubstituted benzenes.
• Density: More dense than nonaromatics,
less dense than water.
• Solubility: Generally insoluble in water. =>
Chapter 16
22
IR and NMR Spectroscopy
• C=C stretch absorption at 1600 cm-1.
• sp2 C-H stretch just above 3000 cm-1.
• 1H NMR at 7-8 for H’s on aromatic
ring.
• 13C NMR at 120-150, similar to alkene
carbons.
=>
Chapter 16
23
Mass Spectrometry
=>
Chapter 16
24
=>
Chapter 16
25
POWER POINT IMAGES FROM
“ORGANIC CHEMISTRY, 5TH EDITION”
L.G. WADE
ALL MATERIALS USED WITH PERMISSION OF AUTHOR
PRESENTATION ADAPTED FOR BURLINGTON COUNTY COLLEGE
ORGANIC CHEMISTRY COURSE
BY:
ANNALICIA POEHLER STEFANIE LAYMAN
CALY MARTIN
Chapter 16
26