Heterocyclic Aromatic Compounds - Faculty Personal Homepage- KFUPM

46 Pages·2008·0.6 MB·English

CHAPTER 19
Polycyclic And
Heterocyclic Aromatic
Compounds
1
Nomenclature of Polycyclic
Aromatic Compounds
Polycyclic aromatic compounds have two or more
benzene rings fused together
2
Monosubstituted Naphthalene is
often designated by Greak Letter
NO
α 2
α
β β
1-nitronaphthalene
β β
β-nitronaphthalene
α α
Cl
OH
2-naphthol
β-naphthol Cl
1,5-dichloronaphthalene
3
More Naming
Br
9,10-dibromoanthracene
Br
9,10-dimethoxy phenanthrene
OCH
3
OCH
4 3
Bonding in Polycyclic Aromatic
Compounds
• In benzene, all C-C bond lengths are equal (
equal distribution of pi electrons around the
benzene ring).
• In the polycyclic aromatic compounds, the C-C
bond lengths are not the same ( e,g. the
distance between carbon 1 and 2 =1.36 A° in
naphthalene is smaller than the distance
between Carbon 2 and 3 = 1.40 A°) (cid:206) there is
no equal distribution of pi electrons around
naphthalene
5
Comparison of bond lengths in
Naphthalene
o
1,42 A
o C-C in ethane: 1.54 A°
1.36 A
C-C in ethylene: 1.34 A°
o
1.40 A
C-C in benzene: 1.40 A°
o
1.39 A
8
1
2
7
6 3
5 4
Two out of three resonance
structures show a carbon 1
carbon 2 double bond
6
Resonance Energy of Polycyclic
Aromatic Compounds
36
61 84 92 Kcal/mol
The resonance energy per ring is less than that of
benzene (cid:206) suggest
• the polycyclic aromatic compounds are more reactive
than benzene.
• reaction at one ring loses less energy because the
product still have one or more intact benzenoid rings.
7
Example
Br
2
Br
Resonqance energy 36 X 2 = 72 Br
92
The product still
has two benzene
rings ; it loses
only 20 Kcal/mol
8
Oxidation of Naphthalene
• Naphthalene can be oxidized to products in
which much of the aromaticity is retained.
O
O
COH
V O
2 5 -H O
2
O
air
heat
COH
O
O
O
CrO
3
1,4-
CH CO H
3 2
naphthquinone
heat
9
O
Reduction of Polycyclic
Aromatic Compounds
The polycyclic aromatic compounds (unlike benzene)
can be hydrogenated without heat or pressure , or
they can be reduced with sodium and ethanol
Na, CH CH OH
3 2 no reaction
heat
Na, CH CH OH
3 2
heat
Na, CH CH OH
3 2
heat
10