Transcript Document

Chemistry 20
Chapter 4
Aromatic Hydrocarbons
Aromatic Compounds
Unsaturated hydrocarbons
Some of them have pleasant odors.
Aromatic compounds
Aromatic Compounds
Aromatic compounds contain benzene ring.
Benzene has:
C6H6
• 6 C atoms and 6 H atoms.
• Two possible Lewis structures.
H
H
C
C
H
C
C
H
C
C
H
H
H
H
C
C
H
C
C
H
C
C
H
H
Aromatic Compounds
Arene: A compound containing one or more benzene rings.
H
H
C
C
H
C
C
C
C
H
H
Naph th ale n e
An thracen e
H
ek ulé structure
A Keku lé structu re
w ing all atoms as a line-angle formula
Aryl group: When we remove a H atom from an arene (Ar-).
H
H
C
C
H
C
C
C
C
H
Phenyl C6H5H
H
Kek ulé structure
A Keku lé structu re
Ph e n an t
Naming of Aromatic Compounds
Aromatic compounds are named:
• With benzene as the parent chain.
• Name of substituent comes in front of the “benzene”.
CH
CH32 CH3
Ethylbenzene
methylbenzene
CH2 CH3
Cl
CH
3
Ethylbenzene
Toluene
chlorobenzene
CH
CH -CH
CH33
CH=CHCH
2 3 22
CH=CHC
2
Toluene
Ethylbenzene Styrene
Tolu
Styrene
ethylbenzene
OH
en ol
Naming of Aromatic Compounds
Some substituted benzenes have common names.
O
CH
OH
NH
OH3
OCH
NHOCH
2
C-H
3
2 3
Toluene
Phen ol
OCH3
A nisole
NH2
A niline
Phen ol A niline
A nisoleBenAzaldehyde
niline Ben
A nisole
O
C-H
Ben zaldehyde
O
C-OH
Benzoic acid
Naming of Aromatic Compounds
When two groups are attached to benzene, the ring is numbered to give
the lower numbers to the substituents.
-
Start numbering from a special name (if we have).
If we do not have, number them to get the smallest set of numbers.
List them by alphabetical order.
COOH
1
Br
2
1
CH2 CH3
CH3
4
3
2
3
CH3
3-methyltoluene
2-Bromobenzoic acid
(o-Bromoben zoic acid)
Ortho
1,3-Dimethylben zene
(m-Xylen e)
Meta
2
1
Cl
1-Ch loro-4-ethylbenzen e
(p-Chloroeth ylb enzene)
Para
Naming of Aromatic Compounds
If we have three or more substituents:
- Start numbering from a special name (if we have).
- If we do not have, number them to get the smallest set of numbers.
- List them by alphabetical order.
1
NO2
2
NO2
OH
CH3
3
4
Cl
4-Ch loro-2nitrotoluen e
Br
6
1
5
Br
2
3
4
Br
2,4,6-Trib romoph enol
4
3
2
Br
CH2 CH3
1
2-Bromo-1-ethyl-4n itroben zene
Naming of Aromatic Compounds
Aryl group: When we remove a H atom from an arene (Ar-).
H
H
C
C
H
C
C
C
C
H
Phenyl C6H5H
H
Kek ulé structure
A Keku lé structu re
ow ing all atoms as a line-angle formula
4
1
Ph enyl group
1-Ph enylcyclohexene
C6 H5
3
2
1
4-Phen yl-1-b utene
Chemical properties of benzene
H
Resonance: stable
H
C
C
H
C
C
H
C
C
H
H
H
H
C
C
H
C
C
H
C
C
H
H
Resonance hybrid: a molecule with two or more Lowis structures.
Contributing structure
Chemical properties of benzene
No addition reactions (almost unreactive)
Aromatic substitution: One of the H atoms is repalecd by some groups.
Halogenation
Nitration
Sulfonation
Chemical properties of benzene
1. Halogenation:
Cl and Br react rapidly with benzene in the presence of an iron catalyst.
H + Cl2
Benzen e
FeCl3
Cl + HCl
Ch lorobenzen e
Chemical properties of benzene
2. Nitration:
In presence of concentrated nitric acid and sulfuric acid,
one of the H atoms is replaced by a nitro (-NO2) group.
H + HNO3
H2 SO4
NO2 + H2 O
N itrob enzene
Chemical properties of benzene
3. Sulfonation:
In presence of concentrated sulfuric acid and heat,
one of the H atoms is replaced by sufonic acid (-SO3H)
group.
H + H2 SO4
Heat
SO3 H + H2 O
Benzenes ulfon ic acid
Phenol
CH2 CH3
OH
• Phenol is the IUPAC name for
benzene with a hydroxyl group.
Ethylbenzene
Phenol
OH
OH
1
OH OH
OH
3
Phen ol
OH
OH
1
NO2
OH
OH
OH
2
3
Cl OH
OH
Phen ol 3-Chloro-2-nitrophenol
3-Methylph
enol 1,2-Benzened
iol edio
1,3-B
3-Methylph enol 1,2-Benzened
iol 1,3-Benzened
iol 1,4-Benzen
ol)
(Catech
ol) uinone
(R
(m-Cres
ol)
(Catech ol) (m-Cres
(Resorcinol)
(Hydroq
(m-Cresol)
Phenol
- Many are used as antiseptics and disinfectants.
- Phenols are found widely in nature.
- Phenols are weak acids (pKa~10).
- Phenols are insoluble in water.
OH + NaOH
Phenol
H2 O
O- Na + + H2 O
S od ium phenoxide
(a w ater-soluble salt)
Phenols as antioxidants
Autoxidation: foods and other materials that contain C=C are
oxidized only in presence of oxygen (no other reactant).
R-H group is converted to an R-O-O-H (hydroperoxide)
group.
H
light
CH2 CH=CH-CH
+ O2
or heat
Oxygen
Section of a fatty
acid hydrocarbon chain
O-O-H
CH2 CH=CH-CH-CH2
A hydroperoxide
Phenols as antioxidants
Phenols are antioxidants and can prevent autoxidation.
Vitamin E is a natural antioxidant.
OH
O
H
3
HO
Vi tami n E
The foods are stored in longer period.
Bu tyl ate dh ydroxytolu e n e (BHT)
Bu tyl
an i