1. Aromatic Hydrocarbons
Chemistry 20
Chapter 4
Aromatic Hydrocarbons

2. Aromatic Compounds Unsaturated hydrocarbons
Some of them have pleasant odors.
Aromatic compounds

3. Aromatic Compounds Aromatic compounds contain benzene ring.
Benzene has:
6 C atoms and 6 H atoms.
Two possible Lewis structures.
C6H6

4. Aromatic Compounds
Arene: A compound containing one or more benzene rings.
Aryl group: When we remove a H atom from an arene (Ar-).
Phenyl C6H5-

5. Naming of Aromatic Compounds
Aromatic compounds are named:
With benzene as the parent chain.
Name of substituent comes in front of the “benzene”.
methylbenzene chlorobenzene ethylbenzene
CH3 Cl
CH2-CH3

6. Naming of Aromatic Compounds
Some substituted benzenes have common names.
CH3
Toluene

7. 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.
3-methyltoluene
Ortho
Meta
Para

8. 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.

9. Naming of Aromatic Compounds
Aryl group: When we remove a H atom from an arene (Ar-).
Phenyl C6H5- 1

10. Chemical properties of benzene
Resonance: stable
Resonance hybrid: a molecule with two or more Lowis structures.
Contributing structure

11. Chemical properties of benzene
No addition reactions (almost unreactive)
Aromatic substitution: One of the H atoms is repalecd by some groups.
Halogenation
Nitration
Sulfonation

12. Chemical properties of benzene
1. Halogenation:
Cl and Br react rapidly with benzene in the presence of an iron catalyst.

13. 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.

14. 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.
Heat

15. Phenol OH Phenol is the IUPAC name for benzene with a hydroxyl group.1 1 2 3 Cl 3
3-Chloro-2-nitrophenol
(m-Cresol)

16. 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.

17. 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.

18. Phenols as antioxidants
Phenols are antioxidants and can prevent autoxidation.
Vitamin E is a natural antioxidant.
The foods are stored in longer period.