1. AROMATIC COMPOUNDS (BENZENE AND TOLUENE)
ORGANIC CHEMISTRY CHM 207
CHAPTER 4:
AROMATIC COMPOUNDS (BENZENE AND TOLUENE)
NOR AKMALAZURA JANI

2. Aromatic compounds
Organic compound that contains a benzene ring in its molecule is known as an aromatic compounds.
Sometimes called arenes.
Molecular formula: C6H6
Represented as a regular hexagon containing an inscribed circle.

3. Structure of Benzene Can be represented in two abbreviated ways.
The corner of each hexagon represents a carbon and a hydrogen atom.

4. Kekulé Structure of Benzene
Molecular formula is C6H6
All the hydrogen atoms are equivalent
Each carbon atom must have four covalent bonds.

5. Resonance Structure
Resonance theory: the structure of benzene is a resonance hybrid structure of two Kekulé cononical forms.
The hybrid structure is often represented by a hexagon containing an inscribed circle.

6. Hexagonal ring – 6 carbon-carbon bonds are equal.
Circle – delocalised electrons of the benzene ring

7. CRITERIA OF AROMATIC COMPOUNDS
Structure must be cyclic, containing some number of conjugated pi bonds.
Each atom in the ring must have an unhybridized p orbital. (The ring atoms are usually sp2 hybridized or occasionally sp hybridized).
The unhybridized p orbitals must overlap to form a continuous ring of parallel orbitals. The structure must be planar (or nearly planar) for effective overlap to occur.
Delocalization of the pi electrons over the ring must lower the electronic energy.
* Antiaromatic compound: fulfills the first three criteria, but delocalization of the pi electrons over the ring increase the electronic energy.

8. Huckel’s rule
Used to determine aromaticity for planar, cyclic organic compounds with a continous ring of overlapping p-orbitals.
If the number of pi (π) electrons in the monocyclic system is (4N+2), the system is aromatic. N is 0, 1, 2, 3…..
Systems that have 2, 6 and 10 pi electrons for N = 0, 1, 2 is a aromatic.
Systems that have 4, 8, and 12 pi electrons for N = 1, 2, 3 are antiaromatic.

9. Naming Aromatic Compounds

10. A substituted benzene is derived by replacing one or more of benzene’s hydrogen atoms with an atom or group of atoms.
A monosubstituted benzene has the formula C6H5G where G is the group that replaces a hydrogen atom.
All hydrogens in benzene are equivalent.
It does not matter which hydrogen is replaced by G.

11. Monosubstituted Benzenes

12. The name is written as one word.
Some monosubstituted benzenes are named by adding the name of the substituent group as a prefix to the word benzene.
The name is written as one word.
nitro group
ethyl group
nitrobenzene
ethylbenzene

13. phenol toluene Certain monosubstituted benzenes have special names.
These are parent names for further substituted compounds.
hydroxy group
methyl group
phenol
toluene

14. carboxyl group
amino group
benzoic acid
aniline

15. Disubstituted Benzenes

16. Three isomers are possible when two substituents replace hydrogen in a benzene molecule.
The prefixes ortho-, meta- and para- (o-, m- and p-) are used to name these disubstituted benzenes.

17. ortho disubstituted benzene
substituents on adjacent carbons
ortho-dichlorobenzene (1,2-dichlorobenzene) mp –17.2oC, bp 180.4oC

18. meta disubstituted benzene
substituents on adjacent carbons
meta-dichlorobenzene (1,3-dichlorobenzene) mp –24.82oC, bp 172oC

19. para disubstituted benzene
substituents are on opposite sides of the benzene ring
para-dichlorobenzene (1,4-dichlorobenzene) mp 53.1, bp 174.4oC

20. When one substituent corresponds to a monosubstituted benzene with a special name, the monosubstituted compound becomes the parent name for the disubstituted compound.
3-nitrophenol
phenol

21. When one substituent corresponds to a monosubstituted benzene with a special name, the monosubstituted compound becomes the parent name for the disubstituted compound.
3-nitrotoluene
toluene

22. Tri- and Polysubstituted Benzenes

23. When a benzene ring has three or more substituents, the carbon atoms in the ring are numbered.
Numbering starts at one of the substituent groups.
The numbering direction can be clockwise or counterclockwise.
Numbering must be in the direction that gives the substituent groups the lowest numbers.

24. 6-chloro clockwise numbering 1-chloro 4-chloro 1,4,6-trichlorobenzene5 1 4 2 3
1,4,6-trichlorobenzene

25. counterclockwise numbering
4-chloro
1-chloro
2-chloro
counterclockwise numbering
chlorine substituents have lower numbers 2 3 1 4 6 5
1,2,4-trichlorobenzene

26. When a compound is named as a derivative of the special parent compound, the substituent of the parent compound is considered to be C-1 of the ring.

27. 2,4,6-trinitrotoluene (TNT)5 1 6 3 4 2 1 6 2 5 3 4
toluene
2,4,6-trinitrotoluene (TNT)

28. When the hydrocarbon chain attached to the benzene ring is small, the compound is named as benzene derivative.
Example:

29. Naming compounds that cannot be easily named as benzene derivatives
Benzene named as a substituent on a molecule with another functional group as its root by the prefix phenyl.
diphenylmethane
4-phenyl-2-pentene

30. The phenyl group, C6H5-

31. If the hydrocarbon chain contains more than three carbon atoms, phenyl is used as part of the name.
Examples:

32. PHYSICAL PROPERTIES OF BENZENE AND ITS DERIVATIVES
Benzene derivatives tend to be more symmetrical than similar aliphatic compounds, and pack better into crystals and have higher melting points.
Density:
- Slightly dense than non-aromatic analogues, but still less dense than water.
- halogenated benzenes are denser than water.
Insoluble in water
Boiling points depends on the dipole moments of compounds.

33. ELECTROPHILIC SUBSTITUTION REACTIONS OF BENZENE
REACTION OF BENZENE
ELECTROPHILIC SUBSTITUTION REACTIONS OF BENZENE
stability of π-electron system is lost when benzene undergoes addition reactions.
benzene and its derivatives undergo substitution reaction rather than addition reactions.
product of substitution reactions: aromatic compounds and not saturated compounds.

34. Mechanism of electrophilic substitution of benzene

35. ELECTROPHILIC SUBSTITUTION REACTIONS

36. ELECTROPHILIC SUBSTITUTION REACTIONS

37. Reagents, electrophiles and catalysts in electrophilic substitution reactions
Halogenation
Cl2 or Br2
AlCl3, AlBr3, FeCl3 or FeBr3
Cl , Br
Nitration
HNO3
H2SO4
NO2
Alkylation
RCl
RCH=CH2
AlCl3 R
RCH-CH3
Acylation
RCOCl
RCO
Sulphonation
SO3
SO3H

38. HALOGENATION OF BENZENE

39. MECHANISM: BROMINATION OF BENZENE

40. MECHANISM: NITRATION OF BENZENE

41. MECHANISM: FRIEDEL-CRAFTS ALKYLATION

42. MECHANISM: FRIEDEL-CRAFTS ACYLATION

43. Ortho-Para and Meta Directing Substituents
When substituted benzenes undergo further substituents, the substituent group present in the benzene derivative will influence electrophilic substitution in 2 ways which are:
i) Reactivity
ii)Orientation

44. EFFECTS OF SUBSTITUENTS ON THE REACTIVITY OF ELECTROPHILIC AROMATIC SUBSTITUTION
Substituent group present in the benzene ring can influence the rate of reaction of further substitutions.
Electron-donating groups make the ring more reactive (called activating groups) thus influence the reaction become faster.
Electron-withdrawing groups make the ring less reactive (called deactivating groups) thus influence the reaction become slower.

45. EFFECTS OF SUBSTITUENTS ON THE ORIENTATION OF ELECTROPHILIC AROMATIC SUBSTITUTION
A substituents group already in the ring influences the position of further electrophilic substitution whether at ortho, meta or para position.
Ortho-para directors: the groups that tend to direct electrophilic substitution to the C2 and C4 positions.
Meta directors: the groups that tend to direct electrophilic substitution to the C3 position.

46. Effetcs of substituent groups on the benzene ring
Activating groups (electron donating)
Deactivating groups
(electron-withdrawing)
-NH R
-OH
-OR
-NHCOCH3 -F
-Cl
-Br -I
ortho-para directors
meta directors

49. REACTIONS OF BENZENE DERIVATIVES
Alkylbenzene such as toluene (methylbenzene) resembles benzene in many of its chemical properties.
It is preferable to use toluene because it is less toxic.
The methyl group activates the benzene nucleus.
Toluene reacts faster than benzene in all electrophilic substitutions.

50. Reactions of toluene Reactions of the methyl group benzene ring
Substitution
-halogenation
Oxidation
Electrophilic
substitutions
- Halogenation
- Nitration
Friedel-Crafts reactions
Sulfonation
Addition reaction
-hydrogenation

51. SIDE-CHAIN REACTIONS

52. OXIDATION REACTION OF ALKYLBENZENE

53. HALOGENATION OF TOLUENE
Side chain substitution
* Bromination of toluene takes place under similar conditions to yield corresponding bromine derivatives.

54. SYNTHESIZING A SUBSTITUTED AROMATIC COMPOUNDS
Synthesis m-chloronitrobenzene starting from benzene
Two substituents: -NO2 (meta-directing) and –Cl (ortho- and para-directing)
Cannot nitrate chlorobenzene because the wrong isomer (o- and p-chloronitrobenzenes) would formed.

56. SYNTHESIZING A SUBSTITUTED AROMATIC COMPOUNDS
Synthesis p-bromobenzoic acid starting from benzene
Two substituents: -COOH (meta-directing) and –Br (ortho- and para-directing)
Cannot brominated benzioc acid because the wrong isomer (m-bromobenzoic acid) would formed.
Oxidation of alkylbenzene side chains yields benzoic acids.
Intermediate precursor is p-bromotoluene

57. Immediate precursor of p-bromotoluene:
Bromination of toluene or
ii) Methylation of bromobenzene

58. Immediate precursor of toluene:
Benzene was methylated in a Friedel-Crafts reaction
Immediate precursor of bromobenzene:
Bromination of benzene

59. TWO WORKABLE ROUTES FROM BENZENE TO p-BROMOBENZOIC ACID

60. USES OF BENZENE AND TOLUENE
- as solvent for oils and fats
- starting material for making other chemicals. For example, benzene is used in the cumene process to produce phenol.
- making organic compounds such as phenylethene (styrene) and nitrobenzene. These organic compounds are then used to make plastics (polystyrene), dyes and nylon.

61. USES OF BENZENE AND TOLUENE
- A common solvent, able to dissolve paints, paint thinners, silicone sealants, many chemical reactants, rubber, printing ink, adhesives (glues), lacquers, leather tanners and disinfectants.
- As a solvent to create a solution of carbon nanotubes.
- Dealkylation to benzene (industrial uses).
- As an octane booster in gasoline fuels used in internal combustion engines.
-As a coolant in nuclear reactor system loops.