1. Aromatic Hydrocarbons
Nomenclature of the Derivatives of Benzene
Physical Properties of Aromatic Hydrocarbons
Preparation of Benzene
Reactions of Benzene
Alkylbenzenes

2. Nomenclature of Benzene Derivatives
Nomenclature of the Derivatives of Benzene
Nomenclature of Benzene Derivatives
1. Monosubstituted benzenes
(a) For certain compounds, benzene is the parent name and the substituent is simply indicated by a prefix

3. Nomenclature of the Derivatives of Benzene
(b) For other compounds, the substituent and the benzene ring taken together may form a new parent name

4. Nomenclature of Benzene Derivatives
There are three different ways that two groups can be attached to a benzene ring, so a prefix—ortho, meta, or para—can be used to designate the relative position of the two substituents.
ortho-dibromobenzene or
o-dibromobenzene
or 1,2-dibromobenzene
meta-dibromobenzene or
m-dibromobenzene
or 1,3-dibromobenzene
para-dibromobenzene or
p-dibromobenzene
or 1,4-dibromobenzene

5. Nomenclature of Benzene Derivatives
If the two groups on the benzene ring are different, alphabetize the names of the substituents preceding the word benzene.
If one substituent is part of a common root, name the molecule as a derivative of that monosubstituted benzene.

6. Nomenclature of Benzene Derivatives
For three or more substituents on a benzene ring:
Number to give the lowest possible numbers around the ring.
Alphabetize the substituent names.
When substituents are part of common roots, name the molecule as a derivative of that monosubstituted benzene. The substituent that comprises the common root is located at C1.

7. Nomenclature of the Derivatives of Benzene

8. Nomenclature of the Derivatives of Benzene
When a substituent is one that when taken together with the benzene ring gives a new parent name, that substituent is assumed to be in position 1 and the new parent name is used

9. Nomenclature of Benzene Derivatives
A benzene substituent is called a phenyl group, and it can be abbreviated in a structure as “Ph-”.
Therefore, benzene can be represented as PhH, and phenol would be PhOH.

10. Example 31-1 Answer Solution:
Nomenclature of the Derivatives of Benzene
Example 31-1
Draw the structural formula for each of the following compounds:
(a) 1,3,5-Trichlorobenzene
(b) 2,5-Dibromophenol
(c) 2,4-Dinitrobenzoic acid
Answer
Solution:
(a) (b)
(c)

11. 31.2 Nomenclature of the Derivatives of Benzene (SB p.150)
Check Point 31-1
Give the IUPAC name for each of the following compounds:
(a) (b)
(c)
(d)
(a) 1,2-Dimethylbenzene
(b) 1-Methyl-2-nitrobenzene or 2-nitrotoluene
(c) 3-Bromo-5-chlorobenzoic acid
(d) 4-Bromo-2,6-dinitrophenol
Answer

12. Give the IUPAC name for each compound.
PhCH(CH3)2
isopropylbenzene
m-butylphenol
2-bromo-5-chlorotoluene

13. Which structure matches the given name?
o-dichlorobenzene

14. 4-chloro-1,2-diethylbenzene

15. Physical properties of aromatic hydrocarbons:
have a fragrant smell
generally less dense than water at 20°C
usually immiscible with water
soluble in organic solvents

16. Boiling point (°C) Melting point (°C) Density at 20°C (g cm–3)
Physical Properties of Aromatic Hydrocarbons
Name
Formula
Boiling point (°C)
Melting point (°C)
Density at 20°C (g cm–3)
Benzene
80.1
5.5
0.878
Methylbenzene
111
–95
0.867
Ethylbenzene
136
–94

17. Boiling point (°C) Melting point (°C) Density at 20°C (g cm–3)
Physical Properties of Aromatic Hydrocarbons
Name
Formula
Boiling point (°C)
Melting point (°C)
Density at 20°C (g cm–3)
1,2-Dimethylbenzene
144
–25.2
0.880
1,3-Dimethylbenzene
139
–47.4
0.864
1,4-Dimethylbenzene
138
13.3
0.861

18. Industrial Preparation Catalytic Reforming of Alkanes
Preparation of Benzene
Industrial Preparation
Catalytic Reforming of Alkanes
Catalytic reforming converts alkanes and cycloalkanes into aromatic hydrocarbons
e.g. Pt
C6H14  C6H6 + 4H2
500°C, 10 – 20 atm

19. Destructive Distillation of Coal
31.5 Preparation of Benzene (SB p.157)
Destructive Distillation of Coal
Heating coal in the absence of air gives out coal gas, ammoniacal liquor, coal tar (قطران الفحم) and coke (فحم الكوك)
Coal tar is a mixture of many organic compounds, mainly aromatic ones
Benzene and methylbenzene can be obtained

20. Decarboxylation of Sodium Salt of Benzoic Acid
Preparation of Benzene
Laboratory Synthesis
Decarboxylation of Sodium Salt of Benzoic Acid
When sodium benzoate is fused with sodium hydroxide, the carboxylate group is removed and benzene is formed

21. Preparation of Benzene
Reduction of Phenol
Phenol vapour is passed slowly over heated zinc dust to produce benzene and zinc(II) oxide

22. Reactions of Benzene
Comparative Investigation of Chemical Properties of Cyclohexane, Cyclohexene and Benzene
Reaction
Cyclohexane (a saturated alicyclic hydrocarbon)
Cyclohexene (an unsaturated alicyclic hydrocarbon)
Methylbenzene (an aromatic hydrocarbon)
Action of Br2 in CH3Cl3 (in dark)
No reaction
Br2 decolourized and no HBr evolved
No reaction with Br2 alone
In the presence of FeBr3, Br2 decolourized and HBr fumes evolved
Action of H2 (with Ni catalyst)
1 mole of cyclohexene reacts with 1 mole of H2 at room temperature
1 mole of methylbenzene reacts 3 moles of H2 at high temperature and pressure

23. Cyclohexane (a saturated alicyclic hydrocarbon)
Reactions of Benzene
Reaction
Cyclohexane (a saturated alicyclic hydrocarbon)
Cyclohexene (an unsaturated alicyclic hydrocarbon)
Methylbenzene (an aromatic hydrocarbon)
Action of acidified KMnO4
No reaction
KMnO4
decolourized
Action of conc. HNO3 and conc. H2SO4
Cyclohexene oxidized and colour darkens
A yellow liquid is formed

24. Reactions of Benzene
Methylbenzene is highly unsaturated, but it is resistant to oxidation and addition reactions
The resistance of oxidation and addition reactions of aromatic compounds is used to distinguish from unsaturated alkenes
Methylbenzene reacts with Br2 in the presence of FeBr3. It is through substitution reaction

25. Electrophilic Aromatic Substitution Reactions
Reactions of Benzene
Electrophilic Aromatic Substitution Reactions
Most characteristic reaction of aromatic compounds:
Electrophilic substitution reactions
The electrophiles attack the benzene ring, replacing one of the hydrogen atoms in the reaction
Electrophiles are either a positive ion (E+) or some other electron-deficient species with a partial positive charge (+)

26. Reactions of Benzene
Nitration
Conc. H2SO4 increases the rate of reaction by increasing the concentration of the electrophile, NO2+ (nitronium ion)

27. Reactions of Benzene
Sulphonation
Benzene reacts with fuming sulphuric(VI) acid at room temperature to give benzenesulphonic acid
Heating aqueous solution of benzenesulphonic acid above 100°C, benzene and sulphuric(VI) acid are formed

28. Reactions of Benzene
Halogenation
Benzene reacts with chlorine and bromine in the presence of catalysts such as AlCl3, FeCl3, FeBr3, to give chlorobenzene and bromobenzene respectively

29. Reactions of Benzene
Alkylation
When benzene is warmed with a haloalkane in the presence of catalysts such as AlCl3, an alkylbenzene is formed
Important step in chemical industry to produce polystyrene, phenol and detergents

30. Reactions of Benzene
Example 31-2
Complete each of the following by supplying the missing reactant or product as indicated by the question mark:
(a)
(b)
(c)
Solution:
(a)
(b) conc. H2SO4, conc. HNO3
(c) fuming H2SO4
Answer

31. Reactions of Benzene
Check Point 31-2
(a) One mole of benzene reacts with three moles of chlorine under special conditions. What is the reaction condition required for the reaction?
(b) Draw the structure of the reaction product in (a).
Answer
(a) UV radiation or diffuse sunlight must be present for the free radical addition reaction to take place.
(b)

32. Alkylbenzenes
Alkylbenzenes are a group of aromatic hydrocarbons in which an alkyl group is bonded directly to a benzene ring
also known as arenes
e.g.

33. This type of oxidation is limited to those molecules with
Alkylbenzenes
Alkylbenzenes are oxidized to benzoic acid by strong oxidizing agents such as hot alkaline potassium manganate(VII)
This type of oxidation is limited to those molecules with

34. Alkylbenzenes
Examples:

35. Alkylbenzenes
The C = C double bond and acyl groups in the side chain are oxidized by hot alkaline potassium manganate(VII)
e.g.

36. Alkylbenzenes
Example 31-3
State the conditions under which methylbenzene can be converted in the laboratory to
(a) C6H5CH2Cl
(b) C6H5COOH
Answer
Solution:
(a) Reagent: Cl2
Condition: in the presence of light
(b) Reagent: (1) KMnO4–, OH–, (2) H3O+
Condition: heating under reflux

37. Alkylbenzenes
Check Point 31-3
Methylbenzene undergoes two different types of chlorination reaction by different mechanisms. Compare the two different types of chlorination reaction in terms of reaction conditions as well as the products formed.
Answer

38. 31.7 Alkylbenzenes (SB p.164)
Two different types of chlorination reaction of methylbenzene are:
Type I: free radical substitution reaction
Type II: electrophilic aromatic substitution reaction

39. Orientation effects of substituents in electrophilic aromatic substitution reactions of monosubstituted Benzenes
Alkyl groups and groups with lone pairs (electron donating groups) direct new groups to ortho-, para-positions and speed-up the reaction (i.e. o & p directors and activating groups).
Halogens direct new groups to ortho-, para- positions but they slow down the
reaction (i.e. halogens are o & p directors and deactivating groups).
Electron withdrawing groups such as nitro, nitrile, and carbonyl direct new
groups to the meta-position and slow the reaction down (i.e. i.e. m directors and deactivating groups).
Thus the order of reactivity of benzene and monosubstituted benzene derivatives in E.Ar.sub. is as in the following chart
Substituted benzene with o,p directors > Benzene > Halobenzene derivatives > Substituted benzene with m- directors

40. Orientation effects of substituents in electrophilic aromatic substitution reactions of monosubstituted Benzenes
Meta directors
Ortho , para directors
-NO2
-SO3H
-COOH, -COOR
-CHO, -COR
-CN
-OH, -OR
-NH2, -NHR, -NR2
-C6H5
-CH3, -R (alkyl)
-F, -Cl, -Br, -I

41. Q2: What is the final product of the following reaction?
Q1: What are the major products of the following reaction?
Q2: What is the final product of the following reaction?
a) o-chlorobenzaldehyde b) m-chlorobenzaldehyde c) p-chlorobenzaldehyde d) a,c
Q3:Which one of the following compounds has aromatic character?

42. The END