Aromatic Hydrocarbons
Nomenclature of the Derivatives of Benzene
Physical Properties of Aromatic Hydrocarbons
Preparation of Benzene
Reactions of Benzene
Alkylbenzenes
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
Nomenclature of the Derivatives of Benzene
(b) For other compounds, the substituent and the benzene
ring taken together may form a new parent name
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
4
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.
5
Nomenclature of Benzene Derivatives
For three or more substituents on a benzene ring:
1. Number to give the lowest possible numbers around the ring.
2. Alphabetize the substituent names.
3. 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.
6
Nomenclature of the Derivatives of Benzene
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
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.
9
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
Answer
(c) 2,4-Dinitrobenzoic acid
Solution:
(a)
(c)
(b)
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
Give the IUPAC name for each compound.
PhCH(CH3)2
isopropylbenzene
OH
m-butylphenol
Br
2-bromo-5-chlorotoluene
Cl
12
Which structure matches the given name?
o-dichlorobenzene
Cl
Cl
Cl
Cl
Cl
Cl
Br
Cl
A
B
C
D
Cl
Cl
C
13
4-chloro-1,2-diethylbenzene
Cl
Cl
Cl
Cl
A
B
C
D
Cl
A
14
Physical Properties of Aromatic Hydrocarbons
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
Physical Properties of Aromatic Hydrocarbons
Name
Formula
Boiling
point (°C)
Melting
Density at
point (°C) 20°C (g cm–3)
Benzene
80.1
5.5
0.878
Methylbenzene
111
–95
0.867
Ethylbenzene
136
–94
0.867
Physical Properties of Aromatic Hydrocarbons
Name
Formula
Boiling
Melting
Density at
point (°C) point (°C) 20°C (g cm–3)
1,2Dimethylbenzene
144
–25.2
0.880
1,3Dimethylbenzene
139
–47.4
0.864
1,4Dimethylbenzene
138
13.3
0.861
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
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 الفحم‬and coke (‫)فحم الكوك‬
• Coal tar is a mixture of many
organic compounds, mainly
aromatic ones
• Benzene and methylbenzene
can be obtained
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
Preparation of Benzene
Reduction of Phenol
Phenol vapour is passed slowly over heated zinc dust to
produce benzene and zinc(II) oxide
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)
No reaction
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
Reactions of Benzene
Reaction
Action of
acidified
KMnO4
Cyclohexane (a
saturated alicyclic
hydrocarbon)
No reaction
Action of conc. No reaction
HNO3 and
conc. H2SO4
Cyclohexene (an
unsaturated
alicyclic
hydrocarbon)
KMnO4
decolourized
Methylbenzene (an
aromatic
hydrocarbon)
No reaction
Cyclohexene
A yellow liquid is
oxidized and colour formed
darkens
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
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 (+)
Reactions of Benzene
Nitration
• Conc. H2SO4 increases the rate of reaction by
increasing the concentration of the electrophile, NO2+
(nitronium ion)
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
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
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
Reactions of Benzene
Example 31-2
Complete each of the following by supplying the missing
reactant or product as indicated by the question mark:
Solution:
(a)
(a)
(b)
(b) conc. H2SO4, conc. HNO3
(c)
(c) fuming H2SO4
Answer
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)
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.
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
Alkylbenzenes
Examples:
Alkylbenzenes
The C = C double bond and acyl groups in the side chain
are oxidized by hot alkaline potassium manganate(VII)
e.g.
Alkylbenzenes
Example 31-3
State the conditions under which methylbenzene can be
converted in the laboratory to
(a) C6H5CH2Cl
(b) C6H5COOH
Solution:
(a) Reagent: Cl2
Condition: in the presence of light
(b) Reagent: (1) KMnO4–, OH–, (2) H3O+
Condition: heating under reflux
Answer
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
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
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
Orientation effects of substituents in electrophilic
aromatic substitution reactions of monosubstituted
Benzenes
Ortho , para directors
Meta directors
-OH, -OR
-NH2, -NHR, -NR2
-C6H5
-CH3, -R (alkyl)
-F, -Cl, -Br, -I
-NO2
-SO3H
-COOH, -COOR
-CHO, -COR
-CN
OH
OH
OH
NO2
HNO3 / H2SO4
+
o-Nitrophenol
53 %
NO2
NO2
NO2
p-Nitrophenol
47 %
SO3 / H2SO4
SO3H
m-Nitrobezenesulfonic acid
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
Q3:Which one of the following compounds has aromatic character?
d) a,c
The END