ARENE CHEMISTRY
Aim: to describe some of the electrophilic substitution chemistry of arenes.
Materials: C.I.E. Chemistry 9701 A/AS Level 2009 page 29
Section 10.2 Hydrocarbons [A Level content only]: A 45 minute lesson should
cover the most of the material in bold.
(j) describe the chemistry of arenes as exemplified by the following
reactions of benzene and methylbenzene:
(i)*substitution reactions with chlorine and bromine
(ii) nitration
(iii) oxidation of the side-chain to give a carboxylic acid
(k)
(i) *describe the mechanism of electrophilic substitution in
arenes, using the mono-nitration of benzene as an example
(ii) describe the effect of delocalisation of electrons in arenes in
such reactions
(l)
predict whether halogenation will occur in the side-chain or aromatic
nucleus in arenes
(m)
apply the knowledge of positions of substitution in the electrophilic
substitution of arenes
Student outcome: Students will gain an important understanding of benzene
chemistry typified by electrophilic substitution reactions, and these concepts
can be applied to other aromatic compounds (e.g. chlorobenzene, phenol).
Anticipated difficulties: It is necessary for students to understand the influence
of delocalised of -electrons on structure and properties and not get confused
with previously taught alkene electrophilic addition reactions [10.2 (d)].
CH3
H
H
C
C
C
Benzene
C6H6
H
H
C
C
C
H
H
Methylbenzene
C7H8
Electrophilic Substitution Reactions of Arenes
Benzene takes part in a variety of substitution reactions with electrophilic
reagents (positively charged).
If chlorine is passed through benzene at room temperature in the presence of
a suitable catalyst (the Lewis acid aluminium trichloride), substitution takes
place. However, due to delocalisation, no electrophilic addition occurs
(compare with alkenes).
Cl
AlCl3
+ Cl2
+
HCl
chlorobenzene
Similarly, bromine in the presence of the aluminium tribromide [or Iron(III)
bromide]:
C6H6 + Br2
AlBr3
C6H5Br + HBr
When treated with a mixture of concentrated nitric acid and concentrated
sulphuric acid at room temperature, nitrobenzene is formed:
NO2
H2SO4
+ HNO3
+
H2O
nitrobenzene
Mechanism of Electrophilic Substitution: Nitration of Benzene:
The reaction occurs in several stages.
1. The Nitronium ion (NO2+) is generated:
H
H2SO4
+
H
O
NO2
HSO4-
+
+
O
H
H
+
O
H
+
NO2
H2O
+ O N O
nitronium ion
(electrophile)
NO2
2. The electrophile reacts with benzene by addition:
H
NO2
H
H
+
NO2
+
NO2+
NO2
+
The positive charge is delocalised over three of the carbon atoms; the ion is
a resonance hybrid (mixture) of three structures above and this delocalisation
stabilizes the developing positive charge making benzenes and arenes
more reactive than alkanes. Remember the more spread out the positive
charge is around the ring and lower in energy! (The curly arrows represent a
movement of electrons).
H
NO2
positive charge spreads around the ring
+
3. In the final step, it is deprotonated to give nitrobenzene:
H
NO2
NO2
+
+
HSO4-
+
H2SO4
In a similar way, we can show its reaction with chlorine. A (+ -) dipole is
created as the halogen approaches the arene because the electron clouds
repel each other.
+
Cl
Cl
Cl
+
-
AlCl3
+
AlCl4-
H
Concept check: Ask the students (to see if they understand):
Why does benzene not undergo electrophilic addition reactions?
The three double bonds are delocalised and are more stable than an alkene.
It reacts in a different manner undergoing electrophilic substitution, for
example with chlorine in the presence of aluminium trichloride catalyst.
Remember in delocalised systems the electrons move around the ring.
Methylbenzene is an arene. Do you think it will undergo electrophilic
substitution reactions? Why
Yes. The ring is delocalised in the same way as benzene. We will look at the
special chemistry of methylbenzene in the next lesson.
What type of reaction is this? Why is aluminium bromide used?
C6H6
+
CH3CH2
Br
AlBr3
C6H5 C2H5 + HBr
ethylbenzene
Benzene reacts with an alkyl halide in the presence of aluminium tribromide
catalyst to give an alkylbenzene (ethylbenzene). It is another example of an
electrophilic substitution reaction.
Evidence of concept understanding beyond A-Level
Pyridine, C5H5N, a colourless liquid, has aromatic properties and undergo
some electrophilic substitution reactions. Other similar heterocyclic
compounds behave in the same way. However, the nitrogen atom is basic
and readily accepts a proton to form an ionic compound. Draw a diagram to
show how the nitrogen atom assists in producing an aromatic ring.
N:
N
..
N
..
Aromatic nature of pyridine
Nitrogen of group V has two SP2 hybrid orbitals, one P orbital and one lone
pair of electrons. One electron from the P orbital is used in delocalised
bonding in the six-membered ring. In the diagram above the electron cloud
therefore becomes delocalised. It is the lone pair of electrons that make
pyridine basic in nature, but do not take part in the delocalised ring!