Chemical Ideas 12.3: Arenes
What’s special about benzene?
The simplest arene, benzene is a colourless liquid with a
molecular formula of C6H6 meaning that it is very
unsaturated…however its behaviour is not like other
unsaturated compounds. Why?
Benzene has 6 delocalised electrons, spread out evenly over
its surface. It is a flat hexagon, with bond angles of 120o.
H
H
C
C
H
C
120o
C
H
C
C
H
H
If you try to draw a dot and cross diagram of Benzene:
Key
H
Electrons from C atoms
H
C
C
C
C
H
H
Each carbon atom forms 3
single bonds. The 4th electron
is delocalised (spread over the
entire system).
C
C
H
Electrons from H atoms
H
The shaded ring =
6 delocalised e-s
(1 from each C atom).
Electron density maps from X-ray diffraction studies give
more evidence for the delocalised structure:
Scale 0 0.1 nm
Notice the uniform density in all the carbon-carbon bonds.
August Kekulé (1865).
Suggested that the delocalised structure exists in 2 forms:
And
called Kekulé structures.
We think of benzene existing between these extremes:
Stability of benzene
The cyclic structure makes benzene much more stable than
expected, compared to alternating single and double bonds
of the kekule structures. The stable ring is preserved when
benzene undergoes reactions; infact reactions in which the
ring is disrupted only happen at much higher temperatures
and more vigorous conditions.
Consider the thermochemical data of the following
reactions:
+ H2
H = - 208 kJ mol-1
H = - 120 kJ mol-1
+ H2
The value for the Kekule structure cannot be measured, so
is hypothetical…
H = 3 x (- 120 kJ mol-1)
+ H2
= -360 kJ mol-1
Thus, when 1 mole of benzene molecules are hydrogenated,
(360 kJ mol-1 - 208 kJ mol-1) = 152 kJ mol-1 less energy is
given out than expected from the Kekule structures.
What are arenes?
‘ar’ = aromatic (sweet-smelling/ aroma / fragrance)…not all
arenes are sweet smelling – benzene smells quite strong and unpleasant!
‘-ene’ = unsaturated, like alkenes.
They are sometimes called ‘aromatic hydrocarbons’ as they
are hydrocarbons like benzene which contain stabilised ring
structures. Hydrogen atoms are often replaced with alkyl or
other functional groups:
Methylbenzene 1,3-dimethylbenzene 1-ethyl-4-methylbenzene
CH3
CH3
CH2-CH3
CH3
CH3
Where ONE hydrogen atom is replaced, it is referred to as a
phenyl group (formula C6H5). Two important aromatic
compounds whose names are based on the phenyl group are
PHENOL and PHENYLAMINE.
Two functional groups are numbered using the lowest
numbers possible.
Functional groups are named in alphabetical order.
Benzenes can join in fused ring systems and share a pair of
carbon atoms. Examples:
Naphthalene, C10H8
anthracene, C14H10
It is important to draw the double and single bonds, to
illustrate how many delocalised electrons there are.
what would be the formulas of the following structures?
Why are they incorrect?
Compounds derived from arenes
Match the following aromatic hydrocarbons to their
structures:
a) chlorobenzene,
b) nitrobenzene,
c) benzoic acid,
d) benzaldehyde,
e) benzyl alcohol,
f) phenol,
g) phenylamine.
e)
d)
a)
CH2OH CHO Cl
g)
NH2
b)
NO2
f)
OH
c)
COOH
 Do problems for 12.3 p292 questions 1- 4.
Chemical Ideas 12.4: Reactions of arenes
The six delocalised electrons in benzene do not belong to
any carbon atom. They are spread out in a cloud above and
below the plane of the benzene ring:
Regions of higher electron density
attract positive ions or atoms with
positive charge within molecules.
Benzene, (like the alkenes) reacts with electrophiles.
Alkenes undergo addition reactions to form saturated
molecules.
H
H
H H
C
C
H
+ Br
Br
H
H
C
C
Br
Br
H
Electrophilic substitution reactions of benzene
The stable ring system is kept intact, in most reactions.
Bromination of benzene
FeBr3
+
Br
Br
Br
+
HBr
Benzene reacts with the Br+ electrophile in an electrophilic
substitution reaction. Bromine becomes polarised as it
approaches benzene and first reacts with iron filings (used
to speed up the reaction) to make iron bromide:
2Fe + 3Br2  2FeBr3
(FeBr3 catalyses the reaction by accepting a lone pair from
Br2 and so making Br+ Br MORE polarised.)
Br+
Br+
Br
FeBr4-
FeBr3
The bromine becomes so polarised that it splits into Br+ and
FeBr4-. The Br+ joins to a carbon atom in the benzene ring
and the H+ it replaces reacts with FeBr4- to make HBr and
reform FeBr3.
Br
Br+
FeBr4- 
+ H
Br + FeBr3
Why doesn’t benzene undergo addition reactions?
This is much more difficult, and would mean disrupting the
stable ring system:
Br
+ Br Br
Br
Benzene undergoes other electrophilic substitution
reactions…
Nitration
Benzene reacts with a nitrating mixture (concentrated
sulphuric acid and nitric acid) below 55oC to form
nitrobenzene.
NO2
+
HNO3
c. HNO3
o
< 55 C
+
H 2O
The electrophile is NO2+:
HNO3 + 2H2SO4  NO2 + 2H2SO4- + H3Onitrating mixture
At higher temperatures, further substitution results in dinitrobenzene and tri-nitrobenzene.
Sulphonation
When benzene is heated under reflux for several hours with
conc. sulphuric acid, benzenesulphonic acid is made:
+
H2SO4
SO2OH
reflux
+
H 2O
O-
The electrophile is SO3:
The SO3 carries a large
positive charge on the
sulphur atom.
S+
-O
O-
Benzenesulphonic acid is a strong acid which forms salts in
alkaline solution:
SO2OH
SO3- +Na
+ NaOH 
+
H2O
Most detergents contain salts like this with long alkyl groups attached to the benzene ring. The
hydrocarbon part dissolves in fats, the ionic part dissolves in water.
Chlorination
Happens in a similar way to bromination… aluminium
chloride is the catalyst:
Overall:
Cl
AlCl3
+ Cl
Cl
+ H
Cl
First step:
Cl+
Cl+
Cl
AlCl4-
AlCl3 reacts
violently
with water,
so conditions
must be
anhydrous.
AlCl3
Second step:
Al
Cl
+
AlCl4 
-
+ H
Cl
+ AlCl3
Friedel-Crafts reactions
Aluminium chloride is used as catalyst to help polarise
hydrogen-containing organic molecules and cause them to
substitute into the benzene ring.
Two examples are alkylation (introducing an alkyl group
into the benzene ring) and acylation (introducing an acyl
group).
Methyl benzene is made by warming benzene with
chloromethane and anhydrous aluminium chloride:
CH3
+ CH3 Cl 
+ H Cl
The mechanism is similar to the halogens, AlCl3 causes
CH3Cl to polarise. This is an example of alkylation.
Acylation (same conditions as alkylation, but treat with acyl
chloride or an acid anhydride)
O
O
C
AlCl3
+ CH3 C
CH3 + H Cl
reflux
Cl
(ethanoyl chloride – an acyl chloride)
O
+ CH3 C
AlCl3
reflux
O
C
CH3 + CH3COOH
O
CH3 C
O
 copy summary and learn these reactions p297
 do problems for 12.4 p 297-298 questions 1-6.