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AROMATIC CHEMISTRY
The term ‘aromatic’ was first used in the nineteenth century to describe a group of
compounds which have pleasant aromas. These compounds, which include
benzene, are very different to aliphatic compounds. The term is still used, since it is
useful to classify aromatic and aliphatic compounds separately, but the word now has
a much more fundamental meaning related to the -bonding in a molecule.
BENZENE
Benzene has the molecular formula C6H6 and therefore contains four double bond
equivalents. However, it does not readily undergo electrophilic addition reactions, like
alkenes do, and was thought at first not to contain double bonds. One of the earliest
structures for benzene was suggested by Ladenburg; another suggestion came from
Dewar.
H
H
H
H
Ladenburg
H
H
Dewar
In 1865, Kekule suggested that a rapid equilibrium existed between two equivalent
forms of benzene. This, he thought, would average out the double and single bonds
and would explain the reluctance of benzene to undergo electrophilic addition
reactions.
Other evidence about the structure of benzene:
X-ray Diffraction
C=C has a bond length of 0.134nm
C-C has a bond length of 0.154nm
X-ray diffraction analysis of benzene shows that:
 the six carbon-carbon bonds are of equal length and are intermediate
between double and single bonds, with a bond length of 0.139nm
 the molecule is planar: all twelve atoms lie in the same plane
Enthalpy of Hydrogenation
If cyclohexene is reduced by hydrogen to cyclohexane, the observed enthalpy
change is -120kJ.mol-1
H = -120 kJ.mol -1
+ H2
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If cyclohexa-1,3-diene is reduced by hydrogen to cyclohexane, the observed
enthalpy change is -234kJ.mol-1
H = -234 kJ.mol -1
+ 2H2
This enthalpy change, which involves the reduction of two double bonds, is as
expected, approximately twice the enthalpy change observed for the reduction of one
double bond.
Therefore, if the hypothetical molecule, cyclohexa-1,3,5-triene were to be reduced to
cyclohexane, the expected enthalpy change would be approximately –350 kJ.mol-1.
H ~ -350 kJ.mol -1
+ 3H2
However, when benzene is reduced to cyclohexane, the observed enthalpy change
is much less than -350 kJ.mol-1, showing that benzene is much more stable than
‘cyclohexa-1,3,5-triene’, which contains 3 isolated double bonds.
H = -208 kJ.mol -1
+ 3H2
This increased stability arises because of the delocalisation energy of benzene.
~ -145 kJ.mol
-1
delocalisation energy
Energy
~ -350 kJ.mol
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-208 kJ.mol
2
-1
The Structure of Benzene
All six carbon atoms are sp2 hybridised, and
therefore have trigonal planar geometry, with
a bond angle of 120o. This gives a regular
hexagon of C-C -bonds (all bond lengths
the same), with all twelve atoms (6xC 6xH)
in the same plane.
Each carbon atom has, in addition, a
half-filled
2p-orbital,
which
is
perpendicular to the plane of the ring.
The six 2p-orbitals overlap to form a
delocalised, cyclic -orbital, which extends
over the whole ring. The orbital has two
lobes, one above the plane of the ring and
one below. This delocalisation has a marked
stabilising effect: the reduction in energy
which it brings about is called the
delocalisation energy.
For a compound to be aromatic and have delocalisation energy, the -orbital must:
 contain 4n + 2 -electrons (Huckel’s Rule), where n is an integer.
 be cyclic
 be planar
Electrophilic Substitution
The benzene ring is a centre of high electron density and will attack electron-deficient
species: electrophiles. However, if benzene were to undergo electrophilic addition,
like alkenes, the stability associated with the delocalisation energy would be lost.
Instead, it undergoes electrophilic substitution and retains the delocalisation energy.
As is usual with an sp2 hybridised carbon atom, the first step in the mechanism is
addition (delocalisation energy is lost), but this is followed, in the second step, by the
elimination of a proton, which restores the delocalised -system.
H
+
E
E
+
The overall result is substitution.
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E
+
+ H
Examples of Electrophilic Substitution
1. Nitration
Nitration introduces a nitro group (-NO2) into the benzene ring. For example, benzene
is nitrated when treated with a mixture of concentrated nitric acid and
concentrated sulphuric acid. The reaction is highly exothermic. The reaction
temperature is allowed to rise to about 50oC, and the mixture is then cooled to retain
this temperature.
NO2
+
HNO3
+ H2O
nitrobenzene
The electrophile is the nitronium ion, NO2+, which is generated by the reaction of
concentrated nitric acid and concentrated sulphuric acid.
2HSO4- + H3O+ + NO2+
HNO3 + 2H2SO4
Initially, nitric acid is protonated by the sulphuric acid:
HSO4- + H2NO3+
HNO3 + H2SO4
base
acid
O
H2NO3+ then breaks down to give water and a nitronium ion:
H2NO3+
H2O + NO2+
H
Finally, the water molecule is protonated by a second molecule
of sulphuric acid.
+
O
N
:O
H
Mechanism:
H
+
NO2
NO2
NO2
+
+ H
+
Uses of nitro compounds
Nitration is used to manufacture
trinitromethylbenzene)
explosives
such
as
T.N.T.
(2,4,6-
Nitro compounds can be reduced to aromatic primary amines by reduction with
Sn/conc. HCl or Ni/H2. Aromatic primary amines are used widely in the manufacture
of dyestuffs, because amino groups are auxochromes.
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2. Acylation (Friedel-Crafts reaction)
Acylation involves the introduction of an acyl group (RCO) into the benzene ring and
therefore involves the formation of a new carbon-carbon bond. The electrophile
required is RCO+.
The reaction is carried out at room temperature by treating benzene with an acyl
chloride in the presence of AlCl3. The product is a ketone.
COR
+
RCOCl
AlCl3
+ HCl
Mechanism:
The acyl chloride, RCOCl, forms a coordinate bond with the AlCl 3. This has the effect
of increasing the polarisation of the C-Cl bond, forming an ion pair.
RCO+ + AlCl4acylium ion
RCOCl + AlCl3
The acylium ion is sufficiently electrophilic to be attacked by the benzene -system.
H
+
COR
COR
+
COR
+
+ H
Finally, the hydrogen ion reacts with AlCl4- to regenerate the catalyst.
H+ + AlCl4-
HCl + AlCl3
Friedel-Crafts acylation reactions can be carried out using carboxylic acid anhydrides
in place of acyl chlorides. One of the first steps in the synthesis of anthraquinone
dyes is the acylation of benzene with benzene-1,2-dicarboxylic anhydride in the
presence of AlCl3.
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