Arenes: Benzene
Arenes are aromatic hydrocarbons containing 1 or more benzene
rings.
A benzene ring is a ring of 6 carbon atoms, each of which is also
bonded to a hydrogen atom, with molecular formula C6H6.
Benzene derivatives can be formed when one or more of the
hydrogen atoms on a benzene ring is replaced by another
atom or group of atoms.
The Kekulé model of Benzene
Nitrobenzene
Bromobenzene
Ethylbenzene
 In 1865, Kekulé proposed this structure of benzene,
with alternate single and double bonds.
 Localised π bond formed by sideways overlap of 2 p
orbitals above and below the plane of the ring.
 Benzene ring contains 3 localised π bonds: each
contains 2 shared electrons between 2 carbon
atoms.
 This model has high electron density
 Kekulé’s structure failed to explain the chemical and
physical properties of benzene fully…
Why was Kekulé wrong?
Benzene does
not undergo
electrophilic
addition. It will
not decolourise
bromine water .
Enthalpy changes of hydrogenation is less exothermic
than expected
The Kekulé structure contained 3 double C=C bonds. When a
double bond reacts with hydrogen, the enthalpy change is
expected to be -120kJmol-1. So, for benzene, containing 3
double bonds, ΔH would be expected to be -360 kJmol-1.
When benzene is hydrogenated however, ΔH is only -208
kJmol-1 . The actual structure of benzene is therefore
more stable than the Kekulé structure.
Bond lengths from X-Ray diffraction
Kekulé structure of alternating single and double bonds
should have 2 different bond lengths: shorter double
bonds of 0.134nm length and longer single bonds of
0.153nm length. X ray diffraction however revealed that
all six of the carbon-carbon bonds in benzene are the
same length: 0.139nm – an intermediate between short
C=C and long C-C bonds.
Delocalised Model of Benzene
Delocalised π bond is
formed by sideways overlap
of 6 p orbitals above and
below the plane of the
ring. The benzene ring
contains 1 delocalised π
bond which contains 6
electrons shared between 6
carbon atoms. It has a
lower electron density than
the Kekulé model.
Each carbon atom in the ring has 4 outer shell
electrons. 3 of these electrons bond to 2 other
carbon atoms and a hydrogen atom. This leaves
a 4th outer shell electron in a 2p orbital above
and below the plane of the carbon atoms. The
electron in the p orbital of the carbon atom
overlaps with the electrons in the p orbitals of
the carbon atoms on either side, resulting in a
ring of electron density above and below the
plane of the carbon atoms.
Electrophilic Substitution
Electrophilic substitution reactions occur in benzene chemistry because of the
delocalised ring of electrons above and below the plane of the carbon atoms.
1. Two of the
delocalised
electrons are
donated to the
positive
electrophile
forming a
covalent bond.
2. An intermediate forms that
contains both the
electrophile and the
hydrogen atom that is being
substituted. The delocalised π
electron is disrupted and the
intermediate is less stable
than benzene.
3. C-H bond breaks and 2
electrons are returned to
the delocalised ring. The
unstable intermediate
rapidly loses a hydrogen as
a H+ ion. The delocalised
ring is reformed and
stability restored.
Nitration of Benzene
Benzene is nitrated to form nitrobenzene.
Conditions: concentrated HNO3; concentrated H2SO4; 55°
The electrophile in this reaction is the nitryl cation, NO2+.
In order to generate this electrophile:
HNO3 + H2SO4  H2NO3+ + HSO4H2NO3+  NO2+ + H2O
Overall: HNO3 + H2SO4  NO2+ + H2O + HSO4-
Reduction of nitrobenzene
Nitrobenzene is reduced to form phenylamine.
Conditions: concentrated HCl; Sn; reflux
The manufacture of
phenylamine therefore
occurs in 2 steps:
1. Nitration of benzene to
form nitrobenzene
2. Reduction of
nitrobenzene to form
phenylamine
+ 6[H]
+ 2H2O
Manufacture of TNT (Trinitrotoluene)
Methyl benzene reacts with nitric acid to form
2,4,6 trinitrotoluene and water.
Halogenation of Benzene
Electrophilic substitution with a halogen in the presence of a halogen
carrier catalyst (FeBr3 or AlCl3)
Benzene is unable to react with a halogen alone because the lower
electron density of the delocalised benzene ring cannot induce a dipole on
the halogen molecule.
Bromination of Benzene
Br2 / FeBr3 / anhydrous
Generation of the Br+ electrophile
using halogen carrier catalyst:
Br2 + FeBr3  Br+ + FeBr4Regeneration of the halogen carrier
catalyst:
FeBr4- + H+  FeBr3 + HBr
Chlorinationof Benzene
Cl2 / AlCl3 / anhydrous
Generation of the Cl+ electrophile
using halogen carrier catalyst:
Cl2 + AlCl3  Cl+ + AlCl4Regeneration of the halogen
carrier catalyst:
AlCl4- + H+  AlCl3 + HCl
Comparing the reactivities of benzene
and cyclohexene
When bromine water is added to
cyclohexene, a cyclic alkene, an
Electrophilic addition reaction
occurs.
The π bond contains localised
electrons between 2 carbon atoms.
This is a region of high electron
density, capable of inducing a dipole
on the bromine molecule.
Cyclohexene will decolourise
bromine water form orange to
colourless.
When bromine water is added to
benzene, there is no reaction
The π bond in benzene is
delocalised over 6 carbon atoms.
It has a lower electron density
than alkenes. The electron density
is insufficient to induce a dipole
on the bromine molecule.
Bromine water will not
decolourise: it will remain orange.
A halogen carrier catalyst is
required to generate the more
powerful electrophile Br+ in order
for a reaction to occur.
Arenes: Phenol
Phenol is hydroxybenzene. It is an aromatic alcohol – an OH
hydroxyl group attached directly to a benzene ring.
Solubility
Phenol has low
solubility
The hydrophilic
OH group can form
hydrogen bonds
with water
Hydrophobic nonpolar benzene ring
cannot form
hydrogen bonds
It is therefore
partially soluble in
water
Bonding in Phenol
 Lone pair of electrons on oxygen atom is
partially delocalised into the ring
 Increased electron density in the ring
 Phenol more able to induce dipole or attract
electrophiles.
Reaction of Phenol
+ H+
When dissolved in water, phenol forms a
weak acidic solution by losing an H+ ion
from the OH group.
Reaction with sodium to form a phenoxide salt:
Effervescence; sodium
dissolves; phenol dissolves
Reaction with sodium hydroxide to form a phenoxide salt:
The phenol
dissolves
Comparing reactivities…
Phenols react with Na and NaOH to
form a phenoxide salt.
Only reacts with
Na, to form an
alkoxide salt.
Aliphatic
Alcohols
Carboxylic
Acids
Phenols
(aromatic
alcohols)
Carboxylic acids are
the most reactive;
they react with Na,
NaOH and Na2CO3 to
form carboxylate
salts.
Halogenation of Phenol
 Phenol undergoes electrophilic substitution
to form 2,4,6-tribromophenol
 Unlike benzene, this reaction will occur at
room temperature without the need for
a halogen carrier catalyst.
 Bromine will decolourise (orange to
colourless) and a white precipitate of
2,4,6- tribromophenol forms.
Phenol is more reactive than benzene because in
phenol, the lone pair of electrons on oxygen is
partially delocalised into the ring. Phenol has a higher
electron density in the ring than benzene so phenol can
induce a dipole in bromine, whereas benzene cannot (in
the absence of a halogen carrier catalyst).
Esterification of Phenols
Ethanoic Anhydride + Phenol 
Phenyl Ethanoate + Ethanoic acid
Conditions: Ethanoic Anhydride /
methanol /warm
Phenol is a weak nucleophile
compared to non-aromatic
alcohols because the lone pair
on the oxygen is partially
delocalised into the ring.
Phenols do not react readily
with carboxylic acids, so
instead a more reactive acid
anhydride is used.
Uses of phenols: Detergents, antiseptics, disinfectants,
preparation of aspirin and other pharmaceuticals, production of
resins for paints, plastics.
Comparing the relative reactivity of ethene,
benzene and phenol towards Br2
Ethene
 Localised π bond: 2 electrons over 2 carbon atoms
 High electron density
 Can polarise (induce a dipole) in Br2 molecule
 Undergoes electrophilic addition reactions
Benzene
 Delocalised π bond: 6 electrons over 6 carbon atoms
 Lower electron density than ethene
 Cannot polarise (induce a dipole) in Br2 molecule
 Undergoes electrophilic substitution reactions with a halogen carrier catalyst
Phenol
 Delocalised π bond; lone pair of electrons on oxygen atom is partially
dissociated into the ring
 Higher electron density in the ring than benzene
 Can polarise (induce a dipole) in Br2 molecule
 Undergoes electrophilic substitution reactions without a halogen carrier catalyst