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A2 Chemistry
F324: Rings, Polymers & Analysis
Plymstock School
4.1.1 - Arenes
Arenes
P.J.McCormack
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4.1.1 Arenes Objective Checklist
Draw the structure of benzene
Explain the terms arene and aromatic
State the empirical and molecular formula of benzene
State five properties of benzene
State the formula of a phenyl group
Explain the models used to describe the structure of benzene
Explain the term pi () bond
Explain what the term delocalised electrons means
Explain in terms of bond lengths why Kekule’s benzene is incorrect
Compare the Kekulé and delocalised models for benzene in terms of p-orbital
overlap forming pi () bonds
Write an equation for the hydrogenation of cyclohexene
Explain using thermodynamic evidence why benzene cannot have alternating single
and double bonds
State the shape and bond angles around the carbon atoms in the benzene ring
State the mechanism by which benzene reacts
State the formula of a nitronium ion
Write a balanced symbol equation to show the formation of a nitronium ion
Write an equation for the nitration of benzene
State the conditions required for the nitration of benzene
Draw the mechanism for the reaction of benzene with a nitronium ion
State the meaning of the term halogen carrier
Write an equation to show the halogenation of benzene
Explain why a halogen carrier is needed to halogenate benzene
State the formula of three halogen carriers
Draw the mechanism for the monohalogenation of benzene
State the molecular formula of phenol
Draw the skeletal formula of phenol
State the physical appearance of phenol at room temperature and pressure
Write an equation for the reaction of phenol with sodium hydroxide
Write an equation for reaction of phenol with sodium
State the uses for compounds containing phenol
Write an equation for the reaction of phenol with bromine to form 2,4,6tribromophenol
Explain the relative ease of bromination of phenol compared with benzene
State three uses of phenols
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Introduction.
The simplest arene is benzene. Arenes are compounds that contain benzene or derivatives of benzene
obtained by replacing hydrogen’s by other groups or fusing benzene rings together. Arenes are
characterised by high stability, non-reactivity, and are unsaturated hydrocarbons but are unlike alkenes.
All arenes contain a delocalised  system of electrons. Benzene has the empirical formula CH a
molecular mass of 78 and molecular formula of C6H6.
Benzene ring C6H6
In 1825, Michael Faraday analysed an oily residue formed inside a gas lamp. He found that the residue contained
previously unknown hydrocarbon, whose molecular formula was later shown as C 6H6.This structure must be
unsaturated.
August Kekulé proposed a ring structure. The idea was attributed to a dream he had about a snake biting
its tail. Kekulé drew the structure of benzene as cyclohexa-1,3,5-triene.
H
H
C
C
H
C
C
H
C
C
H
Sometimes Kekulé’s structure is shown without
the carbon and hydrogen atoms being drawn.
H
Derivatives of Benzene.
Substitution Group
Systematic Name
Other Name
Methyl –CH3
Methylbenzene
Toluene
Chloro, -Cl
Cholorbenzene
-
Nitro, -NO2
Nitrobenzene
-
Hydroxy, -OH
Phenol
-
Amino, -NH2
Phenylamine
Aniline
Carboxylic acid, -COOH
Benzenecarboxylic acid
Benzoic acid
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Naming.
Methyl groups occupy the 1 position and the ring is numbered clockwise. The lowest position of the substituted
groups are used.
-C6H5 this is a phenyl group, so called from the Greek pheno ‘I bear light’ as Faraday isolated benzene from
illuminating gas.
Methylbenzene (toluene)
Bromobenzene
2-methylphenol
2,4,6-trinitrotoluene (TNT)
1,2-dimethylbenzene
Phenol
Ethylbenzene
Phenylamine
Nitrobenzene
Benzenecarboxylic acid (benzoic acid)
Aspirin
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Properties of Benzene.
1. Planar
2. Highly symmetrical
3. Non-polar
4. Lack of polarity results in benzene being a liquid at r.t.p.
5. Immiscible with water
6. Boiling point = 80C
7. Melting point = 6C
8. Reacts by electrophilic substitution.
9. Carcinogenic
The high melting point is due to the ease at which the highly symmetrical benzene ring can pack into a
crystal lattice. Methylbenzene m.p.t = -95C.
4.1.1 (a) Compare the Kekulé and delocalised models for benzene in terms of p-orbital
overlap forming pi () bonds.
Kekulé’s structure suggests that benzene has three single bonds and three double bonds, so would react
in the same way as cyclohexene and undergo electrophilic addition. It further suggests that the single
bonds are long and the double bonds are short bonds.
Problems with Kekulé’s Model.
1. X-ray diffraction shows that benzene is planar which concurs with Kekulé’s structure but:The molecule is a regular hexagon of carbon atoms, with six equal bond lengths.
C-C in cyclohexane = 0.154nm
C=C in cyclohexene = 0.133nm
C-C bond in benzene = 0.139nm
Kekulé’s structure would
look like the diagram on the
right with differing bond
lengths. Benzene is highly
symmetrical so cannot have
alternating double and single
bonds.
2. We find that benzene has a constant bond length, somewhere between a single and double bond
length.
3. Benzene does not behave like an alkene. It reacts by electrophilic substitution rather than by
electrophilic addition, even though it is unsaturated.
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4. The theoretical enthalpy of hydrogenation of Kekulé’s benzene is -360 kJmol-1. The experimental
value is -208 kJmol-1. The structure is more stable (endothermic) than Kekulé’s structure. The
extra stability and equivalent carbon-carbon bond length can be explained by delocalisation.
Bonding in Benzene.
1. The carbon atoms in the ring are bonded to one another and to their hydrogen atoms by sigma
bonds (fig. 1.0).
2. This leaves one unused p orbital on each carbon, each containing a single electron. These p
orbitals are perpendicular to the plane of the ring, with one lobe above and one below the plane
(fig. 2.0).
3. Each p orbital overlaps sideways with two neighbouring orbitals to form a single  bond that
extends as a ring of charge above and below the plane of the molecule (fig.2.1).
4. The electrons in the  bond cannot be said to belong to any particular carbon atom. Each electron
is free to move throughout the entire  system, so the electrons are said to be delocalised. It is
this delocalisation that gives benzene its extra stability. Any system in which electron
delocalisation occurs is stabilised.
H: 1s orbitals
H
C
H
H
C
C
C
C
H
C
atomic orbitals
H
H
Figure 1.0
-bonds are formed by side-by-side overlap of all
six 2p atomic orbitals.
Figure 2.1
Figure 2.0
Electrons tend to repel one another, so a system when they are far apart as possible will involve minimum
repulsion and will therefore be stabilised. Delocalisation of the  electrons has a profound effect on the
both physical and chemical properties.
The Delocalised Theory of Benzene.
This theory suggests that instead of three double and three single bonds in fixed positions (localised), the
six p-orbitals overlap and the six pi () electrons are free to move within this system, creating a ring of
delocalised electrons. This explains why benzene is highly symmetrical and more stable than predicted
from the Kekulé’s structure.
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4.1.1 (b) Evidence for a delocalised model of benzene in terms of bond lengths, enthalpy
change of hydrogenation and resistance to reaction.
Thermodynamic Stability Evidence.
When an alkene (double) bond is reacted with hydrogen the energy change is called the enthalpy of
hydrogenation. The theoretical enthalpy change when one double bond is hydrogenated is -120 kJmol-1.
Kekulé’s structure of benzene has three double bonds therefore the total theoretical enthalpy value is
-360 kJmol-1.
H = -120 kJmol-1
H2
Cyclohexene
Cyclohexane
H = -360 kJmol-1
3H2
Cyclohexa-1,3,5-triene
Cyclohexane
When benzene is hydrogenated experimentally the value for the enthalpy change is -208 kJmol-1. This is 152
kJmol-1 less than the expected enthalpy change of hydrogenation of cyclohexa-1,3,5-triene. From this it can be
concluded that:
1. Benzene is more stable than the Kekulé structure (it has a lower enthalpy of hydrogenation value)
2. The bonding in benzene cannot be composed of alternating double and single bonds.
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The diagram on the right shows the
theoretical values for the hydrogenation of
Kekule’s benzene with a comparison of the
delocalised model.
The delocalised model of benzene is more
stable than the Kekule’s structure so has a
lower enthalpy value.
4.1.1 (c) Electrophilic substitution of arenes with concentrated nitric acid in the presence of
concentrated sulfuric acid.
Electrophile – a
species that accepts a
pair of electron
Electrophilic Substitution.
The benzene ring has a high electron density associated with the delocalised  electrons. Hence an attacking
reagent that is attracted by this negative charge is needed – an electrophile.
Nitration
The substitution of a hydrogen atom for a nitronium ion (NO 2+). When benzene is treated with a mixture of
concentrated nitric and concentrated sulfuric acid and gently refluxed at 50C nitrobenzene is produced.
NO2
+
HNO3
c. H2SO4
50°C
+
H2O
This reaction occurs in several steps. First the nitronium ion is formed.
HNO3 + H2SO4  H2NO3+ + HSO4H2NO3+  NO2+ + H2O
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4.1.4 (d) Outline the mechanism of electrophilic substitution in arenes, using the
mononitration and monohalogenation of benzene as examples
The mechanism by which arenes react is electrophilic substitution. A hydrogen on the benzene ring is replaced
(substituted) for an electrophile such as NO2+ or Cl+.
The general mechanism is:
Mechanism for the Nitration of Benzene.
H
Step 1
NO2+
NO2
NO2
Step 2
+
+
H+
Intermediate
Electrophilic attack by the nitronium ion takes place as the positively charged ion is attracted to the delocalised 
electrons. A covalent bond is formed to one of the carbon atoms disrupting the delocalised  system. The
intermediate has a high activation energy as a considerable amount of energy is required to break the delocalised 
system.
Halogenation.
Benzene does not react with chlorine, bromine, or iodine on their own because the non-polar halogen molecule
has no centre of positive charge to initiate electrophilic attack therefore a catalyst is needed.
The catalyst is called a halogen carrier, and is thought to work by accepting a lone pair from one of the halogen
atoms. This induces polarisation in the halogen molecule. Typical halogen carriers are iron, iron(III) halides or
aluminium halides.
Cl---Cl:---FeCl3
+ The dotted lines show bonds breaking and bonds forming between a chlorine atom and the iron(III) chloride. The
positive end of the halogen is now an electrophile and can attack the benzene ring.
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Cl
+ Cl2
+
HCl
Alkylation: Friedel-Crafts Reaction.
Alkylarenes are made using a halogen carrier and a halogenoalkane to bring about substitution of a delocalised ring.
CH3
+
AlCl3
CH3Cl
Heat
Methylbenzene
As in the reaction with halogens, the halogen carrier (aluminium chloride) accepts an electron pair from the
chlorine atom, polarising the chloromethane molecule.
CH3---Cl:---AlCl3
+
-
The positively charged methyl group attacks the delocalised ring and electrophilic substitution occurs. This is an
example of a Friedel-Crafts reaction.
4.1.4 (e) Explain the relative resistance to bromination of benzene, compared with alkenes,
The Kekulé structure of benzene with its alternating carbon-carbon bonds would suggest that benzene might
readily undergo an addition reaction with dihalogens. When bromine is added to benzene without a halogen carrier
no reaction occurs. This suggests that benzene is not composed of alternating double and single bonds, but of a
delocalised system.
Addition of halogens to benzene is very hard to achieve. This is surprising if we represent benzene by the Kekulé
structure.
+
3Br2
H
H
Br
H
Br H
Br
Br
H
Br
Br
H
Addition of bromine to an alkene such as cyclohexene requires mild conditions (room temperature and pressure).
Cyclohexene produces 1,2-dibromocyclohexane on shaking with bromine water. The mechanism for this reaction
is electrophilic addition.
Br
+
Br2
Br
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Alkanes have a localised electron system with four electrons spread over only two carbon atoms. Benzene has a
lower electron density than alkenes as the 6 delocalised electrons are spread over six carbon atoms in the ring.
The resistance of benzene to undergo bromination compared to cyclohexene can be explained in terms of the
delocalisation of the  electrons in the ring. Benzene requires more vigorous conditions (more energy to
overcome the stability) before undergoing addition reactions because of the chemical stability of the  electron
system, which must be broken for an addition reaction to occur.
When a non-polar molecule approaches benzene there is insufficient -electron density to polarise the bromine
molecule so electrophilic attack does not take place.
Phenol.
Phenols are a class of compounds where a phenyl group is directly attached to a hydroxyl group (-OH). Phenol is a
white crystalline solid at room temperature and pressure. Phenol is only slightly soluble in water and is slightly
acidic. Phenol and derivatives of phenol are used as antiseptics, dyes and are key components of many
pharmaceutical drugs such as paracetamol.
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4.1.1 (f) Reactions of phenol with aqueous alkalis;
Phenol is a weak acid so can be neutralised with aqueous sodium hydroxide. The product formed is called is a salt
sodium peroxide.
C6H5OH + NaOH  C6 H5O- Na+ + H2O
4.1.1(f) Reactions of Phenol with Sodium to form Salts;
2 C6H5-OH + 2Na → 2 C6H5O-Na+ + H2
When reactive metals react with phenol, effervescence is observed due to the production of hydrogen gas. The
organic product is the salt sodium phenoxide C6H5O-Na+.
4.1.1 (f) Reactions of phenol with bromine to form 2,4,6-tribromophenol;
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4.1.1 (g) Explain the relative ease of bromination of phenol compared with benzene.
Bromine reacts readily with phenol to form 2,4,6-tribromophenol a white precipitate. This reaction takes place at
room temperature and pressure. In order for benzene to react with phenol a halogen carrier is required to
polarise the bromine molecule.
Bromine reacts much more readily with phenol than with benzene as the lone pair on the oxygen in phenol
becomes withdrawn into the benzene ring, increasing the electron density, activating the ring. The increased
electron density is able to heavily polarise the bromine molecule which is then attracted to the benzene ring.
One lone pair of electrons
of the oxygen p-orbital
becomes drawn into the
delocalised ring of
electrons.
4.1.1 (h) State the uses of Phenols
Phenols are used in production of plastics, antiseptics, disinfectants and resins for paints
2,4,6-trichlorophenol (TCP)
4-chloro-3,5-dimethylphenol
(Dettol)
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Glossary.
•
Acylation – Substitution of one of the hydrogen atoms of a benzene ring by an acyl group from an acyl
(acid) chloride.
•
Alkylation – Substitution of one of the hydrogen atoms of a benzene ring by an alkyl group from a
halogenoalkane.
•
Aluminium chloride – The catalyst used in Friedel–Crafts alkylation and acylation reactions, chemical
formula AlCl3. In Friedel–Crafts alkylation, AlCl3 reacts with a halogenoalkane to form a carbocation, which
is able to react with the benzene ring in an electrophilic substitution reaction.
•
Arene – A compound containing a benzene ring. Arenes are also called aromatic compounds.
•
Benzene – A cyclical, aromatic hydrocarbon with the formula C6H6. Despite being unsaturated it does not
readily undergo addition reactions but does undergo electrophilic substitution reactions. Its stability is due
to a cloud of delocalized pi electrons above and below the carbon ring.
•
Carbocation – An ion containing a positively-charged carbon atom. Carbocations are intermediates in
electrophilic substitution reactions, for example in Friedel–Crafts alkylation.
•
Cyclic structure – A compound whose structure consists of a ring of atoms. Kekulé first proposed that
hydrocarbon chains might form a ring in his cyclic structure for benzene.
•
Delocalized electrons – Electrons that are not attached to one particular atom, but are shared between
several atoms.
•
Electronegativity – The power of a bonded atom to draw electron density, for example to attract the
pair of electrons in a covalent bond.
•
Electrophile – A species attracted to regions of high electron density, where it accepts a lone pair of
electrons to form a covalent bond.
•
Electrophilic substitution – A substitution reaction in which an electrophile attacks an electron-rich
centre (such as a benzene ring) and accepts a pair of electrons to form a single covalent bond. An example
is the nitration of benzene, in which the nitronium ion NO2+ acts as the electrophile, and substitutes itself
for an H+ ion in connection to the benzene ring.
•
Enthalpy – A measure of the heat energy stored in a chemical system, given the symbol H.
•
Friedel–Crafts reactions – Electrophilic substitution reactions useful in organic synthesis for adding alkyl
or acyl groups to a benzene ring. An aluminium chloride catalyst is required.
•
Hydrogenation– The addition of hydrogen across a carbon–carbon double bond.
•
Nitrating mixture – A mixture of concentrated nitric and sulfuric acids, used to form nitronium ions for
the nitration of benzene.
•
Nitronium ion – The NO2+ ion formed by refluxing a mixture of concentrated nitric and sulfuric acids,
which acts as an electrophile in the nitration of benzene.
•
Pi bond – A type of covalent bond formed by the sideways overlap of p atomic orbitals. It is involved in
double bonds, and is represented by the symbol π.
•
Sigma bond – The strongest type of covalent bond, formed by the end-to-end overlap of atomic orbitals
and represented by the symbol σ.
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•
TNT – Trinitrotoluene or 2,4,6-trinitromethylbenzene. An explosive formed by nitrating methylbenzene
(also called toluene) at a high temperature. It is stable to shock and friction and therefore safer to handle
than many other explosives.
•
Unsaturated – A hydrocarbon that contains one or more carbon–carbon multiple bonds, for example the
alkenes.
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Further Reading & Web Links.
http://goo.gl/h8KCq
Delocalised Model of Benzene
http://goo.gl/zdwRC
ChemGuide Chemistry
http://goo.gl/gqB9z
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Knock Hardy – Benzene Notes
http://goo.gl/CKH0n
OCR Textbook pages 4-19
www.innovativeeducation.org
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