P.J.McCormack - Science and...

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H:\Teaching Documents\Key Stage 5\A-Level Chemistry\F324 Rings Polymers and Analysis\4.1.1 - Arenes\Lesson 1\Arenes

Notes.docx/P.J.McCormack/23-5-13

A2 Chemistry

F324: Rings, Polymers & Analysis

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 2,4,6-

tribromophenol

Explain the relative ease of bromination of phenol compared with benzene

State three uses of phenols

3



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.

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 C6H6.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.

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

C

CC

C

CC

H

H

H

H H

H

Sometimes Kekulé’s structure is shown without

the carbon and hydrogen atoms being drawn.

Benzene ring C6H6

4



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) 1,2-dimethylbenzene Ethylbenzene

Bromobenzene Phenol Phenylamine

2-methylphenol Nitrobenzene Benzenecarboxylic acid (benzoic acid)

2,4,6-trinitrotoluene (TNT) Aspirin

5



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 = 80C

7. Melting point = 6C

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 = -95C.

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

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.

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.

6



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.

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.

C

C

C

C

C

C

H

H

H

H

H

H

H: 1s orbitals

atomic orbitals

Figure 1.0

-bonds are formed by side-by-side overlap of all

six 2p atomic orbitals.

Figure 2.0

Figure 2.1

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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.

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.

Cyclohexene Cyclohexane

H2

H = -120 kJmol-1

3H2

Cyclohexa-1,3,5-triene Cyclohexane

H = -360 kJmol-1

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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.

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 (NO2+). When benzene is treated with a mixture of

concentrated nitric and concentrated sulfuric acid and gently refluxed at 50C nitrobenzene is produced.

This reaction occurs in several steps. First the nitronium ion is formed.

HNO3 + H2SO4 H2NO3+ + HSO4

-

H2NO3+ NO2

+ + H2O

+ HNO3

c. H2SO4

50°C

NO2

+ H2O

Electrophile – a

species that accepts a

pair of electron

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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.

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.

NO2+

NO2

H

+

NO2

+ H+Step 1 Step 2

Intermediate

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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.

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.

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.

+ CH3ClHeat

CH3

Methylbenzene

AlCl3

+ 3Br2

BrH

H BrHBr

H

Br

HBr

HBr

+ Br2

Br

Br

11



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.

12



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 C6H5O- 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.

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)

One lone pair of electrons

of the oxygen p-orbital

becomes drawn into the

delocalised ring of

electrons.

14



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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H:\Teaching Documents\Key Stage 5\A-Level Chemistry\F324 Rings Polymers and Analysis\4.1.1 - Arenes\Lesson 1\Arenes Notes.docx/P.J.McCormack/23-5-13

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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

http://goo.gl/h8KCq

http://goo.gl/zdwRC

http://goo.gl/gqB9z

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Knock Hardy – Benzene Notes

http://goo.gl/CKH0n

OCR Textbook pages 4-19

www.innovativeeducation.org

http://goo.gl/CKH0n

http://www.innovativeeducation.org/

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