4.1.1 Arenes

Arenes are compounds which contain unsaturated rings, with delocalised electrons within the ring. The simplest arene is benzene. Benzene has the formula C6H6 . There are a huge number of derivatives of benzene, each containing a benzene ring. Arenes are sometimes called aromatic compounds (because they are smelly!).

Student activity 1 Name the functional groups present in the following compounds and label which ones are arenes.a) b) c) d)

Benzene

The structure of benzene Kekule’s dream – google link

The bonding in benzene is different from that in saturated compounds or regular double bonded structures. Benzene is a cyclic molecule; it contains carbon atoms which are connected together to form a ring.

All the carbon to carbon bonds in benzene are the same length, 0.139nm, a value between that for a carbon to carbon single bond and a carbon double bond.

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The six carbon atoms are arranged in a hexagonal ring. Each of the carbon atoms in the ring is bonded to two others and to a hydrogen by single bonds. Each carbon therefore uses three of its four electrons to form single bonds. This means that each carbon has one electron left over occupying a p orbital. These p orbitals are perpendicular to the plane of the hexagonal ring of carbon atoms:Diagram showing the arrangement of the p-orbitals, delocalisation, bond angles and lengths.

These p orbitals overlap to give a bond above and below the plane of the ring. The 6 electrons in the bond do not belong to any two carbon atoms in the ring. These electrons are free to move between all the carbon atoms in the ring. They are said to be delocalised (spread over more than two C atoms).

Another persuasive argument for the delocalisation within benzene comes from an analysis of the energetics of hydrogenation.

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Student activity 2True or false statements. Write T or F in pencil next to each statement.

1. Benzene is a planar molecule.2. Arenes always contain 6 carbons.3. The bond length in benzene is shorter than the C=C bond in ethene.4. A bond connects carbon and hydrogen atoms within benzene.5. Delocalised electrons are not held in the orbital of any single atom or pair of atoms.6. The bond angles in benzene are 120o. 7. Because the benzene structure is electron rich, it will tend to attract electrophiles.8. Cyclohexane has the formula C6H6.9. Hydrogenation results in the production of an unsaturated molecule.10. If benzene was actually cyclohexatriene it would provide 152kJmol-1 more energy when hydrogenated.

Reactivity of benzene

The Benzene molecule is a planar hexagon with a cloud of delocalised electrons lying above and below the ring. These electrons come from the fusing of the p-orbitals as shown above. Consequently any reactions which occur must involve the attack of an electrophile on the cloud of electrons.

The delocalised system is stable and arenes react in such a way as to retain their stability. The benzene ring is unaffected by many of the reactions. Benzene tends to undergo electrophilic substitution.

Student activity 31. Which of the following species would you expect to react with benzene (tick)?NH3 H2O Cl2 CH4 OH- NO2

+ 2. Draw skeletal formulae of the following compounds:

a) benzaldehyde;

b) 1,3,5-trimethyl benzene;

c) E-1-phenylprop-1-ene.

Electrophilic Substitution reactions of benzene

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Electrophilic substitution is possible in benzene rings. In this type of reaction two of the delocalised electrons in the ring are donated to an incoming electrophile. An unstable complex containing the electrophile and the leaving group is observed. These complexes are unstable and break down to reform the stable ring structure. The intermediate is not aromatic and is represented using an incomplete circle (often called a horseshoe or smiley).

General Mechanism

The first stage of the process is slow and is known as the rate-determining step of the reaction; the second stage is fast as the unstable intermediate breaks down.

Example 1- The Nitration of Benzene

In this reaction a –NO2 group substitutes for a hydrogen atom in benzene to obtain nitrobenzene, C6H5NO2. Benzene is refluxed on a water bath at 60oC with a nitrating mixture. Although concentrated nitric acid will nitrate benzene slowly alone, a mixture of concentrated nitric and concentrated sulphuric acid will nitrate benzene more quickly. This process is shown in the equation below: -

Equation (using molecular formulae to show the overall reaction).

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If the temperature of the reaction rises above 95oC then a mixture of nitrobenzene and 1,3-dinitrobenzene is produced.Equation (showing position of substitution)

Mechanism:

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In the first stage of the mechanism the nitryl cation has to be produced. This is obtained by the reaction of concentrated nitric and concentrated sulphuric acid. The following reaction takes place to produce a nitronium ion (NO2

+) as an electrophile:

HNO3 + 2H2SO4 → NO2+ + H3O+ + 2HSO4

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This reaction itself goes in two steps:1) HNO3 + H2SO4 → H2NO3

+ + HSO4-

2) H2NO3+ + H2SO4 → NO2

+ + H3O+ + HSO4-

The reaction then proceeds as shown below.

H+ is produced at the end of the reaction. This combines with HSO4- to reform

H2SO4. H2SO4 is therefore acting as a catalyst.

Note: nitrobenzene can be reduced to form phenylamineC6H5NO2 + 6[H] → C6H5NH2 + 2H2O

Example 2- The Halogenation of Benzene (Bromination)

Equation:

Conditions: Br2 in the presence of FeBr3 catalyst: this is called a halogen carrier. The first step in the reaction is the generation of the electrophile which occurs when the catalyst and the reagent react together.

Br-Br + FeBr3 → Br+ + FeBr4-

This action of the catalyst is shown in more detail below:-

The mechanism of this reaction now follows that for nitration:-

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Student activity 4Benzene reacts with another halogen, chlorine in the presence of a catalyst, or a halogen carrier, to form chlorobenzene and hydrogen chloride.

a) Write an equation (using molecular formulae) for the reaction between chlorine and benzene.

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b) Suggest a suitable catalyst

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c) Name the type of reaction taking place

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d) Order the following statements which describe the mechanism.

a. The intermediate formed contains a positive charge and a ring which contains some delocalised electrons.

b. Electrons move out from the delocalised system to the positive chloride ion.

c. A new bond is formed between a carbon of the ring and the chlorine.d. The halogen carrier picks up a negative chloride ion. e. A hydrogen atom donates its shared electrons into the ring to regain

aromaticity.Order: ………………………………………………………

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Uses of benzene compounds: Drugs Dyes Explosives Insecticides PCBsA comparison of the reactions of bromine with benzene and with cyclohexene

Cyclohexene is a typical alkene. Show how it reacts with bromine.

Br─Br

Cyclohexene

What type of reaction is this?

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What observations would you make in this reaction?

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Benzene reacts with bromine in a different way. It will not undergo addition like the alkenes, as the benzene ring is a stable structure requiring a large amount of energy to be broken down. It is the delocalised electrons in the ring that confer stability on benzene. The mechanism and conditions are given in the previous section.

Student activity 5 Teacher note: draw the structures.

Complete the following table, with a tick if it reacts and a cross if it doesn’t.

Compound Reaction with Br2 at room temperature

Reaction with warmNa2Cr2O7/H+

Reaction with Tollens’ reagent

Methylbenzene

Phenylethene

Pentanal

Butanone

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Phenol

Phenol, is a derivative of benzene in which a hydrogen atom has been replaced with a hydroxyl group. Phenols are treated like a separate group of compounds because their chemical properties are different from those of alcohols. Phenol is represented by this structure

Reactions of phenol

1. With bases

Unlike the alcohols, phenols react with sodium hydroxide, because phenol is a weak acid it reacts with strong bases such as NaOH to form salts. A solution of sodium phenoxide is produced.

Equation:………………………………………………………………….

Phenol does not react with weaker bases such as sodium hydrogen carbonate or ammonia. Reacting phenol with a base is a good way of making it water soluble. All salts are water soluble. Why do you think phenol is not very soluble in water?

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2. With sodium

Like the alcohols, phenol reacts with active metals such as sodium liberating hydrogen to make sodium phenoxide.

Equation:………………………………………………………………….

The reaction is more vigorous with phenol than it is with ethanol because phenol is more acidic.

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3. Electrophilic Substitution reactions of phenol

Phenol undergoes electrophilic substitution reactions like benzene. These reactions occur more readily than with benzene. This is because the lone pair on the oxygen of the OH (in a p orbital) can be delocalized into the benzene ring (the ring is said to be activated) and so the electron density increases inside the ring and it is more attractive towards the electrophiles. It is this effect that also causes the formation of the 2,4,6 substitutions in phenol when halogenation takes place.

i) The reaction of phenol with bromine

When bromine water reacts with phenol a white precipitate of 2,4,6-tribromophenol is formed. This is shown in the equation below. These are very mild conditions compared to the bromination of benzene, which requires pure bromine, and benzene in the presence of a catalyst.

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Student activity 6How could you make the following compounds, in 1 step, using reactions we have studied? State the starting compounds and the reagents.

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Uses of PhenolPhenol is used to manufacture a wide range of useful products.

A dilute aqueous solution was first used as an antiseptic in hospital operating theatres – carbolic acid, the ‘hospital smell’. Before disinfectants surgical instruments were often heated until they were red hot.

It is used in disinfectants such as Dettol. Vanillin is found in the seedpods of the vanilla orchid. It is

widely used as flavouring in food like chocolate or ice cream.

It is used as a resin for paints.

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The specification/syllabus

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