F324 Arenes

benzene methylbenzene bromobenzene nitrobenzene

CH3 Br NO2

OH O

O

O

CH3

F324 Arenes

Arenes

NamingArenes are hydrocarbons containing one or more benzene rings. Another term for compounds containing a benzene ring is 'aromatic'. Organic substances are classified as aromatic (having benzene rings) or aliphatic (no benzene rings).

The basic benzene ring, C6H6 is commonly represented as a hexagon with a ring inside. You should be aware that there is a hydrogen at each corner although this is not normally shown.

Arenes occur naturally in many substances, and are present in coal and crude oil. Aspirin, for example, is an aromatic compound, an arene:

Naming of substances based on benzene follows familiar rules:

Structure of benzene compare the Kekulé and delocalised models for benzene in terms of p-orbital overlap forming π

bonds review the evidence for a delocalised model of benzene in terms of bond lengths, enthalpy

change of hydrogenation and resistance to reaction

Electrophilic Substitution describe the electrophilic substitution of arenas with:

(i) concentrated nitric acid in the presence of concentrated sulfuric acid, including equations for the formation of NO2

+

(ii) a halogen in the presence of a halogen carrier (iron, iron halides, aluminium halides), including equations for formation of X+ or δ+X–AlX3

δ–

outline the mechanism of electrophilic substitution in arenes, using the mononitration and monohalogenation of benzene as examples

explain the relative resistance to bromination of benzene, compared with alkenes, in terms of the delocalised electron density of the π bonds in benzene compared with the localised electron density of the C=C bond in alkenes

Phenols describe the reactions of phenol:

(i) with aqueous alkalis and with sodium to form salts(ii) with bromine to form 2,4,6-tribromophenol

explain the relative ease of bromination of phenol compared with benzene, in terms of electron-pair donation to the benzene ring from an oxygen p-orbital in phenol

state the uses of phenols in production of plastics, antiseptics, disinfectants and resins for paints

C

CCC

CC

H

H

H

H

H

H

H

C

H

C C

C C C

H

H

H

H

C

CCC

CC

H

H

H

H

H

HC

CCC

CC

H

H

H

H

H

H

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Numbers are needed to identify the positions of substituents. The carbons around the ring are numbered from 1-6 consecutively and the name which gives the lowest number(s) is chosen:

Determining the structure of benzene (historical)1825 – substance first isolated by Michael Faraday, who also determined its empirical formula as CH

1834 – RFM of 78 and molecular formula of C6H6 determined.Much speculation over the structure. Many suggested structures like:

1865 – Kekulé publishes suggestion of a ring with alternating double and single bonds:

displayed skeletal

This model persisted until 1922, but not all chemists accepted the structure because it failed to explain the chemical and physical properties of benzene fully. If C=C bonds were present as Kekulé proposed, then benzene would react like alkenes. For example benzene would be expected to decolourise bromine water. In fact benzene does not do this, nor does benzene do the other electrophilic addition reactions that alkenes do.

Kekulé's answer was to refine his model to account for this lack of reactivity, suggesting that the double bonds rapidly changed positions round the ring (two forms of benzene in rapid equilibrium) so that approaching electrophiles such as Br2 could not be attracted to a double bond before it moved when the structure changed.

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1922 X-ray crystallography used to measure bond lengths in arenes. Kathleen Lonsdale discovers that all the C-C bonds around the ring are the same length:

C-C bonds 0.153nmC=C bonds 0.134nmbenzene 0.139nm (all six carbon-carbon bonds)

This was important evidence that the Kekulé model was incorrect.

Further modern evidence that Kekulé's model was wrong:The energy change (enthalpy change) when the double bond in an alkene is hydrogenated can be measured.

Therefore when the three double bonds in Kekulé's benzene are hydrogenated we should see an enthalpy change of hydrogenation of 3 x -120 = -360 kJmol-1

When benzene is hydrogenated the enthalpy change is -208kJmol-1, which is 152kJmol-1 less than predicted. The conclusion has to be that the actual structure of benzene has much less energy than the proposed Kekulé structure – i.e. the real structure is 152 kJmol-1 more stable, which helps to explain why benzene is less reactive than alkenes. We can visualise this on an energy level diagram:

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The energy difference between the expected enthalpy of hydrogenation and the actual enthalpy of hydrogenation, which gives rise to the additional stability of benzene compared to the Kekulé's model, is called the delocalisation energy, or resonance energy of benzene.

Currently accepted structure of benzeneThe deficiencies in Kekulé's model led to the currently-accepted delocalised model for the structure of benzene.

In this model: the six carbon atoms are arranged in a planar hexagonal ring with each carbon sigma-

bonded to one hydrogen and two other carbons the shape around each carbon is trigonal planar with 120° bond angles and each

carbon to carbon bond is the same length each carbon has 4 outer shell electrons, three of which are involved in the sigma-

bonds to the two other carbons and the one hydrogen. This leaves a fourth outer shell electron in a 2p orbital above and below the plane of the ring.

The electron in the p-orbital overlaps with the electrons in the p-orbitals of the carbons on either side. This results in a ring of electron density above and below the plane of the benzene ring:

This overlap produces a system of -bonds which spreads (is delocalised) over the six carbon atoms. The p-electrons are no longer held to individual atoms but free to move throughout the ring. Since each carbon contributes one electron, the delocalised -system contains 6 electrons in total.

(Can you think of where you've come across a similar bonding concept with carbon forming 3 bonds and having one electron able to be delocalised ?)

This is what is meant by the circle inside the hexagon when drawing benzene.

C CC

HC

p-orbital

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How benzene reactsBecause it is more stable compared to alkenes, benzene does not (under normal conditions) undergo the addition reactions an alkene would:- it doesn't decolourise bromine water- it doesn't react with hydrogen halides such as HCl- react with other halogens (Cl2, I2)

Why? In an addition reaction, electrons from the delocalised system would be needed to form the bond to the atom/group being added (just like in an alkene the double bond electrons are needed). This would disrupt the delocalisation of the ring structure and result in the reaction product being less stable than benzene.

While benzene is less reactive than alkenes, that does not mean it is completely unreactive. Instead of addition reactions, benzene and its derivatives take part in substitution reactions where one atom/group (often one of the hydrogen atoms) connected to the ring is replaced by another. The delocalised ring is not disrupted, so the stability of the ring remains intact.

Substitution reactions of benzeneThe region of high electron density above and below the plane of the ring attracts electrophiles, so these are ELECTROPHILIC SUBSTITUTIONS

1) Nitration of benzeneEffect: One of the hydrogen atoms on the ring is replaced by a nitro (-NO2) group

Conditions: "nitrating mixture" of conc. HNO3 and H2SO4 at 50°Cthe H2SO4 is a catalyst

Notes:If the mixture gets hotter than 50°C then a further nitro- group may be added to the ring (i.e. the product reacts with further nitric acid)

Nitrobenzene is an important starting point for manufacturing dyes, pesticides and pharmaceuticals such as paracetamol.

Mechanism:The conc. sulphuric acid is used to generate an electrophile, NO2

+ from the conc. nitric acid. This ion is called the nitronium ion (or nitryl cation)

HNO3 + H2SO4 NO2+ + HSO4

- + H2O

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The nitronium ion is the electrophile which attacks the benzene ring, doing an electrophilic substitution reaction:

Finally the H+ which is produced reacts with the HSO4- ion, regenerating the H2SO4

catalyst:H+ + HSO4

- H2SO4

2) Halogenation of benzeneBenzene doesn't react with halogens on their own, but does react with halogens in the presence of a halogen-carrier catalyst.

e.g. when bromine water is added to benzene and a little FeCl3 is added, the bromine is decolarised and white fumes of HBr are seen.

Common halogen carriers for chlorination are FeCl3, AlCl3 and Fe metal which reacts with the Cl2 present to form FeCl3. Similarly for bromination, the common halogen carriers would be FeBr3 or AlBr3 or Fe.

Effect: a hydrogen is replaced on the benzene ring by a halogen atom

Conditions: halogens react with benzene at room temperature and pressure in the presence of a suitable halogen carrier

Uses: chlorobenzene is used as a solvent and in the production of pesticidesbromobenzene is used in the preparation of pharmaceuticals

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Mechanism: The role of the halogen-carrier is to generate Cl+ or Br+ ions, which are more powerful electrophiles than Cl2 or Br2 (see later).

e.g. AlBr3 + Br2 Br+ + AlBr4-

The Cl+ or Br+ ion is the electrophile which attacks the benzene ring, doing an electrophilic substitution reaction:

Finally the H+ reacts with the FeBr4- or AlBr4

- regenerating the halogen carrier catalyst:

e.g. H+ + AlBr4- AlBr3 + HBr

Comparing and contrasting the reactions of alkenes with those of benzeneIt is important to be able to discuss the differences and similarities in how an alkene (e.g. cyclohexene) react with an electrophile such as Br2 compared to how benzene reacts.Start by comparing the mechanisms:

CYCLOHEXENE reacts by ELECTROPHILIC ADDITION (AS F322 module)

The two electrons in the - bond of an alkene are localised between the two carbon atoms. The high electron density here can polarise the bromine, causing the reaction.

Whereas BENZENE reacts by ELECTROPHILIC SUBSTITUTION

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The six electrons in the -system above and below the plane of the benzene ring are delocalised over the six carbon atoms, so the electron density is lower. The bromine cannot be polarised sufficiently to react, and the lower electron density does not attract the electrophile so strongly.

Key differences:Benzene has delocalised -electrons spread spread over all six carbon atoms in the ring (6 electrons spread over 6 bonds). Alkenes have -electrons localised in the double bond (2 electrons localised in 1 bond), so benzene has a lower -electron density than alkenes.

When a non-polar molecule such as bromine approaches the benzene ring there is insufficient -electron density above and below any two carbon atoms cause the necessary polarisation of the bromine molecule, so a halogen carrier is needed to generate Br+ to attack the ring

The -electron density in an alkene's double bond is sufficient to polarise the bromine molecule so that it can act as an electrophile without needing a halogen carrier.

The extra stability of the benzene ring caused by the delocalisation of the -electrons means that it reacts by electrophilic substitution (which does not require the -system to be disrupted) so that the ring stays intact. Alkenes do not have this extra stability so they react by electrophilic addition, the -bond being destroyed in the process.

PhenolsIn a phenol there is an –OH group directly attached to the benzene ring.

e.g.

phenol 2-ethylphenol

Compare this to an aromatic compound where the –OH group is not attached to the benzene ring – these are aromatic alcohols, and not phenols.

e.g. 2-phenylethanol (not a phenol)

OH OH

CH2CH3

CH2CH2OH

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Properties of phenol pink crystalline solid (at room temperature and pressure) slightly soluble in water because the –OH group forms hydrogen bonds with water,

but the presence of the benzene rings makes phenols less soluble than alcohols weakly acidic in aqueous solution:

C6H5OH(aq) ⇌ C6H5O-(aq) + H+

(aq)

Uses a 5% solution of phenol (carbolic acid) was used as the first antiseptic in surgery

(Joseph Lister, 1870). alkylphenols such as 2-ethylphenol are found in surfactants and detergents chlorophenols such as 2,4-dichlorophenol are found in antiseptics/disinfectants such

as Dettol or TCP (trichlorophenol) salicylic acid is a phenol used in the preparation of aspirin and

other pharmaceuticals bisphenol is used in the production of epoxy resins for paints used in the manufacture of dyes phenols are nitrated in the production of explosives phenol can be reacted with methanal to form a resin which is the basis of

thermosetting plastic materials such as bakelite and melamine, and is the binding material in plywood and MDF

Reactions of phenol 1) forming saltsa) Phenol is neutralised by solutions of alkalis such as sodium or potassium hydroxide to form a salt + water:

C6H5OH + NaOH C6H5O-Na+ + H2O phenol sodium hydroxide sodium phenoxide water

b) Phenol reacts with reactive metals to form a salt. Effervesence is seen as hydrogen gas is given off.

C6H5OH + Na C6H5O-Na+ + ½ H2

phenol sodium sodium phenoxide hydrogen

2) reaction with bromineLike benzene, phenol undergoes electrophilic substitution with bromine. Unlike benzene. the reaction takes place at room temperature without the need for a halogen carrier catalyst.

When bromine water is added to an aqueous solution of phenol, the bromine is decolourised and a white precipitate of 2,4,6-tribromophenol is formed:

O

OH

OH

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In the same way, phenol readily undergoes nitration to form 2,4,6-trinitrophenol.

Why is bromination of phenol easier than bromination of benzene ? A lone pair (occupying a p-orbital) on the oxygen atom of the –OH group is drawn

into the benzene ring (adding two more electrons to the -system) This creates a higher electron density in the ring structure – the ring is activated The increased electron density increases the polarisation of Br2 molecules, which are

thus attracted more strongly towards the ring than in benzene, and able to attack as electrophiles.

N.B. the increased electron density in the ring increases the reactivity of phenols towards all electrophiles compared to benzene, not just bromine.

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