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A2 Advanced Organic Chemistry - Arenes


Arenes - "Periodic Table" Videos


Arenes - Nomenclature

(1) Prefix :

Count the number of carbon atoms in the side chain or use reactive group name.

(2) Suffix :

Add the word ___benzene directly to the prefix to obtain the compound's name.

Certain types of compounds have unique names.

Examples -

kekule structure of benzene or  displayed formula for benzene benzene
displayed formula for methylbenzene toluene methylbenzene
displayed formula for nitrobenzene nitrobenzene
displayed formula for benzoix acid benzoic acid
displayed formula for phenol phenol

When more than one group is present around the ring, one of them has to be assigned the job of being the major group. The other groups are then attached to carbon atoms relative to that major group.

Examples -

1,3-dimethylbenzene

There is another naming system for multi-substituted benzene rings that is used in industry and university. This involves giving the 2nd position the name ortho-, the 3rd position the name meta- and the 4th position the name para-, see the diagram below,

So, 1,3-dimethylbenzene is also known as meta-dimethylbenzene or m-dimethylbenzene. Also, since xylene is the old name for a dimethyl substituted benzene the molecule would be called meta-xylene or m-xylene.

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Arenes - Reactions

(1) Nitration :

This is a relatively easy process to accomplish with a mixture of concentrated sulphuric and concentrated nitric acids. This mixture reacts together to produce an unstable nitronium ion, NO2+ :

HNO3 + 2H2SO4 → NO2+ + 2HSO4- + H3O+

This ion is attracted to the electron rich benzene ring - causing an electrophilic substitution reaction to occur :

The products of the reaction are nitrobenzene and hydrogen ions.

For a detailed look at the mechanism of this particular reaction see the Reaction Mechanism section later.

(2) Halogenation :

All normal unsaturated compounds, i.e. alkenes, will react with halogens, in the absence of light, to give the dihalogenated addition product (see the section on alkenes).

The nature of the benzene ring and its delocalised electrons prevents this sort of reaction from occurring.

Aromatic compounds will react with halogens (i.e. Cl2 and Br2), however, they require a catalyst - a metal halide (e.g. iron(III) bromide or aluminium chloride) or iron filings.

These catalysts act to polarise the halogen bond (see alkene reaction mechanism) forming a d+ charge on a halogen atom (Cl or Br) :

This species is then able to react with the benzene ring in exactly the same manner as the nitronium ion does in the nitration reaction (see reaction mechanism below).

The products will be the monohalogenated benzene compound (i.e. chlorobenzene or bromobenzene) and the hydrogen halide (i.e. HCl or HBr).

The above example shows only iron(III) bromide and bromine as an example of one set of reactants. Aluminium chloride and chlorine works in exactly the same manner.

(3) Friedel-Crafts reaction :

This type of reaction involves using a haloalkane to add an alkyl group to a benzene ring. As with the halogenation above a metal halide catalyst is needed to generate the necessary alkyl electrophile :

where R=any alkyl chain e.g. CH3CH2.

This complex then reacts with the benzene ring in a similar manner to the other electrophiles looked at so far, to form an alkyl benzene compound :

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Arenes - Reaction Mechanism

Electrophilic Substitution -

This is a two stage process -

(i) The first stage involves the addition of the electrophile, NO2+ in this case, to the benzene ring. One of the double bonds breaks and gives its electrons to form a new C-N bond. This leaves a carbon atom in the hexagon with a positive charge on it, because it has lost one of its own electrons.

(ii) The second stage is the elimination of the hydrogen atom, which comes out as a proton, H+, when it gives the electrons in the C-H bond back to the benzene ring.

In the above diagram, each curly arrow shows the movement of two electrons in forming a new bond between two atoms.

During the overall reaction the benzene ring's electrons stay delocalised, there is only a transient step in the middle where the stable electron arrangement is disturbed.

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Arenes - Delocalisation of electrons

The carbon-carbon double bond in an alkene consists of a σ-bond formed by the overlap of two sp2 orbitals on the carbon atoms and a π-bond formed by the overlap of the two spare pz orbitals on the adjacent carbon atoms :

In a benzene ring there are six carbon atoms bonded together in the shape of a hexagon by six σ-bonds -

This gives six spare pz orbitals arranged around the ring. They are so close together that instead of just two overlapping to form a normal π-bond, the electrons are able to move around the whole of the hexagon (above and beneath) forming a delocalised ring of electrons :

This very stable arrangement is extremely loathe to be broken during any reactions. This leads to the substitution reactions occurring and the fact that addition reactions, i.e. those that would break the delocalised system of electrons, do NOT occur.

A space filling model of benzene shows the closeness of the atoms in the molecule :

Comparison with alkenes :

Arenes Alkenes
Electrophilic reactions Electrophilic reactions
Substitution reactions Addition reactions
Stable ring of 6 electrons Stable π-bond of 2 electrons
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Arenes - Industrial synthesis of arenes

The reactions noted above are used in industry to make a wide variety of useful materials.

The nitration reaction is used directly to make nitrated explosives such as TNT, trinitrotoluene, or more formally 2,4,6-trinitromethylbenzene, C6H2(NO2)3CH3. It can also be used as an intermediate step in the production of pharmaceuticals and dyes, where the nitro groups are reduced to form amine groups (see the nitrogen page for details about forming amines).

The Friedel-Crafts reaction can be used as an intermediate stage in the production of other materials. For example, reacting benzene with bromoethane and subsequently changing the ethyl group into an unsaturated group yields phenylethene (styrene).

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Arenes - Reactions of phenol

(1) Reaction with sodium :

As with other alcohols, i.e. compounds containing the hydroxyl group, -OH, phenols react with sodium metal to produce sodium phenoxide and hydrogen :

(2) Reaction with hydroxides :

Unlike other alcohols, phenol is acidic enough to react with strong bases such as sodium hydroxide. Again, this yields sodium phenoxide (and water) :

However, phenol will not react with weak bases, such as sodium carbonate.

(3) Relative acidity :

Water vs. ethanol vs. phenol vs. carboxylic acids -

Water and ethanol have virtually the same pH value of about 7. Neither of these two will react with strong or weak bases. They will react with metals near the top of the reactivity series i.e. potassium, sodium, lithium, calcium and magnesium.

Phenol is much more acidic than water or ethanol and as well as reacting with the reactive metals mentioned it will also react with strong bases i.e. hydroxides. It does not react with weak bases such as carbonates and ammonia.

Carboxylic acids have a much more acidic proton than phenols and so they react with reactive metals, strong bases and weak bases. This gives one more method of telling the difference between organic compounds in analytical tests.

(4) Reaction with bromine :

Unlike other arenes, the benzene ring in phenol is about 100 times more reactive than in benzene itself, because of the hydroxyl group attached to it. Therefore phenol does decolourise bromine water, though it still undergoes a substitution reaction (rather than the addition of a normal alkene) :

This reaction works with any benzene ring with at least one oxygen atom attached to it, for example,

anisole dibenzo-18-crown-6
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Arenes - Uses of phenol

Phenol and benzene substituted derivatives of it are used in antiseptics and disinfectants, e.g. TCP, 2,4,6-trichlorophenol.

Analgesics, such as Aspirin and Oil of Wintergreen, are also derivatives of phenol.

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written by Dr Richard Clarkson : © Saturday, 1 November 1997

Updated : Saturday, 17th March, 2012

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