CD - COLOUR BY DESIGN

Aromatic Compounds

Benzene and Other Arenes

The most important aromatic hydrocarbon is benzene, C₆H₆. A benzene molecule consists of a planar ring of six carbon atoms. The first structure proposed for benzene was by Kekule. He proposed alternate single and double bonds in the ring, with the structure halfway between the two:

Neither of these structures represents the true structure. All bond angles are 120^o and all the carbon-carbon bond lengths are the same, 0.139nm. Since the bond lengths in carbon-carbon single and double bonds are 0.154nm and 0.132nm respectively, these bonds in benzene are intermediate between single and double bonds.

Each carbon atom has four electrons available for bonding, three of which are used to form covalent bonds with two adjacent carbon atoms and one hydrogen atom. This leaves one electron on each carbon atom. These six electrons are spread out evenly and shared by all six carbon atoms in the ring. They are said to be delocalised around the ring, a structure represented by drawing a circle inside the ring.

The delocalisation of electrons makes benzene more stable than you would expect if it had a Kekule type structure, with three double bonds and three single bonds.

Naming Aromatic Compounds

The hydrogen atoms in a benzene ring can be substituted by other atoms or groups of atoms. Some examples are given below:

Substituent Group		Systematic Name	Common Name
Methyl	-CH₃	Methyl benzene	Toluene
Chloro	-Cl	Chlorobenzene	-
Nitro	-NO₂	Nitrobenzene	-
Hydroxy	-OH	Phenol	-
Amino	-NH₂	Phenylamine	Aniline
Carboxyl	-CO₂H	Benzenecarboxylic acid	Benzoic acid

Where more than one hydrogen atom is substituted, numbers are used to indicate which of the six possible positions are used, hence:


methyl-2-chlorobenzene	methyl-3-chlorobenzene	methyl-4-chlorobenzene

Isomers with two substituents are called 1,2- ; 1,3- ; and 1,4- .

Reactions of Arenes

Arenes take part in both substitution and addition reactions. As there is a high electron density in the benzene ring, benzene tends to attract positive ions, or atoms with a partial positive charge within molecules, so benzene, like alkenes, reacts with electrophiles. However, the reactions are much slower with benzene. Benzene undergoes substitution reactions with electrophiles.

Substitution Reactions

Bromination
Benzene is substituted by bromine in the presence of a catalyst such as iron or iron (III) bromide.

    +    Br₂            +    HBr

The iron is converted into iron (III) bromide. It is thought that the FeBr₃ helps to polarise the bromine molecule by accepting a lone pair (of electrons) from one of the bromine atoms.



The Br⁺ attacks the benzene ring, and an H⁺ ion (proton) is lost from the ring.

     +    FeBr₄^-            +    HBr    +    FeBr₃
Nitration
Benzene reacts with a mixture of concentrated nitric acid and concentrated sulphuric acid at 50^oC to produce nitrobenzene.

+ HNO₃ + H₂O

At higher temperatures, further substitution of the ring occurs to give di- and tri- substituted compounds.

There is strong evidence that the nitrating agent is the nitronium ion, NO₂⁺.

This is formed from a reaction between nitric acid and sulphuric acid.

HNO₃ + 2H₂SO₄ NO₂⁺ + 2HSO₄^- + H₃O⁺
Sulphonation
If benzene and concentrated sulphuric acid are refluxed for several hours, benzene sulphonic acid is produced. The electrophile in this case is SO₃, which carries a large positive charge on the sulphur.

+ H₂SO₄ + H₂
Chlorination
A chlorine atom can be substituted into the benzene ring in a similar way to bromine. Aluminium chloride is used as a catalyst.

+ Cl₂ AlCl₃

chlorobenzene + HCl
Friedel-Crafts Reactions

Aluminium chloride can be used as a catalyst to bring about substitution of an alkyl group into a benzene ring. If benzene is warmed with chloromethane and anhydrous aluminium chloride, methylbenzene is formed. The reaction is called alkylation.

+    CH₃Cl AlCl₃

reflux +    HCl

A similar reaction takes place if benzene is reacted with an acyl chloride, the product this time being a ketone. The reaction is called acylation.

    +    CH₃COCl AlCl₃

reflux +    HCl

Addition Reactions

Hydrogen
Hydrogen adds on to benzene in the presence of a platinum catalyst at room temperature or a nickel catalyst at 150^oC.

+ 3H₂
cyclohexane
Chlorine
In uv light, chlorine adds on to benzene. The need for light suggests a free-radical mechanism.

+ 3H₂
cyclohexane