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 120o 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.
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|
|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:
Isomers with two substituents are called 1,2- ; 1,3- ; and 1,4- .
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.
Benzene is substituted by bromine in the presence of a catalyst such as iron or iron (III) bromide.
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.
Benzene reacts with a mixture of concentrated nitric acid and concentrated sulphuric acid at 50oC to produce nitrobenzene.
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.
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.
A chlorine atom can be substituted into the benzene ring in a similar way to bromine. Aluminium chloride is used as a catalyst.
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.
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.
Hydrogen adds on to benzene in the presence of a platinum catalyst at room temperature or a nickel catalyst at 150oC.
In uv light, chlorine adds on to benzene. The need for light suggests a free-radical mechanism.
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