Reactions of Arenes
Arenes (aromatic hydrocarbons) usually undergo electrophilic substitution reactions. However, under special conditions they can also undergo addition and oxidation reactions.
Electrophilic Substitution Reactions
Some electrophilic substitution reactions of arenes are given below :
Arenes react with halogens in the presence of a Lewis acid such as anhydrous FeCl3, FeBr3 or AlCl3 to produce haloarenes.
You may be wondering how Cl2 acts as an electrophile. The reason lies in the following reaction :
Generation of Cl+ electrophile :
The above reaction clarifies the role of Lewis acids. It is used in the above reaction to generate the desired electrophile.
Benzene on heating with a mixture of concentrated nitric acid and concentrated sulphuric acid yields nitrobenzene.
Generation of NO2+ electrophile : The following two steps may be followed to generate the NO2+ (nitronium ion) electrophile.
When benzene is heated with fuming sulphuric acid, benzene sulphonic acid is formed.
Benzene or its homologues on treatment with an alkyl halide in the presence of anhydrous aluminium chloride forms an alkylbenzene.
Generation of CH3+ electrophile :
The reaction of benzene or its homologues with an acyl halide or acid anhydride in the presence of anhydrous aluminium chloride yields acyl benzene.
Generation of CH3−C+=O electrophile :
Addition Reactions in Arenes
Formation of Cyclohexane
Hydrogenation of benzene at high temperature and/ or pressure in the presence of nickel yields cyclohexane.
Formation of benzene hexachloride (gammaxane)
In the presence of ultra-violet light, three chlorine molecules add to benzene to produce benzene hexachloride (C6H6Cl6) or gammaxane.
Combustion of Arenes
Arenes easily burn on heating in air or oxygen producing CO2 and H2O with sooty flames.
Revision includes earlier concepts
Let's do some revision. The reactions used in the answer are the ones that we have learned so far. To find the answer, all you need to do is click the 'Answer' button; however, it is not recommended. Try to put some effort before clicking the answer button.
Conversion : Prepare toluene from ethyne.
Do not forget the reactions. We will need them to do conversions in later chapters.