Logo : organicmystery.com Online Organic Chemistry Tutorial

Reactions of Arenes

«Previous

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 :

Halogenation

Arenes react with halogens in the presence of a Lewis acid such as anhydrous FeCl3, FeBr3 or AlCl3 to produce haloarenes.

Electrophilic substitution reaction : halogenation

You may be wondering how Cl2 acts as an electrophile. The reason lies in the following reaction :

Generation of Cl+ electrophile :

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.

Nitration

Benzene on heating with a mixture of concentrated nitric acid and concentrated sulphuric acid yields nitrobenzene.

Electrophilic substitution reaction : nitration

Generation of NO2+ electrophile : The following two steps may be followed to generate the NO2+ (nitronium ion) electrophile.

Generation of nitronium ion
Generation of nitronium ion

Sulphonation

When benzene is heated with fuming sulphuric acid, benzene sulphonic acid is formed.

Electrophilic substitution reaction : sulphonation

Friedel-Crafts Alkylation

Benzene or its homologues on treatment with an alkyl halide in the presence of anhydrous aluminium chloride forms an alkylbenzene.

Friedel-Crafts alkylation

Generation of CH3+ electrophile :

generation of CH3+ electrophile

Friedel-Crafts Acylation

The reaction of benzene or its homologues with an acyl halide or acid anhydride in the presence of anhydrous aluminium chloride yields acyl benzene.

Friedel-Crafts Acylation
Friedel-Crafts acylation

Generation of CH3−C+=O electrophile :

Generation of acetyl cation

Addition Reactions in Arenes

Formation of Cyclohexane

Hydrogenation of benzene at high temperature and/ or pressure in the presence of nickel yields cyclohexane.

hydrogenation of arenes

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.

Formation of gammaxane

Combustion of Arenes

Arenes easily burn on heating in air or oxygen producing CO2 and H2O with sooty flames.

Combustion of Arenes

 

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.

«Previous