As already mentioned benzene prefers to undergo substitution reactions in spite of the high degree of unsaturation. This is due to high stabilisation of benzene ring by resonance (or by delocalization of n electrons). The substitution reactions in benzene are initiated by electrophilies. Thus, characteristic reactions of benzene are electrophilic substitution reactions.
The molecule of benzene is symmetrical and the six carbon atoms as well as the hydrogen atoms occupy similar positions in the molecule. If one atom of hydrogen is substituted by a monovalent group or a radical (say methyl group), the resulting monosubstitution product exists in one form only. The position assigned to the substituent group does not matter because of the equivalent positions of the six hydrogen atoms. Thus, we have only one compound having the formula C6H5X where X is some monovalent group.
Some examples of monosubstituted derivatives of benzene are given below:
In addition to substitution reactions, benzene also undergoes addition reactions, but under more drastic conditions. Benzene and its homologues also undergo oxidation reactions and side chain substitution reactions.
I. ELECTROPHIUC SUBSTITUTION REACTIONS
Chlorine or bromine react with benzene in the presence of Lewis acids like ferric or aluminium salts of the corresponding halogen, which act as catalysts.
The replacement of hydrogen atom of benzene by a sulphonic acid group ( -SO3H) is known as sulphonation. The reaction is carried out by treating benzene with concentrated tetraoxosulphate (VI) acid containing dissolved sulphur(VI) oxide or with chlorosulphonic acid.
A nitro group ( – N02) can be introduced into a benzene ring using a nitrating mixture (a mixture of concentrated tetraoxosulphate(VI) acid and concentrated trioxonitrate(V) acid).
4. Friedel-Crafts Alkylation
On treatment with an alkyl halide in the presence of Lewis acids, such as anhydrous aluminium chloride as catalyst, benzene forms an alkylbenzene.
This reaction is called Friedel-Craft’s alkylation.
Directive Influence in Arenes
In benzene all the six hydrogens are equivalent. Therefore, any of the six positions can be occupied during the formation of monosubstituted products. However, when monosubstituted product is to be converted into disubstituted product, the substituent already present in the ring directs the incoming group to a particular position. This is referred to as directive influence of the group.
1. The substituents or groups which direct the incoming group to ortho and para positions are called orlho and para directing groups. Some examples are:
–CH3 , -Cl, -OH, -Br, -NH2 , etc.
For example, if we carry out nitration of toluene, the mixture of 2 and 4 nitrotoluene is formed
2. The substituents or groups which direct the incoming group to meta position are called meta directing groups. Some examples are:
-NOZ2 -CN, -CHO, -COOH, -SO3H, etc.
For example, nitration of nitrobenzene produces 1, 3-dinitrobenzene.
II. ADDITION REACTIONS OF ARENES
Benzene and its homologues undergo some addition reactions characteristic of alkenes and alkynes, but under more drastic conditions (like higher temperature and pressure). This is due to the fact that these compounds are relatively more stable and behave like saturated hydrocarbons.
1. Addition of Hydrogen
Benzene reacts with hydrogen in the presence of catalyst, nickel or platinum at 475-500 K or produce cyclohexane.
2. Addition of Halogens
Benzene reacts with chlorine in sunlight and in the absence of substance like A1Cl3, FeCI3 etc., to form benzene hexachloride (B.H.C.). Benzene hexachloride is an important pesticide.
III. OXIDATION REACTIONS OF ARENES
Arenes on combustion give carbon (IV) oxide and water, and large amount of heat is produced.
2C6H6 + 1502 à 12CO2 + 6h2O +Heat
Aromatic hydrocarbons burn with sooty flame due to high carbon content of these compounds.
2. Controlled Oxidation
Among arenes, benzene is too stable to be oxidised even with hot KMnO4 solution. The higher homologues, however, can be oxidised. During this reaction, the ring remains intact but the side alkyl chain is oxidised to – COOH group irrespective of the length of the side chain. Some examples are: