Electrophilic substitution reactions of benzene

Benzene gives substitution reactions as described below.

Nitration

When benzene is heated with concentrated HNO₃ in presence of concentrated sulphuric acid at 333 K, we get nitrobenzene.

[A mixture of conc. HNO_3+conc.H₂SO_{4 is called as 'Nitrating Mixture'.]}

benzene nitrobenzene

At a higher temperature, another H-atom can be replaced by NO₂ group to produce dinitrobenzene.

nitrobenzene m-dinitrobenzene

Sulphonation

When benzene is heated with concentrated sulphuric acid at 353 K for 8 hours, benzene sulphonic acid is formed.

benzene benzene sulphonic acid

Oleum gives m-disulphonic acid.

benzene oleum benzene disulphonic acid

Mechanism

The accepted mechanism for sulphonation is given below:

Halogenation

In halogenations, the nature of the products depends upon the reaction conditions.

Benzene in the presence of a halogen carrier e.g., FeCl₃ or iodine, reacts with chlorine and bromine even in dark and at room temperature to give mono- and di-halo derivatives,

benzene chlorobenzene

chlorobenzene o-dichlorobenzene p-dichlorobenzene

Bromine reacts with benzene to form bromo derivatives. Iodine does not react with benzene.

Mechanism

The accepted mechanism of substitutional halogenation of aromatic compounds is described below.

Friedel-Crafts reaction

This reaction is used for introducing an alkyl (R-) or acyl (RCO-) group into the benzene ring. Benzene, in the presence of anhydrous aluminium chloride (AlCl₃), reacts with substances such as haloalkane (RX), alcohol (ROH) and acid chlorides (RCOCl) etc., to give the corresponding derivative of benzene. Some examples of alkylation are,

benzene chloromethane toluene

bromoethane ethylbenzene

These two reactions are called alkylation of benzene.

Some examples of acylation are,

acetophenone

Introduction of COCH₃ group in benzene ring is called acylation of benzene.

Directive influence in arenes

During the formation of monosubstituted products in benzene as all the six hydrogens are equivalent any of the six positions can be occupied. But, when the monosubstituted product is to be converted into disubstituted one, the existing substituent present in the ring directs the incoming group to a particular position. This is referred to as directive influence of the group. Depending upon their directive influence, various groups/substituents/functional groups can be divided into two categories as described:

• The substituents or groups, which direct the incoming group to ortho and para positions are called ortho and para directing groups. For example:

-CH₃,- C₂H_5-, -Cl, -OH, -Br, -NH₂, -NHR, -NR₂, -NHCOCH₃, -OCH₃ etc.

If we carry out nitration of toluene, the mixture of ortho and para nitrotoluenes are formed.

Toluene o-Nitrotoluene p-Nitrotoluene

These groups (except halogens) increase the electron density at the ring through resonance effect. Thus the reactivity of benzene ring towards electrophilic substitution reactions increases.

• The substituents or groups which direct the incoming group to meta position are called meta directing groups. For example:

-NO₂, -CN, -CHO, -COR, -COOH, -COOR, -SO₃H, etc.

The nitration of benzoic acid produces m-nitrobenzene.

Benzoic acid m-Nitrobenzoic acid

These groups withdraw the electrons from benzene ring through resonance effect, reducing the electron density at the benzene ring. They decrease the reactivity of benzene ring towards electrophilic substitution reaction and make it less susceptible to the electrophilic attack.

Oxidation

Benzene is slowly oxidised by chromic acid, acidified KMnO₄ etc., to CO₂ and H₂O.

C₆H₆ + 15 [O]6CO₂ + 3H₂O DH = -650kJ

When benzene vapours mixed with air are passed over V₂O₅ (vanadium pentoxide) at 400°C, maleic anhydride is obtained.

maleic anhydride

Oxidation of alkyl sidechain

The alkyl sidechain in the molecule of an arene can be oxidised under different conditions.

• With hot acidified KMnO₄ or K₂Cr₂O₇, the sidechain gets oxidised to COOH group irrespective of the length of the sidechain. For example,

toluene benzoic acid

phenylethane benzoic acid

• With weak oxidising agents such as acidic manganese dioxide (MnO₂) or chromylchloride (CrO₂Cl₂), the side chain is oxidised to aldehyde (-CHO) group.