Explain aromatic electroplhilic substitution reactions of benzene.

Explanation

(1) Halogenation : Benzene reacts with bromine or chlorine in the presence of Lewis acids like `Fe Cl _(2), Al Cl _(3)` et. , to give corresponding halobenzene.

Similarly with bromine, bromobenzene is formed.
(2) Nitration: Benzene undergoes nitrogoes nitration when hated with a mixture of 1:1 (by vulume) concentrated nitric acid, and concentrated sulphuric acid (nitration mixiture) at a temperature below `60^(@)C.`

(3) Berizene reacts with fuming sulphuric acid (oleum) and gives benzene sulphonic acid.

(4) Friedel - Craft.s alkylation and acylation : Benzene reacts with alkyl halides and acyl h in the presence of Lewis acids `(Al Cl _(3), Fe Cl _(3))` and gives alkyl benzenes and acyl benzenes.

Benzene reacts with acetyl chloride in te presence of anhydrous aluminium chloride and acetophenone.

General Mechanisms : Electrophilic substitution reaction (s) proceeds in two steps as
l. Generation of electrophile
II. a) Formation of carbocation intermediate
b) Removal of proton from carbocation intermediate. . "
l. Generation of electrophite` E^(+) :` In the reactions halogenation, alkylation and acylatio benzene anhydrous `AlCl _(3)` the Lewis.acid produces electrophile `X^(+)` say `Cl^(+), R^(+) and RCO^(+)` by reacting the reagent chlorine, alkyl halide and acylhalide respectively.

ll a) Formation of carbocation : Electrophile generated above attacks one of the benz carbons to change it to `sp ^(3)` hydridised. The carbocation (Arrhenium ion) is stabilised through resonal.

Sigma complec loses aromatic character due to delocalisation of electrons stopping at `sp ^(3)` carbon
b) Losing of proton : To regain, aromatic character the `overset(+) (C)` loses one proton to `sp ^(3)` carbon on attacked of `(Al Cl _(4)) ` in case of halogenation , alklation and acylation and `HSO _(4) ^(-)` in case of nitration .