Electrophilic substitution reactions involve replacing a hydrogen atom in an aromatic ring with an electrophilic group. Nitration replaces hydrogen with a nitro group using nitric and sulfuric acid. Halogenation uses a halogen and Lewis acid. Sulphonation uses fuming sulfuric acid and oleum. Friedel-Crafts alkylation and acylation use an alkyl or acyl halide with aluminum chloride to add those groups. The mechanism involves generating an electrophile, forming a carbocation intermediate, and removing a proton. Ortho and para directing groups increase electron density, favoring substitution at those positions, while meta directing groups decrease electron density, favoring meta position. Polynuclear hydrocarbons from

1. ELECTROPHILIC
SUBSTITUTION
REACTIONS

2. NITRATION
 The replacement of a hydrogen atom in the ring by a nitro (-NO2) group is
called nitration. It is carried out by heating benzene with the nitrating mixture
consisting of concentrated nitric acid and sulphuric acid to about 330K.

3. HALOGENATION
The replacement of a hydrogen atom in the ring by a halogen atom (F, Cl, Br or
I) is called halogenation. Arenes react with halogen in the presence ofa
Lewis acid like anhydrous FeCl3, FeBr3 or AlCl3 to yield haloarenes.

4. SULPHONATION
The replacement of a hydrogen atom in the ring by a sulphonic acid (-SO3H)
group is called sulphonation. It is carried out by heating benzene with fuming
sulphuric acid and oleum.

5. FRIEDEL CRAFTS ALKYLATION
ALKYLATION:-
 When benzene is treated with an alkyl halide in the presence of anhydrous
aluminium chloride, alkylbenene is formed.
FREIDEL

6. FREIDEL CRAFTS ACYLATION
 The reaction of benzene with acyl halide or acid anhydride in the presence of
Lewis acid (AlCl3) Yields acyl benzene
CRAFT

7. Mechanism of electrophilic substitution reaction
According to experimental evidences, SE (S= substitution; E= electrophilic)
reaction are supposed to proceed via the following three steps:
a)Generation of the electrophile.
b)Formation of carbocation intermediate.
c)Removal of proton from the carbonation intermediate.
GENERATION OF AN ELECTROPHILE
The attacking reagent may not be strong electrophile. Therefore, first of allan
electrophile is generated by some preliminary reaction. For example ,
during chlorination of benzene, an electrophile (Cl +) is generated by
reacting with anhydrous AlCl3 used ascatalyst.
Cl2 +AlCl3
Cl+ +AlClˉ
4

8. FORMATION OF ACARBOCATION
The electrophile E+ approaches the electron cloud of the aromatic ring and
forms a bond with carbon, creating a positive charge on the ring. This results in
the formation of a sigma complex (called arenium ion).
The arenium ion gets stabilized by resonance
Due to resonance positive charge spread all over the benzene ring.

9.  REMOVAL OF APROTON
The carbocation formed loses a proton to the nucleophile (Nuˉ) present in the
reaction mixture to form a substitution product. During this step, the aromatic
character of the benzene ring is restored and this step is fast.
The loss of proton allows the two electrons from the carbon-hydrogen bond to
move to regenerate the aromatic ring and thus restoring the aromatic
character.

10. ADDITION REACTIONS
1) ADDITION OF HYDROGEN :
Benzene reacts with hydrogen in the presence of a catalyst such as nickel, or
platinum at 473 to 573 K under pressure to form cyclohexane.
2) ADDITION OF HALOGEN :
Benzene reacts with chlorine or bromine in the presence of sunlight and
absence of halogen carrier to form benzene hexachloride.

11. DIRECTIVE EINFLUENCE OF A FUNCITONAL GROUP IN A
MONSUBSTITUTED BENZENE
When monosubstituted benzene is subjected to further substitution, three
possible disubstituted products are not formed in equal amounts. Twotypes
of behaviour are observed
. Either ortho and para products or meta product is predominantly formed.This
behaviour depends on the nature of the substituent already present in the
benzene ring and not on the nature of the entering group.
This is known as directive influence of substituents.
a)Ortho and para directing groups
b)Meta directing group

12. ORTHO AND PARA DIRECTING GROUP
The groups which direct the incoming group to ortho and para position are
called ortho and para directing groups.
The directive influence of –OH (phenolic) group.
 The resonance structures of phenol show that the overall electron density on
the benzene ring increases in comparison to benzene. Therefore, it is an
activating group.

13. Meta directing group
ct.
The groups which direct the incoming group to meta position are called meta
directing groups. Some examples of meta directing groups are –NO2, -CN,-
CHO, -COR, -COOH, -COOR, -SO3H, etc. Let us take an example of Nitro
group.
Nitro group reduces the electron density
in the benzene ring due to its strong-I effe
Nitrobenzene is the resonance hybrid
of the following structures.

14. Benzene and polynuclear hydrocarbon containing more than two benzene
rings fused together are toxic and said to possess cancer producing
(Carcinogenic) property. Such polynuclear hydrocarbons are formed on
incomplete combustion of organic materials like tobacco, coal and
petroleum. They enter into human body and undergo various biochemical
reaction and finally damage DNA and causecancer.