Electrophilic substitution reaction

1. Molecular Orbital Diagram of Benzene
&
Electrophilic Substitution Reaction
Mr. Yashvant V. Khandare

2. Molecular Orbital Diagram of Benzene
 Molecular Formula of Benzene is C6H6.
 All the carbon atoms are present in sp2
hybridization state.
 C-C bonds -- sp2-sp2 overlaps
 C-H bonds -- sp2-s overlaps
 All bonds lie in a same plane at an angle
120℃.

3. s

4. Molecular Orbital Diagram of Benzene
 Each carbon contains one unhybridized p-
orbital with single electron.
 P-orbitals perpendicular to sigma bonds and
parallel to each other.
 These overlap sidewise on either to form
delocalized 𝝅 𝑴. 𝑶.
 Containing electron cloud of six delocalized
electrons which are revolving above and
below the plane.

5. Molecular Orbital Diagram of Benzene
s

7. Substitution reaction
A chemical reactions in which an atom,
ion, or group of atoms or ions in a
molecule is replaced by another atom,
ion, or group.

8. Electrophilic Substitution Reaction
Definition :- Electrophilic substitution
reactions are chemical reactions in which
an electrophile displaces a functional group
in a compound, which is typically, but not
always, a hydrogen atom.

9. Definition :- Replacement of hydrogen atom
of benzene nucleus by nitro (NO2) group is
called nitration of benzene.
Ex. Benzene on heating with mixture
(conc,HNO3 & conc. H2SO4) gives
nitrobenzene.
Nitration

10. Nitration

11. Mechanism :
It consist of following three steps
a) Formation of electrophile : Nitrating mixture gives
nitronium ion (NO2+) as electrophile.
b) Formation of 𝝈 complex: Nitronium ion (NO2+)
attacks on benzene to give intermediate carbonium ion
called 𝝈 complex.
c) Formation of final product : In this step, there is
removal of proton by attack of base HSO4- to give
nitrobenzene. It is fast step.

12. s
HSO4-
H2SO4

13. Halogenation
&
Friedel Craft Reaction

14. Halogenation
 Replacement of hydrogen atom of benzene nucleus
by halogen is called halogenation of benzene.
 For example benzene reacts with chlorine in
presence of catalyst anhydrous AlCl3 to give
chlorobenzene.

15. Chlorination of Benzene

16. Mechanism
Step-I : Formation of Electrophile
Chlorine reacts with AlCl3 to give chloronium
(Cl+) as electrophile.
Cl2 + AlCl3 --- Cl+ + AlCl4
-
Chloronium ion

17. Step-II : Formation of σ-Complex
Chloronium ion attacks on benzene to give intermediate
carbonium ion called sigma complex.
It is stabilized by resonance. It is slow & rate determining step.

18. Resonating Structures

19. Step-III :- Formation of Final Product
In this step, there is a removal of proton by
attack of base AlCl4
- to give chlorobenzene.
It is fast step.
AlCl4
-
AlCl3

20. Friedel Craft Reactions
a)Friedel Craft Reaction:
Replacement of hydrogen atom of benzene
nucleus by alkyl group is called alkylation of benzene.
For example, benzene on heating with alkyl halide in
presence of catalyst anhydrous AlCl3 or FeCl3 gives
alkyl halide.

21. Friedel Craft Reaction
Alkylation & Acylation

22. Friedel Craft Reactions
a)Friedel Craft Reaction:
Replacement of hydrogen atom of benzene
nucleus by alkyl group is called alkylation of benzene.
For example, benzene on heating with alkyl halide in
presence of catalyst anhydrous AlCl3 or FeCl3 gives
alkyl halide.

27. Friedel Craft Reaction
Acylation

28. Friedel Craft Acylation
Replacement of hydrogen atom of benzene nucleus
by acyl (R-CO) group is called acylation of benzene.
Example :- Benzene on heating with acetyl chloride in
presence of anhydrous AlCl3 gives acetophenone.

29. Mechanism
It consist of following three steps:
Step-I : Formation of Electrophile
Acetyl Chloride reacts with AlCl3 to give acetyl
carbonium ion as electrophile.

30. Step-II : Formation of σ-Complex
Acetyl Carbonium ion attacks on benzene to give intermediate
carbonium ion called sigma complex.
It is stabilized by resonance. It is slow & rate determining step.

31. Step-III :- Formation of Final Product
In this step, there is a removal of proton by
attack of base AlCl4
- to give Acetophenone.
It is fast step.