The document discusses different types of electrophilic substitution reactions: SE1, SE2, and SEi. SE1 reactions follow first-order kinetics and involve two steps - rate-determining ionization and fast combination. SE2 reactions also follow first-order kinetics, but occur in a single step through a transition state. SE2 reactions can result in retention or inversion of configuration. SEi reactions are concerted mechanisms where the electrophile assists in removing the leaving group, leading to retention of configuration.

1. ELECTROPHILIC SUBSTITUTION REACTION
An electrophilic substitution reaction is a chemical reaction in
which the functional group attached to a compound is
replaced by an electrophile.

2. TYPES OF ELECTROPHILIC SUBSTITUTION
REACTION
There are two types of electrophilic substitution reaction:
1. SE1 ( Electrophilic substitution unimolecular reaction)
2. SE2 (Electrophilic substitution bimolecular reaction)

3. SE1 (Electrophilic Substitution Unimolecular
Reaction)
The substitution electrophilic unimolecular mechanism are termed SE1
reactions. These mechanisms follow first order of reaction kinetics with
respect to the substrate.
The SE1 mechanism involves two steps:
A. The first step is a rate slow determining ionization step leading to
formation of the carbanion.
B. The second step involves fast combination of electrophile resulting
in to the products.

4. SE1 (Electrophilic Substitution Unimolecular
Reaction)
STEP1:
STEP2:
R-
+
X X
+
R
SLOW
R-
+ Y+ R Y

5. SE1 (Electrophilic Substitution Unimolecular
Reaction)
Stereochemistry of SE1 mechanism: The chlorination of acetone to α-
chloroacetone.
STEP1:
(carbanion)
STEP2:
CH3COCH3 + Cl2
ClCH2COCH3 + Cl-
(α-chloroacetone)
(acetone)
+
OH
–
H CH2 C
O
CH2 C
O
CH3
CH3
(α-chloroacetone)

6. The electrophilic substitution bimolecular mechanism are termed SE2
mechanisms. These reactions follow bimolecular rate kinetics, where
the reaction rate is first order in substrate and first order in the
electrophile when both reactions are in dilute solutions.
Stereochemistry of SE2 mechanism: The SE2 reaction mechanism
proceeds via a single transition state where the old bonds and the
newly bonds are both present simultaneously. There are two possible
ways to electrophile attack to the reaction centre.
SE2(ELECTROPHILIC SUBSTITUTION
BIMOLECULAR REACTION)

7. SE2(ELECTROPHILIC SUBSTITUTION
BIMOLECULAR REACTION)
a. The first one is attack from the front, which is called SE2 (front) that
results in retention of configuration.
b. The second option is attack of the electrophile from the rear, which
is called SE2(back) that leads to inversion of configuration.

8. DIFFERENCE BETWEEN SE1 & SE2 REACTION
SE1 REACTION
1. The electrophilic substitution uimolecular
mechanism are represented by SE1.
2. SE1 reaction follows first order kinetics
with respect to the substrate.
3. The SE1 mechanism involves two steps: the
first step is a slow rate determining
ionization step leading to formation of the
carbanion. The second step involves fast
combination of electrophile resulting in to
products.
SE2 REACTION
1. The electrophilic substitution bimolecular
mechanism are represented by SE2.
2. SE2 reaction follows first order in substrate
and first order in the electrophile.
3. The SE2 reaction mechanism proceeds via a
single step where the old bonds and the
newly formed bonds are both present
simultaneously.

9. DIFFERENCE BETWEEN SE1 & SE2 REACTION
SE1 REACTION
4. There are only way to electrophile attack to
the reaction centre.
SE2 REACTION
4. There are only two ways to electrophile
attack to the reaction centre:
a. The first one is attack from the front, which
is called SE2(front) that results in retention
of configuration.
b. The second option is attack of the
electrophile from the rear, which is called
SE2(back) that results in inversion of
configuration.

10. SEi (ELECTROPHILIC SUBSTITUTION INTERNAL
REACTION)
When the electrophile attacks from the front and a portion of it assists
in the removal of the leaving group by forming a bond with the leaving
group at the same time when the new bond is formed, the mechanism
is called SEi (electrophilic substitution internal reaction).
Mechanism of SEi: In SEi mechanism, the electrophiles with suitably
placed functional groups attacks the sp3 hybridized substrate from the
front, whereby a portion of the electrophile might assist in the removal
of the leaving group. It is a concerted mechanism leading to retention
of configuration.
Z
Z
C X
Y
Y
X C
(retention of configuration)