Hasham Anas Presentation

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Presented to
Dr. Maryam Munir
From Hasham Anas

 Nucleophilic Substitution Reactions in Aliphatic System
➢ DEFINITION:
Nucleophilic substitution reactions are a
class of reactions in which an electron rich nucleophile
attacks a positively charged electrophile to replace a
leaving group.
 SN1 REACTION
➢ DEFINITION:
The SN1 reaction is a substitution reaction
in organic chemistry, the name of which refers to the
Hughes-Ingold symbol of the mechanism. "SN" stands for
"nucleophilic substitution", and the "1" says that the
rate- determining step is unimolecular.
Such reactions are commonly seen in
reactions of secondary or tertiary alkyl halides under
strongly basic conditions or, under strongly acidic
conditions, with secondary or tertiary alcohols.
 Mechanism:
This type of mechanism involves two steps.
1. The first step is the ionization of alkyl halide in the

presence of aqueous acetone or ethyl alcohol.
2. The second step is the attack of the nucleophile.
An example of a reaction-taking place with an SN1

reaction mechanism is the hydrolysis of ter-butyl
bromide forming tert-butanol:
This SN1 reaction takes place in three steps:
i. Formation of a tert-butyl carbocation by separation of a leaving

group (a bromide anion) from the carbon atom: this step is slow.
ii. Nucleophilic attack: the carbocation reacts with the nucleophile. If the
nucleophile is a neutral molecule (i.e. a solvent) a third step is required to
complete the reaction. When the solvent is water, the intermediate is an
oxonium ion. This reaction step is fast.
iii. Deprotonation: Removal of a proton on the protonated nucleophile by
water acting as a base forming the alcohol and a hydronium ion. This
reaction step is fast.
Characteristics of SN1 Reaction:
❖ This is a two-step reaction process.

i. Carbon-halogen bond breaks which result in a positively
charged carbon (carbocation intermediate).
ii. Nucleophile attacks the carbocation and forms a new bond.
❖ Unimolecular and follows first-order kinetics.

❖ Rate of the reaction depends on the concentration of the
substrate (alkyl halide).
❖ Has a racemization stereochemistry, i.e., both retention and
inversion products are formed.
❖ A polar protic solvent is used to enhance the reactivity.
Factors Affecting SN1 Reaction Mechanism:
i. The reaction is favoured by a highly substituted alkyl halide

and ideally the one which will not lead to rearrangement.
ii. A non-basic nucleophile is preferred (to reduce the

elimination E1 side reaction).
iii. A good leaving group is preferred (like Iodine or Bromine).
iv. Polar protic solvents are most effective and suitable ones for
the reaction.
SN2 REACTION
Definition:
SN2 Reaction is a Nucleophilic Substitution reaction (A class
of reactions wherein the electron-rich nucleophile attacks a
positively charged electrophile to replace a leaving group) in
which two components are involved in the rate-determining
step. With simultaneous bond-making and bond-breaking steps,
SN2 reactions are bimolecular
➢ What is SN2 Reaction? The SN2 reaction is a type of reaction

mechanism that breaks one bond and synchronously forms one
bond, i.e. it is a single-step reaction. The SN2 reaction is a
substitution reaction, the name of which refers to the Hughes-
Ingold symbol of the mechanism. “SN” stands for “substitution
nucleophilic“, and the “2” says that the rate-determining step
is bimolecular.
SN2 REACTION
Mechanism:
There is a single transition state, according to the SN2 process,
since bond- breaking and bond-making happen simultaneously.
Note that the nucleophile must approach from the backside of
the carbon (Wherein the front side has the leaving group) for this
to occur (so-called backside attack).
2. In an SN2 reaction, there is no intermediate, just a transition

state. A transition state does not have a real lifetime, as starting
materials transition into products; it is the highest energy point
on the reaction coordinate.
Inhibition by Steric Hindrance:
✓ Reactions following SN2 mechanisms are particularly sensitive
to steric factors since they are retarded by steric hindrance
(crowding) at the site of reaction.
✓ In general, the order of reactivity of alkyl halides in SN2

reactions is methyl > 1° > 2°.
✓ The 3° alkyl halides are so crowded that they do not generally

react by an SN2 reaction mechanism.
Inhibition by Steric Hindrance:

➢ Bond forming and bond breakage take place in the same step.
➢ Bimolecular and follows second-order kinetics.
➢ Tertiary are unreactive.
➢ Rate of the reaction depends on the concentration of the substrate.
➢ A polar aprotic solvent is used to enhance the reactivity.
Factors Affecting SN2 Reaction:
➢ An unhindered Substrate (the back of the substrate must be as
unhindered as possible).
➢ A strong Nucleophile and a weaker leaving group.
➢ A polar aprotic Solvent with low dielectric constant.
Application of substitution reaction:
One of the most important reactions in synthetic organic

chemistry is an electrophilic aromatic substitution. These
reactions are used to synthesize essential intermediates that can
be used as precursors for pharmaceutical, agrochemical, and
industrial products manufacturing.

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From Hasham Anas

This type of mechanism involves two steps.

1. The first step is the ionization of alkyl halide in the

An example of a reaction-taking place with an SN1

i. Formation of a tert-butyl carbocation by separation of a leaving

❖ This is a two-step reaction process.

ii. Nucleophile attacks the carbocation and forms a new bond.

❖ Unimolecular and follows first-order kinetics.

i. The reaction is favoured by a highly substituted alkyl halide

ii. A non-basic nucleophile is preferred (to reduce the

iii. A good leaving group is preferred (like Iodine or Bromine).

➢ What is SN2 Reaction? The SN2 reaction is a type of reaction

2. In an SN2 reaction, there is no intermediate, just a transition

✓ In general, the order of reactivity of alkyl halides in SN2

✓ The 3° alkyl halides are so crowded that they do not generally

Inhibition by Steric Hindrance:

Application of substitution reaction:

One of the most important reactions in synthetic organic

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