Aliphatic nucleophile substitution reaction
First of all, I will discuss the important definitions
of the aliphatic nucleophile substitution
reaction.
Substitution
When a
functional group is replaced by another functional group then this process is
known as substitution.
For
example: When the halide
ion of alkyl halide is replaced by hydroxide ion then alkyl halide is converted
into an alcohol class.
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| substitution |
Nucleophile
A reagent with an electron pair that has the ability to attack
an electron-deficient center is called a nucleophile. A nucleophile is also known
as nucleus-loving species.
For example OH-, Cl-
Electrophile
It is also known as electron-loving species. It has the ability to accept an electron pair.
Leaving
group
The group that leaves the molecule after the substitution
of the new group is known as leaving group. Another name of leaving group is
nucleofuge.
Substrate
An organic compound upon which nucleophile attacks and a new product is formed is known as substrate.
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| substitution |
Nucleophilic
substitution reaction
A type of reaction in which nucleophile substitute the
part of the molecule and convert it into a new class is known as substitution
reaction. It is denoted by SN, here S stands for substitution and N
stands Nucleophile. There are 2 types of aliphatic nucleophilic reactions.
vSN1
vSN2
SN1
mechanism
This mechanism consists of two steps. This is also
known as a unimolecular substitution reaction.
Ø
In
the 1st step ionization of
substrate take place results in the formation of a carbocation.
Ø
In
the 2nd step, nucleophile attack on the carbocation and converted it
into a new class of compounds.
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| SN1 mechanism |
Most of
the time solvent itself acts as a nucleophile that’s why this is also known as
solvolysis.
The step
which is slow one is known as the rate-determining step. In this reaction first
step is the rate-determining step because this is a slow one.
Kinetics
of SN1 mechanism
In the rate-determining step, only one molecule
(substrate) is involved so the rate depends upon the concentration of substrate
only. It is a first-order reaction.
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| Kinetics of SN1 mechanism |
SN2
mechanism
This mechanism is a bimolecular reaction. It occurs in
one step only. Substitution occurs directly without the formation of any
intermediate.
- vIn this mechanism, nucleophiles attack from the
opposite of leaving group. Nucleophiles attack from one side and at a time
leaving the group to leave the substrate from the other side.
- treaction is a concerted one. Bond formation between
nucleophile and substrate and bond cleavage between leaving group and substrate
occurs simultaneously.
 |
| SN2 mechanism |
This mechanism involves only one step so this step is
known as the rate-determining step.
Kinetics
of SN2 mechanism
Two molecules are involved in rate-determining steps so the rate of this mechanism depends upon the concentration of substrate and nucleophile
both. So this reaction is the second-order reaction.
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| Kinetics of SN2 mechanism |
Factors
affecting the nucleophilic substitution reaction
There are many factors that affect these mechanisms
which are as follow;
Effect
of substrate
As I already discussed, in both mechanisms the rate
of the reaction depends on the concentration of substrate. Substrate structure
is very important in this regard. SN1 mechanism consists of two
steps. In the 1st step, carbocation is formed so the ease of
formation of carbocation depends on the stability of carbocation. The greater
the stability of carbocation, the more rapidly it will be formed. So SN1 mechanisms are favorable
for tertiary substrate as compared to the secondary and primary substrate.
Tertiary
< < secondary < primary < methyl
SN2 mechanism occurs in one step so, the substrate which has less hindrance is suitable for this mechanism because in this mechanism the attacking of the nucleophile and leaving of leaving group occurs at a time. SN2 mechanism is more favorable for primary substrate as compared to the tertiary and secondary substrate.
Effect
of nucleophile
Nucleophile does not affect the rate of SN1
mechanism because it is not the part of rate-determining step. It attacks in
the step. It affects the rate of the SN2 mechanism because it is the
part of rate-determining step. Those molecules which are good electron donors
are better nucleophiles.
Effect
of leaving group
In both the mechanism, leaving groups leave along with
their electron pair so those leaving groups which easily accommodate the
electron pair after separating from the substrate are considered as good
leaving groups. Conjugate bases of strong bases are good leaving groups. Those
leaving who have the ability to delocalize the electron pair are also considered
a good leaving group. PhO-
is a better leaving group as compared to RO- because
delocalization occurs in phenoxide ions.
Effect
of solvent
Both the mechanism occurs in solution form so the reaction
medium also play important role in the rate of mechanism. SN1
mechanism is more favorable in polar solvents because polar solvents are more
helpful in ionization (1st step). They are also helpful in the
stabilization of charged species produced in the first step.
Polar
solvents may also decrease the nucleophilicity of nucleophiles due to solvation.
Solvation is less in aprotic solvents. So SN2 mechanism favors in
aprotic solvents than the polar solvents.
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