Critical Thinking

Mechanism configuration. On the other hand, if the

Mechanism
of Nucleophilic substitution reaction

            The reaction
mechanism of nucleophilic substitution is classified as

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a)     Bimolecular
Nucleophilic substitution reaction (SN2)

b)     Unimolecular
Nucleophilic substitution reaction (SN1)

Stereo
chemical aspects of SN reaction

            To
understand the stereo chemical aspects of Nucleophilic substitution reactions
we must understand basic idea of stereo chemistry (Refer unit 11 Class XI for
explanation)

            The
stereo chemistry plays a significant role in nucleophilic substitution reactions

i)                  
The SN2 reaction proceeds
with complete stereo chemical inversion ie the optical activity of the product
is opposite to that of reactant.

ii)               
The SN1 reaction proceeds
with racemisation ie the product formed is optically inactive but can be
resolved into two optically active isomers.

Let us discuss the
following terms to know the above stereo chemical aspects by following
illustration.

1)     Chiral carbon

J.Vant Hoff and C.LeBel both
independently in the year 1874 pointed out that the four valancies of a carbon
atom are directed towards the corners of a regular tetra hedron and if all the
atoms of groups attached to a carbon atom are different, such a carbon is
called       asymmetric carbon or chiral carbon or stereo centre.

2)     Enantiomers

Optical isomers whose molecular
structures are non super imposable mirror images of each other and which rotate
the plane polarised light usually but in opposite direction are called enantiomers.

3)     Racemisation

A mixture containing equal amounts
of enantiomers (dextro and leavo forms) is called racemic mixture and are
optically inactive. The process of conversion of one enantiomer (+ or -) into a
racemic mixture is called racemisation.

4)     Inversion and retention

If the relative configuration of
atoms/groups around the chiral centre in an optically active molecule remains
the same after and before the reaction, the reaction is said to proceed with retention of configuration. On the
other hand, if the relative configuration of the atoms/groups around a stereo
centre in the product is opposite to that of reactant, the reaction is said to
proceed with inversion of configuration.

Example

            If
(Y) is the only product, the process is called the retention of configuration
because (Y) has the same configuration as reactant.

            If
(Z) is the only product, the process is called inversion of configuration
because (Z) has the configuration opposite to the reactant (X).

            If
an equimolar mixture of (Y) and (Z) is formed, then the process is called
racemisation and the product is optically inactive because one isomer will
rotate light in the direction opposite to another.

            Thus
form the above it is clear three different products may be formed when a
chemical reaction involves bond cleavage or bond formation at a chiral carbon
(optically active)

            In
short, in case of optically active haloalkane the product formed by SN2
reactions will have inversion of configuration and in SN1 reaction
racemisation takes place.

SN2
Mechanism

SN2
stands for bimolecular Nucleophilic substitution

                        “S”
stands for substitution

                        “N”
stands for nucleophilic

                        “2”
stands for bimolecular (two molecules are involved  in the rate determining step)

            The
rate of SN2 reaction depends upon the concentration of both alkyl
halide as well as the nucleophile.

            Rate
of reaction = K alkylhalideNucleophile

            This
suggest that the reaction is second order and occurs in one step.

            This
reaction involves the formation of a transition state in which both the
reactant molecules are partially bonded to each other. The attack of
nucleophile occurs from the back side and the halide ion leaves from the front
side. The carbon at which substitution occurs has inverted configuration during
the course of reaction just as an umbrella has tendency to invert in a wind
storm. This inversion of configuration is called Walden inversion; after paul waldsen who first discovered the
inversion of configuration of a compound in SN2 reaction.

            We
will understand SN2 reaction mechanism by taking an example of
reaction between chloromethane and aqueous KOH.

                                                                                                                                                            (14.44)

 

 

                                                                                                                                                                                           

 

           

 

 

 

            SN2 reaction of optically
active haloalkane are always accompanied by inversion of configuration at the
asymmetric centre.

Example

            (-) – 2 – Bromo octane is heated
with sodium hydroxide (+) – 2 – Octanol is formed in which   – OH group occupies a position opposite to
what bromine had occupied,

                                                (-)
2 – Bromo octane                                   (+)
2 – Octanol (product)

            (-) – 2 – bromo octane and (+) – 2 –
octanol have opposite sign of optical rotation even though they are not
enantiomers.

SN1
Mechanism

            SN1
stands for unimolecular Nucleophilic substitution

                        ‘S’ stands for substitution

                        ‘N’ stands for nucleophilic

                        ‘1’ stands for unimolecular (one
molecule is involved in the rate determining step)

            The rate of SN1 reaction
depends upon the concentration of alkyl halide and is independent of the
concentration of the nucleophile.

            Rate of reaction = Kalkyl halide

            This suggest that the reaction is
first order and occurs in two steps.  

            We will understand SN1
reaction mechanism by taking a reaction between tertiary butyl bromide with
aqueous KOH.

            This reaction takes place in two
steps as shown below

Step – 1 Formation of
carbocation

            The polar C – Br bond breaks slowly
and carbocation and bromide ion are formed. This step is slow and hence it is
the rate determining step.

Step – 2 Nucleophilic
attack on carbocation

            The carbocation formed immediately
reacts with the nucleophile. This step is fast and hence does not affect the
rate of the reactions.

            This step can be illustrated as
shown below.

            As shown above, the nucleophilic
reagent OH- can attack carbocation from both the sides, they will be
mirror image of each other.

            In the above example substrate
tertiary butyl bromide is not optically active, hence the obtained product is
optically inactive. If halo alkane substrate is optically active then product
obtained will be optically inactive racemic mixture. As nucleophilic reagent OH-
can attack carbocation from both the sides, equal proportion of dextro and levo
rotatory optically active isomer which forms optically inactive racemic
mixture.

Example

            Hydrolysis of optically active 2 –
Bromo butane which give racemic (or)  butan-2-ol

Step – 1 : Formation of
carbocation

 

Step – 2 : Nucleophilic
attack on both faces (front and rear) with almost equal giving 50 : 50
                mixture of two
enantiomers (Racemic mixture)

            The order of reactive reactivity of
haloalkanes towards SN1 and SN2 reaction is given as.

SN2 reaction

Tertiary halide, Secondary halide, Primary halide,
CH3X

SN1 reaction

Comparision of SN2
and SN1 mechanism

S.No

Concept

SN2

SN1

1

Number
of steps

One
step process

Two
step process

2

Kinetics

Second
order

First
order

3

Molecularity

Bimolecular

Unimolecular

4

Transition
state

One
step, one transition state

Two
steps, two transition

5

Direction
of attack of Nucleophile

Only
back side attack

Back
side attack and front side attack

6

Type
of alkyl halides

Primary
alkyl halide

Tertiary
alkyl halide

7

Stereo
chemistry if substrate is optically active

Inversion
of configuration

Racemisation

8

Nature
of solvent

Low
polar solvent favours this mechanism

High
polar solvent favours this mechanism.

x

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