SN1 & SN2 Reaction Mechanism

SN1 & SN2 Reaction Mechanism

Aug 26, 2022, 16:45 IST

SN1 & SN2 Reaction Mechanism

Unimolecular Nucleophilic Substitution SN1

In unimolecular Nucleophilic substitution reaction, leaving group (a nucleophile) is first expelled in slow (rate determining) step followed by attack of another nucleophile. Some important characteristics of SN1 mechanism are:

(i) Better the leaving group, faster will be rate of SN1 reaction because leaving group is expelled in rate determining step.

R―I > R―Br > R―Cl (SN1 reactivity)

(ii) As the product of rate determining step is carbocation, we can interpret that more the stability of carbocation, more will be rate of SN1 reaction 3o R―X>2o R―X>1o R―X

3o R―OH > 2o R―OH > 1o R―OH

Iii. When solvent polarity is increased, increase in solvation of neutral reactant is almost unaffected, solvation of partially charged transition state increases slightly but the solvation of charged intermediate increases considerably (as shown in energy profile of rate determining step). Therefore, increase in solvent polarity decreases the activation energy and increases the rate of SN1 reaction. Hence, SN1 reaction is highly favoured in polar solvent.

(iv) As a carbocation (containing six valence electrons and a vacant orbital) is formed, rearrangements are possible before the attack of nucleophile.

(v) Nuˉ can be a weaker nucleophile than Lˉ. A strong Nuˉ is not required for SN1 reaction. Moreover, increase in nucleophilic strength of Nuˉ or its concentration will not increase the rate of SN1 reaction.

(vi) As carbocation is planar, attack of Nuˉ from both the sides of plane is equally favoured. Therefore, SN1 reaction proceeds with 50% inversion of configuration (attack of nucleophile from opposite side of leaving group) and 50% retention of configuration (attack of nucleophile from the side of leaving group) i.e. SN1 reaction proceeds with 100%

racemization (racemization is the process of formation of racemic mixture which is the equilibrium mixture of two mirror images of each other).it must be remembered that even if solvent is highly polar (which is generally used for doing SN1 reactions), the attractive force between the carbocation and Lˉ cannot be completely destroyed. They will have some attractive force and will have some extent of ion pairing (although very small). Therefore, attack of nucleophile from opposite side of leaving group will be more favoured (though very slightly). In other words, inversion of configuration will be slightly more than retention of configuration i.e. SN1 reaction leads to almost 100% racemization with little inversion. (vii)Unimolecular elimination (E1) will be the competing reaction. Bimolecular Nucleophilic Substitution (SN2 or SN2 ):

Mechanism of SN2