2. DIFFERENT TYPES OF MECHANISM OF
NUCLEOPHILIC SUBSTITUTION
REACTIONS
Nucleophilic substitution reaction has two mechanisms.
➢Sn1 mechanism
➢Sn2 mechanism
Where ‘s’ represents chemical substitution, ‘n’ represents nucleophilic and the number stands
for the kinetic order of a reaction.
3. Sn1 Mechanism
• Sn1 stands for substitution nucleophilic unimolecular.
• The rate determining step i.e., the slowest step of the reaction is unimolecular.
• Sn1 is a two step process.
• A stable carbocation forms.
• Polar solvents like water favours this reaction.
• Sterically hindered species prefer this reaction.
• Rate of Sn1 :three degree>two degree>one degree.
• Retention of the configuration occurs.
4. Sn2 Mechanism
• Sn2 stands for substitution nucleophilic bimolecular.
• The rate determining step i.e., the slowest step of the reaction is bimolecular i.e., it occurs
when the nucleophile attacks the sp3 hybridised carbon and the leaving group leaves.
• Sn2 is a one step process.
• An intermediate unstable compound occurs.
• Non-Polar solvents like carbon tetrachloride favours this reaction.
• Sterically hindered species do not prefer this reaction.
• Rate of Sn2 : one degree>two degree>three degree.
• Inversion of the configuration occurs.
5. DIFFERENCE BETWEEN Sn1 AND Sn2
Sn1 Mechanism
1. Sn1 stands forsubstitution nucleophilic unimolecular.
2. The rate determining step i.e., the slowest step ofthe reaction is
unimolecular.
3. Sn1 is a two step process.
4. A stable carbocationforms.
5. Polarsolvents like water favoursthis reaction.
6. Sterically hindered species prefer this reaction.
7. Rate of Sn1 :three degree>two degree>one degree.
8. Retention of the configurationoccurs.
Sn2 Mechanism
1. Sn2 stands for substitution nucleophilic bimolecular.
2. The rate determining step i.e., the slowest step of the reaction is bimolecular i.e., it
occurs when the nucleophile attacks the sp3 hybridised carbon and the leaving
group leaves.
3. Sn2 is a one step process.
4. An intermediate unstable compound occurs.
5. Non-Polar solvents like carbon tetrachloride favours this reaction.
6. Sterically hindered species do not prefer this reaction.
7. Rate of Sn2 :one degree>two degree>three degree.
8. Inversion of the configuration occurs.
