SN1 and SN2 Reactions.
- SN1 stands for substitution nucleophilic unimolecular.
- SN1 is a two step mechanism.
- In the two step SN1 reaction, the substrate, R – X, first ionizes reversibly into R+and X– and then the intermediate carbocation combines with the attacking nucleophile, Nu– to form the product.
- Rate depend only on the concentration of the substrate.
- The nucleophile can attack the electrophilic carbon from either side (front and backside).
- It is expected to first order kinetics.
- In SN1 reaction, the product may be partially inverted with respect to that of starting substrate, largely inverted or inverted to a very small extent. The product is racemate usually.
- Reaction is favoured by polar solvents.
- Reaction can occur with weak base.
- SN1 reactions are generally given by tert-alkyl halides; 3° > 2° > 1° > CH3.
- SN2 stands for substitution nucleophilic bimolecular.
- SN2 is a One step mechanism.
- The attack of nucleophile on carbon and deparature of halide ion takes place simultaneously in a single step.
- Rate depends on the concentration of substrate as well as on the concentration of attacking nucleophile.
- The nucleophile attacks the electrophilic carbon from the backside.
- It is expected to be second order jinetics. If however one of the reactants is the solvent, the SN2 mechanism will lead to first order kinetics.
- SN2 Reactions always proceed with complete inversion of configuration (backside attack).
- Reaction is favoured by nonpolar solvents.
- Reaction requires strong bases.
- SN2 are generally given by primary alkyl halides. CH3 > 1 > 2° > 3°