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 Reaction.
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°
