In SN2-REACTIONS, the formation of the carbon-nucleophile bond [C–Nu] and the cleavage of the carbon- halogen bond [C–X] occur simultaneously. These are nucleophilic substitution reactions which occur in one step.
The reaction proceeds in one step. The attack of nucleophile and release of halide group take place at the same time.i.e
Nu– + R-X ® [Nu…R…X] ® R–Nu + X–transition state
Reactions of secondary alkyl halide may be SN1 or SN2 depending upon the nature of solvent.
Rate of SN1-reactions
The rate of an SN2 reactions depend on the concentrations of nucleophile and the substrate i.e Rate of reaction = k[substrate][Nu].Since both substrate and nucleophile are present in rate expression therefore ifst is a bimolecular reaction. It may be defined as “a bimolecular nucleophilic substitution reaction which occur in one step.
Factors affecting the nature of SN reactions
The mechanism of SN-reactions depend upon:
- The structure of alkyl group
- Nature of the solvent
Structure of alkyl group:
Reactions of primary alkyl halides take place in SN2 mechanism. Reactions of tertiary alkyl halides take place in SN1 mechanism. Reactions of secondary alkyl halides take place both in SN1 and SN2 mechanism but it depends upon the nature of solvent. The order of stability of carbonium ion is as under: R3C+[tertiary carbonium ion]>R2C+H[secondary carbonium ion]>RC+H2[primary carbonium ion]
Nature Of solvent: A polar solvent favors SN1 mechanism due to capability of ionization whereas a non- polar solvent favours SN2 mechanism.