Nucleophilic Substitution Reactions I



          Those  reactions in which an atom or group of atoms in a molecule is substituted by a nucleophile are called nucleophilic substitution reactions. It is denoted by SN where S stands for substitution and N for nucleophilic.

 Aliphatic Nucleophilic Substitution (SN1 SN2)

Nucleophile      Substrate                             product             leaving group

                 Basically it is the reaction of an electron pair donor (the nucleophile, Nu) with an electron pair acceptor (the electrophile). A sp3 hybridized electrophile must have a leaving group (X) in order for the reaction to take place.

          The alkyl halides are good substrates for nucleophilic substitution reactions because the halide ion is a good leaving group and therefore can be readily substituted by a nucleophile. Since the alkyl group at which substitution takes place is generally aliphatic in nature, these reactions are commonly known as aliphatic nucleophilic substitutions. Sometimes, the solvent itself act as nucleophile; the reaction is then called solvolysis. Thus a reaction in which water is used as solvent and it also act as nucleophile, is called hydrolysis, while a reaction involving methanol as a solvent and also as a nucleophile, is known as methanolysis.



There are two types of substitution reactions.

1)  SN1 reactions

2)  SN2 reactions

SN1 Reaction:

The substitution  nucleophilic unimolecular reactions are called SN1 reactions. SN1 reaction consists of two steps. The first step involves slow ionization of the substrate (alkyl halide) resulting in the formation of a carbocation that rapidly combines with the nucleophile to form the product in the second step. Mostly the solvent itself acts as nucleophile.The SN1 reaction is generally called solvolysis.

SN2 Reaction:

            The SN2 reaction (also known as bimolecular nucleophilic substitution) is a reaction in which bond making and breaking occurs simultaneously. In SN2 reaction a lone pair from a Nu attacks an electron deficient electrophilic carbon of alkyl halide and removing the leaving group. Thus the incoming group replaces the leaving group in one step. Since two reacting species are involved in the slow, rate-determining step of the reaction, hence it is bimolecular nucleophilic substitution, or SN2.


Reactivity of SN reactions:


          We have considered two pathways i.e., SN1 and SN2 that are generally involved in the aliphatic nucleophilic substitution reactions. Which of these pathways is followed by a particular reaction, and at what rate, depends on the following factors

1) The substrate

2) The nucleophie

3) The leaving group

4) The solvent