Nucleophilic substitution reaction in alkyl halide

Nucleophilic Substitution Reaction of Alkyl Halides (R – X)

First let us understand what a nucleophilic substitution reaction is:

A nucleophile is:       nucleo + philic

                 means       nucleus loving 

That means a compound which loves nucleus i.e. A^-  (- because a negative charge attracts toward nucleus)

In substitution reaction, one functional group ( X ) is substituted (replace) by another group (Y)

So, when a nucleophile reacts with haloalkane (R- X) , substitution reaction takes place and halogen atom (X) is called leaving group departs as halide ion. 
Since the substitution reaction is initiated by a nucleophile, it is called nucleophilic substitution reaction.

Mechanism:

Substitution reactions are categories into unimolecular substitution reaction (S_N1) and bimolecular substitution reaction (S_N2)

a)    S_N2 (Bimolecular substitution reaction):

                                                  

·       It is bimolecular reaction means rate of reaction depends upon the concentration of both reactants (B- and RA)

·       No intermediate [X] is formed in this reaction because formation of bond and breaking of bond take place simultaneously.

·       In this reaction, configuration of carbon atom inverted while pushing the leaving group out, so this process is called inversion of configuration.

·       Order of reactivity of alkyl halide decreases with:

1⁰ > 2⁰ > 3⁰

As in primary halide, carbon atom (which bears halide group) has only small H atoms so Nu- easily approach to C atom while in secondary and tertiary halide, bulky R groups present which create hindrance to Nu- to approach C atom. 

b)                   S_N1 (Unimolecular substitution reaction):

·       This reaction is carried out in polar solvent (R⁺A^-)

                                                  

·       It is unimolecular reaction means rate of reaction depends upon the concentration of only one reactant (which is RA).

·       This reaction occurs in two steps:
In step I, a carbocation is formed while in II step, Nu- attacks to carbocation and alkyl halide is formed.

·       Greater the stability of carbocation (which is R – CH₃⁺), greater will be its ease of formation from alkyl halide and faster will be rate of reaction. So order of reactivity increases with:

1⁰ < 2⁰ < 3⁰

Rate of reaction is very fast in tertiary (3⁰) carbocation because of its high stability

So we can say order of reactivity of alkyl halide:

Reactivity of alkyl halide will be:

R – I  >  R – Br  >  R – Cl  >  R – F