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.
Since the substitution reaction is initiated by a nucleophile, it is called nucleophilic substitution reaction.
Mechanism:
Substitution reactions are categories into unimolecular
substitution reaction (SN1) and bimolecular substitution reaction (SN2)
a) SN2 (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) SN1 (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.
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