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

Finkelstein Reaction

Finkelstein Reaction:

The Finkelstein reaction is named after the German chemist Hans Finkelstein.  In this reaction, one halogen atom exchanges for another. It is basically single-step bimolecular nucleophilic substitution reaction (SN2 reaction).

According to classic Finkelstein reaction an alkyl chloride or an alkyl bromide is converted into alkyl iodide by the treatment with a solution of sodium iodide in acetone.

Sodium iodide is soluble in the acetone but sodium chloride and sodium bromide are not. So when sodium chloride or sodium bromide are formed in the reaction, it get precipitated and this help reaction to driven to completion due to mass action.

For example, bromoethane can be converted to iodoethane:

Every alkyl halide reacts differently when it undergoes the Finkelstein reaction. The reaction works well for primary (except for neopentyl) halides, and exceptionally well for allyl, benzyl, and α-carbonyl halides. Secondary halides are far less reactive. Vinyl, aryl and tertiary alkyl halides are unreactive.

Find the total number of alkene

Question: How many numbers of alkenes possible by dehydrobromination of 3-bromo 3-cyclopentylhexane using alcoholic KOH?

Solution:

When any alkyl halide is treated with alcoholic KOH, it removes hydrogen and a halogen atom from it and forms alkene. This is basically a dehydrohalogenation reaction. (You can recall it’s basic from here). The halogen atom removes from the α-carbon and hydrogen atom from β-carbon of the alkyl halide.

 In 3-bromo 3-cyclopentylhexane, α-carbon atom is at position 3 (to which halogen atom attached) and three β-carbon atom which are at 2, 4 and 3 (C of cyclopentane). 

Because of three types of β-hydrogen, there are more than one alkenes are expected.

So let us find out.

11)   Hydrogen remove from cyclopentane i.e. position 3:

2

  2   2) Hydrogen remove from position 2:

Here two alkenes are obtained, one is cis from and other one is trans.

  3) Hydrogen remove from position 4:

Here also two alkenes are obtained, one is cis from and other one is trans.

So the total numbers of alkenes obtained from dehydrobromination of 3-bromo 3- cyclopentylhexane are 5.