A substitution reaction is a chemical reaction where on one group is replaced by another group. Generally substitution reaction takes place where there is polar carbon-heteroatom bond present in the molecule. The atom or group which is replaced in the reaction is known as leaving group and the product formed is called as substitution product. The group or atom which replaces the leaving group is can be nucleophile, electrophile or a free radical.
Figure 1: Substitution Reaction
![]() |
Figure
2: Classification of Substitution Reactions |
1. Electrophilic Substitution Reactions
An
electrophilic substitution reaction is chemical reaction in which leaving group
is replaced by an electrophile. This reaction is observed in the compound which
contains electron rich carbon-carbon double bond. The electrophile is an
electron deficient species which is formed in the reaction. The elctrophile
accepts the pair of electron from carbon-carbon double bond form new
carbocation intermediate. Finally the leaving group get cleaved to form stable
molecule.
![]() |
Figure 3: Electrophilic substitution reaction
Based
upon type of substrate molecule there are two types of electrophilic
substitutions reactions are found namely; Electrophilic
Aromatic Substitution Reaction and Electrophilic
Aliphatic Substitution Reaction.
1.1
Electrophilic Aromatic Substitution Reaction
In
electrophilic aromatic substitution reaction the substrate is an aromatic
compound and electrophile is replaces a hydrogen atom from the aromatic ring. For
example, halogenation of benzene compound is electrophilic aromatic
substitution reaction.
![]() |
Figure
4: Bromination of Benzene |
In
this case first step is generation of an electrophile. Here Br2
donate pair of electron to FeBr3 to form electrophilic complex. Then
second step is addition of electrophile on to aromatic ring to form carbocation
intermediate. Then the last step is deprotonation and regeneration of acid
catalyst.
![]() |
Figure
5: Mechanism of Bromination of Benzene |
Other examples of electrophilic aromatic substitution are nitration of benzene and sulphonation of benzene.
![]() |
Figure 6: Examples of electrophilic aromatic substitution
1.2 Electrophilic Aliphatic Substitution Reaction
In electrophilic aliphatic substitution reaction the substrate is an aliphatic compound and electrophile replaces hydrogen atom from the molecule. For example, ketone halogenation in basic condition is an electrophilic aliphatic substitution reaction.
![]() |
Figure 7: Alpha bromination of ketone
2.
Free Radical Substitution Reactions
Free
radical substitution reaction consists of replacement of leaving group by free
radical. This reaction has three steps which are shown below.
Initiation: Generation
of radical.
Propogation: Formation of new
intermediate radicals.
Termination: This
is final step in which stable molecule is forms and reaction stops.
Halogenation
of alkane in presence of halogen gas and light is an example of free radical
substitution reaction.
![]() |
Figure
8: Haloganation of Alkane |
In this reaction is first step is initiation in which bromine radical is formed from bromine gas in presence light. Then bromine radical takes a hydrogen atom from alkane to form HBr and new alkyl radical. This step is known as propogation. The last step is termination where another bromine radical and alkyl radical combines to form alkyl halide.
![]() |
Figure 9: Mechanism of Haloganation of Alkane
3.
Nucleophilic Substitution Reactions
The
nucleophilic substitution reaction consists of replacement of leaving group by
nucleophile. There are three types of nucleophilic substitution can be found in
organic chemistry these are; Nucleophilic aliphatic substitution, Nucleophilic
acyl substitution and Nucleophilic aromatic substitution.
3.1
Nucleophilic Aliphatic Substitution
The
nucleophilic aliphatic substation reaction is can be seen in aliphatic
molecules. In general alkyl halides shows this type of reactions. Based upon
molecularity of the reaction there are two types nucleophilic aliphatic
substitution reactions present in nature namely Unimolecular Substitution Reaction (SN1) and Bimolecular Substitution Reaction (SN2)
3.1.1
Unimolecular
Substitution Reaction (SN1)
This
is two step reaction in which first step is formation of carbocation
intermediate takes place. In second step a nucleophile attacks on the
carbocation to produce substitution compound. Tertiary alkyl halides and allyl
halides favours this type of substitution. Also there is requirement of weak
nucleophile to attack on the carbocation.
For example, reaction of tert-butyl
bromide with methanol is a unimolecular substitution reaction. The rate of the
reaction is depending upon concentration of alkyl bromide. In this reaction
first step is heterolytic cleavage of carbon-halogen bond to form tertiary
butyl carbocation intermediate. Then methanol molecule attack on the
carbocation to form substitution product.
![]() |
Figure 10: Unimolecular Substitution Reaction (SN1)
3.1.2 Bimolecular
Substitution Reaction (SN2)
This
is one step reaction in which a strong nucleophile attack on tetrahedral carbon
to replace the leaving group. Primary and secondary alkyl halides are prone to
show SN2 reaction. The rate of the reaction is depending upon the
concentration of both nucleophile and alkyl halide. For example, reaction of
isopropyl iodide with sodium ethoxide produces substitution product. In this
reaction iodine is a leaving group and ethoxide is a nucleophile.
![]() |
Figure 11: Bimolecular Substitution Reaction (SN2)
3.2 Nucleophilic Acyl Substitution
This
type of reaction is shown by acyl derivative like acyl chloride and anhydride. In
this reaction a strong nucleophile attacks on the acyl carbon to form
tetrahedral intermediate. Then the next step is removal of leaving group to
give neutral molecule.
Reaction
of acyl chloride and methyl amine to form amide compound is an example of
nucleophilic acyl substitution reaction. In this reaction first step is
addition of methyl amine (nucleophile) on to carbonyl carbon to produce charged
species. Then in second step removal of chloride ion takes place. Finally
deprotonation of amine nitrogen gives formation of amide product.
![]() |
Figure 13: Nucleophilic Acyl Substitution
3.3 Nucleophilic Aromatic Substitution
In
this reaction the leaving group is present on the aromatic ring. There are
three types of substitution reaction can be found in this category of
reactions.
3.3.1
Addition
elimination reaction (SNAr)
Here
in the first step a nucleophile add to the electron deficient aromatic ring to
form anionic intermediate. Then in second step elimination of leaving group
takes place to form stable aromatic ring. The main requirement of the reaction
is that the aromatic ring must contain one or more electron withdrawing
group.
![]() |
Figure 14: SNAr (addition-elimination) Mechanism
For
example, reaction of 1-chloro-2-nitrobenzene with sodium hydroxide under
heating condition gives 2-nitrophenol. Here the mechanism of the reaction is SNAr.
In the first step hydroxide ion add to the electron deficient ring to form
anionic intermediate. And the second step is elimination of leaving group to
form aromatic ring.
Figure 15: Reaction of 1-chloro-2-nitrobenzene with sodium hydroxide
3.2.2 Aromatic SN1 Mechanism
In aromatic SN1
reaction first step is formation of carbocation intermediate takes place. The
second step consists of addition of nucleophile occurs to give substitution
product. This reaction is similar to aliphatic SN1 reaction.![]() |
Figure
16: Aromatic SN1 Mechanism |
For
example; reaction of diazonium salt with sulphuric acid gives substitution
product by aromatic SN1 mechanism. In this reaction nitrogen gas
releases as leaving group to form arenium ion. Then the sulphuric acid acts as
nucleophile and attack on arenium ion to give substitution product.
![]() |
Figure 17: Reaction of diazonium salt with sulphuric acid
3.3.3 Benzyne
Mechanism
Benzyne
mechanism is similar to E2 elimination reaction. According to this mechanism a
strong nucleophile abstract a proton which is adjacent to leaving group.
Simultaneously the leaving group cleaves from the substrate molecule to form
triple bond species it is known as benzyne
intermediate. Then the nucleophile attack on electron deficient benzyne to
form another anionic intermediate. Finally protonation of anionic species gives
stable compound.
![]() |
Figure 18: Benzyne Mechanism
Reaction
of chlorobenzene with sodium amide (NaNH2) /liq. NH3 proceeds
through benzyne intermediate followed by addition of nucleophile (NH2)
delivers substitution product.
![]() |
Figure 19: Reaction of chlorobenzene with sodium amide (NaNH2) /liq. NH3
That's all for this topic. If you have any questions please feel free to ask me in the comment box. Thank you..!
No comments:
Post a Comment