Chemical Reactions- Nucleophilic Substitution Reactions

Do you know there are more than 1600 different types of haloalkanes existing today? Out of all the halogenated organics, bromoalkanes are the most common ones. Haloalkanes are responsible for an important and wide range of chemical reactions. We will study these chemical reactions in two parts. Let us go through these nucleophilic substitution reactions one by one.

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Substitution Reactions of Haloalkanes

Chemical reactions in haloalkanes primarily fall into 3 different categories. They are:

• Nucleophilic Substitution Reaction (SN¹ and SN² reactions)
• Elimination Reactions
• Reaction with Metals

Nucleophilic Substitution Reactions

In this type of chemical reactions of haloalkanes, the nucleophile will attack a site having electron deficiency and will substitute the halogen or X there.

In case of alkyl halides, the electronegative halogen member will cause electrons withdrawal from the σ bond and attract the electrons toward it thereby polarizing the bond. As a result of which a partial positive charge will develop on the carbon atom making the carbon an electron deficient site.

Browse more Topics Under Haloalkanes And Haloarenes

• Introduction and Classification
• Nomenclature and Nature of C-X bond
• Physical properties
• Methods of Preparation
• Chemical Reactions – Nucleophilic Substitution Reactions
• Chemical Reactions and Stereochemistry
• Polyhalogen compounds
• Reactions of haloarenes

Now if a nucleophile will attack the electron deficient carbon atom. It will cause the departure of the halogen member from the compound as a halide ion. The order of the leaving of halogen group will be I⁻ > Br⁻ > Cl⁻ > F⁻. The type of product formed will depend upon the nucleophile attacking the electron deficient carbon.

Formation of Product on the Basis of a Nucleophile

Type of Nucleophile (Nu−)	Formation of Class of Compounds		Resultant compound (R − Nu)
H2O	Alcohol		R − OH
NaI	Haloalkane		R − I
R′M+	Alkane		R − R′
KCN	Nitrile		R − CN
AgCN	Isonitrile		R − NC
NaOR′	Ether		R − O − R′
R2′ NH	Tertiary amine		R − N − R2′
K − O − N = O	Alkyl nitrite		R − O − N = O
LiAlH4(H)	Hydrocarbon		R − H
AgNO2	Nitroalkane		R − NO2

 

 

 

 

 

 

 

 

 

 

There are certain groups that consist of two nucleophilic centres. We call this type of nucleophiles as ambident nucleophiles.

Ambident Nucleophiles

Groups such as nitrites and cyanides consist of two nucleophilic centres, hence they are ambident nucleophiles.

Substitution Reactions of Haloalkanes with Nucleophiles

KCN

When KCN reacts with a haloalkane (R-X), C acts as the nucleophile. The reaction results in the formation of alkyl cyanides as the product. We know that KCN is ionic in nature. Thus, KCN in the solution will dissociate into K⁺ and CN^–and the negative charge will be present on the carbon atom.

The reaction results in the formation of Alkyl cyanide. Therefore, it will have a C-C bond which is highly stable. The reaction can be represented as

AgCN

The reaction of haloalkane (R-X with AgCN) results in the formation of the product alkyl isocyanides. In this case, N act as the nucleophile.  The bond present between Ag and C is covalent in nature because of the less difference in electronegativity between C and Ag.

The bond in AgCN is “Ag-C≡ N.” Therefore, carbon cannot act as the nucleophile but the nitrogen has a  lone pair of electrons which makes it a better nucleophile. Thus, in AgCN nitrogen acts as a nucleophile.

Reactions of Haloalkanes – Nucleophilic Substitution Reactions

There are generally two types of nucleophilic substitution reaction.

• SN¹
• SN²

Substitution Nucleophilic Unimolecular (SN¹)

In this section, we will study about SN¹ chemical reaction and what conditions haloalkanes require for undergoing this reaction. The meaning of SN¹ reaction lies in the name itself “substitution, nucleophilic, and unimolecular.” Therefore, this reaction will follow the first order kinetics. In another term, we can say that the rate determining step in this reaction is unimolecular.

Moreover, the rate of reaction will depend upon the concentration of the one species which in this case is haloalkane or alkyl halide. For instance, if a tertiary alkyl halide and reacts with a nucleophile result in the formation of tertiary alcohol and halide ion.

From the reaction, we can decipher that SN¹ reaction is a two-step process and it leads to the formation of carbocation intermediates.

• The First Step: Polarization and cleaving of C-X bond occur to form carbocation intermediates. The first step is the reversible process. It is also the rate determining step of the reaction.
• The Second Step: Nucleophile attacks the carbocation to form the respective product.The planar nature of carbocation results in both inversion and retention configuration.

Conditions of SN¹ Reaction

The rate of the reaction will depend on the concentration of the alkyl halide and it will not depend upon the nucleophile. This is because the rate always depends on the slowest step which in this case is the breakdown of the C-Br bond in order to form a carbocation. Therefore, the reaction will be a first-order reaction. The breakdown energy for breaking the bond is obtained from solvation of the leaving group.

Water and alcohol are types of polar protic solvent. These solvents have the capacity to attract the halogen group thereby facilitating the breakdown of C-X bond in the reaction. This will result in the formation of carbocation intermediate. Additionally, the stabilization of leaving the group is possible with the help of the protic solvents through hydrogen bonding. Hence, the first step is

The stability of carbocation will determine the speed of the rate of reaction (more the stability faster the reaction). Therefore the more easily the leaving group (X^–) can leave the compound the more easily the nucleophile can attack the compound and leads to substitution process.

Order of Reactivity

Therefore, the reactivity of halides towards S_N1 reaction is R − I > R − Br > R − Cl > R – F. We know that C-I bond of the alkyl iodides can be easily broken and it is easy to release I^– or the leaving group. Therefore, it becomes easy for the nucleophile to attack the alkyl halides and result in the substitution. On the other hand, it is tough to break C-F bond because it is very strong and in turn, it is difficult for the nucleophile to attack.

The reactivity order of alkyl halides in the case of S_N1 reaction is- 3⁰ > 2⁰ > 1⁰ > methyl. The same reason is responsible for more reactivity of compounds such as benzylic halide and allylic halides towards S_N1 reaction because it leads to the formation of highly stable resonance structures of carbocation intermediates.

Substitution Nucleophilic Bimolecular (SN²)

SN² chemical reactions follow second order kinetics. The rate determining step depends on both the concentration of alkyl halides (R-X) and the nucleophile present in the reaction. The SN² reaction is a one-step process and there is no formation of intermediates. The basic mechanism of the reaction is

Mechanism of SN² Reaction

We will study the mechanism of SN² chemical reactions with the help of an example. Let’s take an example of CH₃Cl haloalkane reacting with the nucleophile OH^–.

This reaction is a one-step process, unlike SN¹ reaction. In this reaction, there is no formation of intermediates. The reaction undergoes a transition state where the nucleophiles are attached to the alkyl halides. Hence it is very clear the rate determining step is dependent on the concentration as well as the nucleophile.

However, the transition state is highly unstable because in the transition state carbon is bonded to 5 atoms. This is because there is the simultaneous breaking of C-Cl bond and the formation of C-OH bond. Moreover, the nucleophile attacks the alkyl halide substrate in a backside method thereby inverting the configuration of the product.

Order of Reactivity

Since the attack of the substrate occurs from back-side method, it is not favourable to use a bulky nucleophile or a bulky substrate. This is because the S_N2 mechanism is very prone to steric inhibition. Hence the order of reactivity of alkyl halide towards this type of reaction is 1⁰ > 2⁰ > 3⁰.

The order of reactivity follows this particular order because as the crowding around the actual C-X bond increases the steric inhibition increases. Thus, it decreases the reactivity in the SN² reaction. Thus it is very easy to react if the alkyl halide substrate is methyl halide. It is also quite convenient to undergo SN² reaction in ethyl halide but it is difficult to undergo SN² reaction in case of a secondary and tertiary halide such as isopropyl halide and t-butyl halide.

The order of reactivity of the halides are R − I > R − Br > R − Cl > R − F

Conditions of the SN² Chemical Reactions

There is a requirement of the strong nucleophile to undergo SN² reaction. The reaction takes place in the presence of solvents that are polar aprotic such as DMSO, DMF. The SN² reaction does not occur in presence of polar protic solvent because these types of solvent deactivate the charged nucleophile during the bond formation of polar protic solvent with the strong nucleophile.

Thus, it decreases the reactivity of the nucleophile and ceasing the possibility of an SN² reaction.

Solved Example for You

Q. List the important difference between SN¹ Reaction and SN² Reaction

Solution:

Difference between SN¹ Reaction and SN² Reaction

SN1 Reaction	SN2Reaction
Two-step process	One step process
Formation of carbocation intermediates	Lack of carbocation intermediates formation.
The strength of Nucleophile is not important	Requirement of a Strong nucleophile.
The requirement of Polar protic solvent.	The requirement of Polar Aprotic solvent.
Configuration- Retention and inversion.	Configuration-Inversion