Nucleophilic Substitution: Type, Mechanism, Characteristics & Examples

Nucleophilic Substitution is a type of chemical reaction in which electron-rich chemical species replace a functional group. The nucleophile will replace an electron-deficient molecule. The electron-deficient molecule is called an electrophile.

• Nucleophilic Substitution is a type of normal displacement reaction.
• The molecule leaving the functional group is called the substrate.
• Bromoalkanes are the most frequent of all the halogenated organics. 
• Haloalkanes play a key role in a variety of chemical processes. 
• Nucleophilic Substitution consists of a lone pair of electrons.
• The electron will be removed from the σ bond.
• These types of reactions are attracted towards the nucleus of an atom.
• SN₁ and SN₂ reactions are two types of Nucleophilic Substitution.
• The strength of a nucleophile molecule is called nucleophilicity.

Key Terms: Nucleophilic Substitution, Substitution Reaction, Hydrogen Bonding, Electronegativity, SN₂ reaction, SN1 Reaction, Molecule, Haloalkanes, Electron, Electron pair

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What is Nucleophilic Substitution Reaction?

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A nucleophilic substitution reaction is one in which one nucleophile substitutes another in an organic process. It is very similar to conventional chemical displacement reactions, in which a more reactive element replaces a less reactive one in a salt solution. 

• The "leaving group" is the molecule on which substitution occurs.
• Substrate is the molecule on which the electron pair is displaced from the carbon. 
• The departing group is a neutral molecule or an anion when it exits.
• The nucleophilicity of a nucleophile is its reactivity or strength in nucleophilic substitution processes. 
• A stronger nucleophile replaces a weaker nucleophile from its component in a nucleophilic substitution process. 
• Nucleophilic Substitution can be approximately depicted as follows.
• Edward D. Hughes and Sir Christopher Ingold first studied the nucleophilic substitution.

R−LG+Nu^Θ →R−Nu+LG^Θ

• Where,
• R → Alkyl group
• LG → Leaving group (less nucleophilic)
• Nu^Θ→ Stronger nucleophile

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Nucleophilicity

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Nucleophilicity is defined as the nucleophiles' ability to associate their lone pairs with a positive center. It's a kinetic word that refers to the nucleophile's rate of attack on the substrates (R – LG). The following factors can be used to compare the nucleophilicity of different nucleophiles.

Basic Strength of Nucleophiles

Basic strength refers to a species' ability to give electron pairs once more. The only difference between nucleophilicity and basic strength is that nucleophilicity is a kinetic term, but basic strength is a thermodynamic word. It is concerned with how much the reaction's equilibrium is changed to the right.

B^Θ + H⁺ → BH

• Where B^Θ →Base accepts protons or loses electrons.
• Strong bases are often stronger nucleophiles; for example, compare the nucleophilicities of I, Br, Cl, and P.
• The strength of conjugate acids is calculated to compare the basic strengths of the above nucleophiles.
• Inverse to that order is the order of their basic strength (the conjugate base of strong acids is weak bases, and vice versa).
• It results in the order of nucleophilicities.
• The conjugate acids of F, Cl, Br, and I are HF, HCl, HBr, and HI, respectively. 
• The following is the order of acidic strength among the four acids:
• HI > HBr > Hcl > HF HI > HBr > HCl > HF HF HI > HB (Bond enthalpy). 
• As a result, the order of nucleophilicity and basic strength is F >Cl>Br>I.

Electronegativity of the Nucleophilic Atom

When the lone pair of electrons are loosely held, it is common that a nucleophilic attack will be successful. This explains why nucleophiles with lone pairs on highly electronegative atoms are less nucleophilic or weaker nucleophiles.

Steric Hindrance

Nucleophiles that are crowded, also known as substantially impeded nucleophiles, are unable to migrate, making them weaker nucleophiles. Due to the existence of steric hindrance, primary alkoxide ions are stronger than tertiary alkoxide ions.

Polar Solvent Effects

The nucleophilicity of nucleophiles in polar solvents such as alcohol acids, H₂O, and others is determined by the solvation effect. Ion mobility is reduced in heavily hydrated ions, which diminishes nucleophilicity.

• Hydration is a process that occurs when all of the water molecules in the vicinity of an ion congregate in one location.
• In the case of polar solvents, the order of nucleophilicity is not the only factor to consider.
• Hydration also plays a role in determining the strength of the nucleophiles.

Leaving Capacity of the Leaving Group

The pace of a nucleophilic substitution reaction is determined by the nucleophilicity of the incoming nucleophile and the leaving capacity of the replacing or substituted leaving group. The faster the reaction, the greater the capacity of the leaving group to leave.

• Weaker bases are better-leaving groups, which is the only guideline for determining leaving capacity.
• For example Fº< Clº< Brº< Iº is the order of leaving capacity.

Nucleophilic Substitution Reaction

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Mechanisms of Nucleophilic Substitution

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The pace of a nucleophilic substitution reaction is determined by the nucleophilicity of the arriving nucleophiles and the leaving capacity of the leaving group. The following are the two mechanisms of nucleophilic substitution reactions.

Substitution Nucleophilic Unimolecular (SN₁)

The SN₁ is a chemical reaction that is also known as an unimolecular nucleophilic substitution reaction. In this reaction, haloalkanes are substituted. The kinetics of this reaction will be first order. The rate-determining step in this reaction is unimolecular.

• The rate of reaction is determined by the concentration of the single species, which is haloalkane or alkyl halide in this case. 
• For example, when a tertiary alkyl halide interacts with a nucleophile, tertiary alcohol and halide ions are formed.
• In the SN₁ reaction, the final product is formed by the nucleophile attacks.
• These types of reactions take place in tertiary and secondary alkyl halides.

​Conditions of SN₁ Reaction

The pace of the SN₁ reaction will be determined by the concentration of alkyl halide rather than the nucleophile. This is because the rate is always determined by the slowest step, which in this case is the breakage of the C-Br bond to generate a carbocation.

• As a result, the reaction will be of the first order.
• Solvation of the leaving group yields the breakdown energy for breaking the bond.
• Polar protic solvents include water and alcohol.
• The solvents can attract the halogen group, allowing the C-X bond to be broken more easily in the reaction.
• A carbocation intermediate will form as a result of this.
• The stability of leaving the group is also feasible with the use of protic solvents and hydrogen bonding.

Substitution Nucleophilic Bimolecular (SN₂)

Second-order kinetics govern SN₂ chemical reactions. The rate-determining step is influenced by the number of alkyl halides (R-X) and the nucleophile present in the reaction. There are no intermediates formed in the SN₂ Reaction because it is a one-step reaction. With the help of an example, we will look at the mechanism of SN₂ chemical reactions. 

• Consider the reaction of the haloalkane CH₃Cl with the nucleophile OH–.
• Unlike the SN₁ reaction, this is a one-step procedure. 
• There are no intermediates formed in this reaction. 
• The nucleophiles are connected to the alkyl halides during the reaction's transition state. 
• As a result, the rate-determining step is influenced by both the concentration and the nucleophile.
• However, carbon is extremely unstable because it is bound to five atoms in the transition state. 
• This is due to the simultaneous creation of the C-OH bond and the breaking of the C-Cl link. 
• Furthermore, the nucleophile attacks the alkyl halide substrate from the backside, inverting the product's structure.

Conditions of the SN₂ Chemical Reactions

A strong nucleophile is required for the SN₂ reaction to take place. The reaction takes place in the presence of polar aprotic solvents like DMSO and DMF. The polar protic solvents deactivate the charged nucleophile during the bond formation of the polar protic solvent with the strong nucleophile.

• The SN₂ reaction does not occur in the presence of the solvents.
• As a result, the nucleophile's reactivity is reduced, and the likelihood of an SN₂ reaction is eliminated.

Nucleophilic Substitution Reaction

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Things to Remember

• The nucleophilic substitution reaction is affected by the steric impacts and caused by the inversion of a chiral centre.
• Methyl halides are the most reactive compound, followed by primary compounds, secondary halides, and tertiary halides.
• There is no reaction in the vinyl compound when they form C=C.
• A neutral product will result from the reaction of the negatively charged nucleophile with the substrate.
• The positively charged product will result from the reaction between the neutral nucleophile and the substrate.
• Few compounds like alcohols, fluorides, ether, and amines do not undergo SN2 reactions.
• The SN2 mechanism is also prevented if the leaving group is relatively tiny.
• Solvents capable of contributing hydrogen bonds will slow down the SN2 reaction.

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