Substitution Reaction – Conditions, Types, and Examples

What is Substitution Reaction?

A substitution reaction is a chemical reaction in which one atom or group of atoms in a molecule is replaced by another atom or group of atoms. There are two types of substitution reactions: nucleophilic substitution reactions and electrophilic substitution reactions.

• In a nucleophilic substitution reaction, a nucleophile displaces a leaving group to form a new bond. The nucleophile is typically a negatively charged atom or molecule.
• In an electrophilic substitution reaction, an electrophile displaces a nucleophile to form a new bond. However the electrophile is typically a positively charged atom or molecule.

What happens in a Substitution Reaction?

A substitution reaction is a chemical reaction in which one atom or group of atoms in a molecule is replaced by another atom or group of atoms.

Substitution Reaction Example

In a substitution reaction, one atom or group of atoms in a molecule is replaced by another atom or group of atoms. The most common type of substitution reaction is the S N 2 reaction, in which a molecule is attacked by a nucleophile. In this reaction, the nucleophile displaces an atom or group of atoms from the molecule. For example, in the reaction shown below, the nucleophile (S) attacks the carbon atom in the molecule, displacing the chlorine atom.

The S N 2 reaction is a two-step process. In the first step, the nucleophile attacks the molecule and forms a bond to the carbon atom. In the second step, the bond between the carbon atom and also the chlorine atom is broken, and the chlorine atom replaced by the nucleophile.

Substitution Reaction Conditions

The substitution reaction conditions determine the rate of the reaction, as well as the type of substitution that will occur. The most important factor in setting the substitution reaction conditions is the nature of the reagent that substituted. The most common type of substitution reaction is a nucleophilic substitution reaction, in which a nucleophile attacks the carbon atom that substituted.

There are a number of factors that can affect the rate of a substitution reaction. The most important factor is the nature of the reagent that substituted. The most common type of substitution reaction is a nucleophilic substitution reaction, in which a nucleophile attacks the carbon atom that substituted. Other factors that can affect the rate of a substitution reaction include the concentration of the reagents, the temperature of the reaction, and also the presence of solvents or other reagents that can stabilize the transition state of the reaction.

Types of Substitution Reactions

There are three types of substitution reactions:

1. Nucleophilic substitution
2. Electrophilic substitution
3. Free radical substitution

What are Nucleophiles?

Nucleophiles atoms or molecules that have a lone pair of electrons and attracted to positively charged atoms or molecules, called nuclei. In particular, they attracted to the nuclei of atoms that have lost one or more electrons, making them positively charged.

What is the Nucleophilic Substitution Reaction?

The nucleophilic substitution reaction is a type of organic reaction in which a nucleophile displaces a leaving group from a carbon atom. The nucleophile typically a negatively charged atom or molecule, while the leaving group is typically a positive atom or molecule.

The Nucleophilic Substitution Reaction Mechanism

The reaction between an alkyl halide and a nucleophile proceeds via a two-step mechanism:

1. In the first step, the alkyl halide attacked by the nucleophile, forming a new carbon-carbon bond. This step called the S N 1 reaction.
2. In the second step, the leaving group ejected from the molecule, and also the newly formed carbon-carbon bond broken. This step called the S N 2 reaction.

SN2 Reaction – Mechanism of SN2 Reaction

The SN2 reaction mechanism involves the attack of a nucleophile (Nu) on the carbon atom of a substrate molecule. The Nu displaces the leaving group, which then expelled from the molecule.

The following steps illustrate the SN2 reaction mechanism:

1. The nucleophile (Nu) attacks the carbon atom of the substrate molecule.
2. The leaving group displaced from the molecule.
3. The leaving group expelled from the molecule.

SN1 Reaction – Mechanism of SN1 Reaction

The SN1 reaction is a substitution reaction that involves the nucleophilic substitution of an alkyl group by a halide ion. The mechanism of the SN1 reaction involves the attack of the nucleophile on the carbon atom of the alkyl group. This produces a carbocation intermediate that attacked by the halide ion. This produces the final product and also the halide ion eliminated.

Example of Nucleophilic Substitution Reaction:

The example below shows the substitution of a chloride ion with a bromide ion in a benzene ring. This is an example of a nucleophilic substitution reaction.

What Are Electrophiles?

An electrophile a molecule that attracted to electrons, and in particular, to electron-deficient atoms or groups of atoms. Electrophiles are therefore able to interact with other molecules by stealing electrons from them, a process known as oxidation.

What Meant by the Electrophilic Substitution Reaction?

An electrophilic substitution reaction is a type of chemical reaction that occurs when an electron-rich molecule, called an electrophile, reacts with a molecule that has an electron-deficient atom, called a nucleophile. Therefore the electrophile donates an electron to the nucleophile, which forms a new covalent bond between the two molecules.

Types of Electrophilic Substitution Reaction:

1. Electrophilic addition:

In this type of substitution reaction, a molecule with a double or triple bond attacked by an electrophile, which adds to the molecule. Therefore the result is a single, or multiple, bond between the attacking molecule and the original molecule.

2. Electrophilic substitution:

In this type of substitution reaction, an atom or group of atoms in a molecule attacked by an electrophile, which replaces the atom or group of atoms.

Electrophilic Aromatic Substitution Reaction

• In electrophilic aromatic substitution, a benzene ring substituted by an electrophile. The electrophile is usually an aromatic ring itself, such as an aryl halide or an aryl sulfonate.
• The electrophilic aromatic substitution reaction occurs in two steps. The first step is the formation of the electrophile. The second step is the substitution of the electrophile into the benzene ring.
• The electrophile usually formed by the reaction of an aromatic ring with an electrophile. For example, the reaction of an aryl halide with an alkali metal produces an aryl cation.
• The aryl cation then substituted into the benzene ring. For example, the reaction of an aryl cation with benzene produces phenol.

Electrophilic Aliphatic Substitution

• In electrophilic aliphatic substitution, an electron-rich alkyl group substituted by another alkyl group that is electron-poor. The electron-poor group called the electrophile.
• The most common electrophile is the halogen atom. Other electrophiles include the sulfonate ion, the sulfate ion, and the carbanion.