Definition: What is a Substitution Reaction?

A substitution reaction is an organic chemical reaction during which a functional group replaces an atom or another functional group attached to a carbon atom in a compound.

Components of a Substitution Reaction

An electron-rich species donates a pair of electrons to an electron-poor species and forms a new product and a new base. Therefore, a substitution reaction contains four components.

1. Nucleophile: the electron-rich species donating a pair of electrons to carbon
2. Electrophile: the electron-deficient species accepting a pair of electrons
3. Product: the species that is formed from a substitution reaction
4. Leaving group: the group that leaves the compound

Types of Substitution Reaction

There are three general classes of substitution reactions, depending on the following factors.

• Reactant or substituent
• Intermediate – carbocation, carbanion, or free radical
• Substrate (compound) – aliphatic or aromatic

1. Nucleophilic Substitution

When electron-rich species (nucleophile) provides an electron pair for bonding with the compound being transformed, it is called nucleophilic substitution. Some examples of nucleophiles are Cl^–, Br^–, I^–, OH^–, RO^–, CN^–, H₂O, and NH₃.

Types of Nucleophilic Substitution:

• SN1 reaction
• SN2 reaction
• Nucleophilic aliphatic substitution
• Nucleophilic aromatic substitution
• Nucleophilic acyl substitution

2. Electrophilic Substitution

When the substituent is electron-deficient (electrophile) and accepts an electron pair for bonding with the compound to be transformed, it is called electrophilic substitution. Some examples of electrophiles are H₃O⁺, NO₂⁺, and SO₃.

Types of Electrophilic Substitution:

• Electrophilic aliphatic substitution
• Electrophilic aromatic substitution

3. Radical Substitution

Radical substitution takes place when an atom or group of atoms in a molecule is replaced by a free radical. Some examples of free radicals are Br, Cl, I, and OH.

Applications of Radical Substitution:

• Barton-McCombie deoxygenation
• Wohl-Ziegler reaction
• Hunsdiecker reaction
• Dowd-Beckwith reaction
• Barton reaction
• Minisci reaction

Examples of Substitution Reaction

Halogenation reactions are widespread examples of substitution reactions. Alkane can react with a halogen gas in the presence of ultraviolet light giving alkyl halide. Examples include benzene and methane undergoing chlorination resulting in chlorobenzene and chloromethane (methyl chloride), respectively. Displacement of a good leaving group on an aromatic ring by a nucleophile is an example of nucleophilic aromatic substitution.

Mechanism of Substitution Reaction

Each type of substitution reaction has a unique mechanism. There is no general way to represent them.

Applications of Substitution Reaction

The following organic reactions use the mechanism of substitution reaction.

• Williamson ether synthesis
• Friedel-Crafts acylation
• Friedel-Crafts alkylation
• Halogenation
• Fischer esterification
• Mitsunobu Reaction
• Haloform Reaction
• Sandmeyer Reaction