Coupling reactions are important chemical reactions used in organic chemistry to join two molecules together. Think of it like linking two building blocks to make something bigger and more useful. These reactions help chemists build complex compounds by connecting smaller parts, especially carbon atoms. Coupling reactions are widely used in making medicines, dyes, plastics, and other useful materials. Because they are so common and helpful, understanding how they work is a key part of learning organic chemistry. In this article, we’ll explore what coupling reactions are, the different types, how they happen, and where they are used in real life.

What is Coupling Reaction

The term ‘coupling reaction’ means the class of organic reactions that involve the joining of two chemical species. It is usually done with the help of a metal catalyst. There is an important type of coupling reaction, which is the reaction of an organic halide with an organometallic compound having the general formula R-M that helps in the new carbon-carbon formation of chemical bonding. The compound formed as a result will have the formula R-R’; if the organic halide in this reaction has the general formula R’-X.

E.g., When there is a reaction between benzene diazonium chloride and phenol, the phenol molecules at their para-position are coupled with the diazonium salt to form p-hydroxyazobenzene. It is shown in the below figure:

1. Homo-Coupling:

In a Homo-coupling reaction, two chemical species are combined to yield a single product. E.g., Glaser coupling, Wurtz coupling, pinacol coupling.

An example of homo-coupling reactions is the Ullmann reaction, shown in the below figure.

2. Hetero-Coupling:

It is also known as cross-coupling. In Hetero-coupling reactions, two different chemical species join together to afford a single product. E.g., Suzuki coupling, Grignard reaction.

There is another example of Hetero-coupling reactions; the Heck reaction. The Heck reaction of an aryl halide and an alkene is shown below.

3. Azo Dye

It is the organic compound with functional group R−N=N−R′ where R and R’ are aryl groups. They belong to the azo compounds family. They have the property that they are insoluble in water and other solvents. They are used in the preparation of leather objects, textiles, and many other food products.

Coupling Reaction Mechanism 

In coupling reactions, two similar or different species react together to yield a new product. There is a common intermediate and a metal catalyst. In these reactions, there is an energy transfer from one side to another. Metal catalysts take away the electrons from other molecules, and that is why metal catalysts are preferred in coupling reactions over other catalysts. These metal catalysts increase the reaction rate without affecting the thermodynamics of the reaction.

Let’s understand this with the help of the following example:

E.g.,

\mathrm{C}_2 \mathrm{H}_4 \mathrm{Cl}_2+\mathrm{C}_6 \mathrm{H}_7 \mathrm{BO}_2\left(\text { in presence of Pd and } \mathrm{CH}_2 \mathrm{Cl}_2\right) \longrightarrow \mathrm{C}_8 \mathrm{H}_8

Applications of Coupling Reaction

There are so many applications of the coupling reactions. Let us discuss some of them.

• Many conjugated polymers are prepared through coupling reactions where metal catalysts are used.
• Coupling reactions are used in several pharmaceutical products.
• The Suzuki coupling reaction is used in the synthetic production of complex compounds. For example, it is used in the production of caparratriene, which is used in leukaemia treatment.
• We can obtain many natural products with the help of different coupling reactions. For example, with the help of the Sonogashira cross-coupling reaction, benzylisoquinoline alkaloids can be synthesized.

Advantages and Limitations of Coupling reactions

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