Dimerization is a chemical process in which two smaller molecules, often identical, combine to form a larger molecule called a dimer. This occurs when the molecules bond with each other through covalent bonds or weaker forces such as hydrogen bonds. Dimerization is crucial in creating new molecular structures and is essential in both chemical reactions and biological processes. ^[1-4]

Types of Dimerization  ^[1-4]

Covalent Dimerization

Covalent dimerization occurs when two molecules combine to form strong covalent bonds.  These bonds result in larger and more complex molecules from smaller ones.

Non-Covalent Dimerization

Non-covalent dimerization occurs when two molecules come together without forming covalent bonds. Instead, they are held together by weaker forces, such as hydrogen bonds, van der Waals forces, or ionic interactions. Non-covalent dimerization is important because it allows molecules to interact temporarily and reversibly.

Examples of Dimerization  ^[1-4]

Alkenes and Alkynes

The unsaturated bonds in alkenes and alkynes make them reactive, and they can undergo dimerization to form larger molecules.

Examples

• Two ethene (C₂H₄) molecules combine in the presence of a catalyst to form butene (C₄H₈). This reaction happens because the double bonds in ethene break, and new single bonds are formed to join them together. This type of dimer, called a chemical dimer, is permanent unless broken down by another chemical reaction.
• Acetylene (C₂H₂) dimerizes to form vinylacetylene (C₄H₄) under high pressure and temperature. 

Carboxylic Acids

Carboxylic acids, which contain the -COOH group, can form dimers through hydrogen bonding. In this type of dimerization, the hydrogen atom of the -OH group in one molecule forms a hydrogen bond with the oxygen atom of the C=O group in another molecule. 

Example

A classic example is the dimerization of acetic acid (CH₃COOH). In its pure liquid form or nonpolar solvents, acetic acid exists mostly as dimers. Such a type of dimer, called physical dimer, is not permanent. These hydrogen bonds are relatively weak compared to covalent bonds, so the dimer can break apart and re-forms depending on the conditions.

The dimerization of acetic acid is why its boiling point is much higher than expected for a molecule of its size.

Polymerization

Dimerization can also be the first step in polymerization, where many small molecules (monomers) join to form a large molecule (polymer). In this context, dimerization involves just two monomers combining to form a dimer. 

Example

In the production of polyethylene (-(C₂H₄)-)_n, ethene (C₂H₄) molecules can first dimerize and then continue reacting to form long polymer chains. While dimerization itself only creates a two-unit molecule, it sets the stage for larger reactions that result in important materials like plastics and resins.

Mechanism of Dimerization

The mechanism of dimerization depends on the type of molecules involved and the conditions under which the reaction takes place. Below is a simple explanation using the example of ethene’s (C₂H₄) dimerization to form butene (C₄H₈). ^[1-4]

Step 1: Activation

A catalyst, like a metal complex, activates ethene molecules. The catalyst provides the energy or surface needed to weaken the double bond in ethene. This makes the molecule more reactive and ready to combine with another ethene molecule.

Step 2: Bond Formation

One ethene molecule interacts with another, forming a temporary complex. The double bond in one of the ethene molecules breaks, allowing new single bonds to form. These bonds connect the two ethene molecules, resulting in a larger molecule.

Step 3: Stabilization

The newly formed molecule rearranges itself to achieve a stable structure. This rearrangement results in butene (C₄H₈), a product of dimerization. The catalyst is not consumed in the reaction and can be reused.

Applications of Dimerization ^[1-4]

• Polymer Production: Dimerization is an important step in making polymers like polyethylene and polystyrene. These materials are used to make everyday items like plastic bags, bottles, and packaging.
• Biological Processes: In living organisms, dimerization affects how molecules like DNA and proteins work. For example, some proteins need to dimerize (combine with another protein) to become active and perform their functions in the body.
• Medicine Development: Understanding dimerization helps scientists design drugs. Some medicines target the dimerization of proteins to treat diseases like cancer or infections.
• Environmental Chemistry: Dimerization can affect how pollutants behave in the environment. For example, gases like nitrogen dioxide (NO₂) can dimerize to form dinitrogen tetroxide (N₂O₄), influencing air pollution and atmospheric reactions.
• Material Science: Dimerization is used to create advanced materials like resins, adhesives, and coatings, which are essential in construction, electronics, and manufacturing.