Radical Reactions

A step of a radical reaction

Core Concepts

A radical reaction is characterized by the involvement of highly reactive species called radicals. Radical reactions are a fundamental class of chemical reaction. In this article, we will explore their mechanisms, specifically focusing on the initiation, propagation, and termination phases. Additionally, we will delve into the mechanisms of halogenation, including chlorination and bromination.

Topics Covered in Other Articles

What is a Radical Reaction?

Radical substitution reactions involve the replacement of a functional group or atom in an organic compound by a radical species. Instead of using traditional reagents, radicals act as the agents of change in these reactions. They selectively target specific atoms or groups within the compound and undergo substitution, resulting in the formation of new radicals as byproducts.

Methyl benzene

Radical reactions take place by bonding changes through homolysis, producing radicals with unpaired electrons, contrasting them with ionic reactions that involve heterolysis and ions.

generic radical reaction

This example demonstrates the transfer of a single electron (instead of an electron pair) in radical reactions. In this case, group A and group B each acquire one electron from the covalent bond that connected them.

In this reactions, the collision of radicals with other molecules often leads to the pairing of their unpaired electrons. This can be achieved through atom abstraction, where an atom is removed via homolytic bond cleavage while simultaneously forming a bond with another radical. It can also happen due the addition of the radical to a π Bond (double bond).

Radical Reaction Chain Mechanism

This chain reaction refers to a sequential, stepwise mechanism where each step produces a reactive intermediate that triggers the subsequent cycle of the reaction, as in the previous example with the hydrogen atom abstraction. In this process we have 3 steps: Initiation, Propagation and Termination.


This is the first step of the mechanism, here radicals are created.

initiation radical

As we mentioned before radicals are created by compounds going through homolysis, to explain this process we can use a halogen molecule since they have weak bonds, this type of bonds undergo homolysis when irradiated with UV light or when heated.

chlorine homolysis

Also heating a compound with an oxygen-oxygen single bond can cause homolysis due to its inherent weakness.

peroxide homolysis


In this second step the radicals react and we get new radicals.

During this step we have the “chain” reactions. Initially, a reactive free radical forms and actively interacts with stable molecules, thereby creating fresh free radicals. These newly generated free radicals, in turn, contribute to the cycle by producing additional free radicals, thus perpetuating the propagation.

radical reaction propagation

Common propagation steps involve hydrogen abstraction or the addition of the radical to double bonds. This continuous generation and reactivity of radicals play a fundamental role in sustaining and amplifying the chain reaction, allowing it to proceed in a self-propagating manner.

For instance, consider the scenario where a halogen atom abstracts a hydrogen atom from an alkane. As a result, the halogen atom receives an electron from the hydrogen atom, allowing it to pair with its unpaired electron, in this case we will generate another radical intermediate like an alkyl radical (R) that will undergo further reactions.

hydrogen atom abstraction
Hydrogen atom abstraction
radical addition
Radical addition to bond


In the final step we have the consumption of radicals.

The coupling of any two radicals signifies the depletion of reactive intermediates and the termination of the chain reaction. The propagation phase takes responsibility for generating new molecules as it allows for multiple repetitions of the second step. This enables the creation of a large number of compounds within a single radical chain reaction.

radical termination

Radical Halogenation Reaction

Halogenation reactions are chemical processes that involve the incorporation of a halogen atom (such as fluorine, chlorine, bromine, or iodine) into a compound, in the presence of heat or light. These reactions commonly occur through substitution in a radical-mediated process, where the halogen replaces an existing atom or functional group within the molecule.

This is the general reaction in this case:

Halogenation can be achieved using different methods and under varying reaction conditions. It can be carried out using elemental halogens or halogen-containing reagents.These reactions have widespread applications in organic synthesis, introducing halogen atoms to influence properties of molecules.

halogenation addition


Chlorination is a specific example of halogenation, focusing on the introduction of a chlorine atom into a compound.

We can use the chlorination of methane to understand this type of reaction better:

radical chlorination reaction
Methane Radical Chlorination Reaction


Initiation: Chlorine Dissociation

  1. The chlorine molecule (Cl2) dissociates when exposed to heat or ultraviolet (UV) light.
  2. Each chlorine atom retains one electron, resulting in the formation of two highly reactive chlorine radicals (Cl•).
radical reaction chloride initiation
Initiation of Reaction

Propagation: Hydrogen Abstraction and Halogen Abstraction

  1. In the hydrogen abstraction step, a chlorine radical (Cl•) abstracts a hydrogen atom from methane (CH4), this leads to the formation of a methyl radical (CH3•) and a hydrogen chloride molecule (HCl).
  2. In the halogen abstraction step, the methyl radical (CH3•) reacts with a chlorine molecule (Cl2), abstracting a chlorine atom and producing chloromethane (CH3Cl) and a chlorine radical (Cl•). The chlorine radical (Cl•) formed in this step can continue the propagation process by abstracting a hydrogen atom from another methane molecule, initiating a new cycle.
chlorination propagation

Termination: Coupling of Two Radicals

After several iterations of the propagation steps, termination occurs when two radicals combine, in this case a methyl radical (CH3•) can combine with a chlorine radical (Cl•).This coupling reaction results in the formation of chloromethane (CH3Cl) without any leftover radicals.

methyl chloride termination

In summary, the chlorination of methane through halogenation involves generating chlorine radicals in the initiation step, followed by propagation steps that include hydrogen abstraction and halogen abstraction. Finally, termination reactions occur, leading to the formation of the desired chloromethane product and the termination of the chain reaction.

Multiple Halogen Substitution Reaction

During the chlorination of methane, an equimolar ratio of methane (CH4) and chlorine (Cl2) undergoes a rapid reaction when subjected to heat or light irradiation. The reaction ultimately yields a mixture of products, which includes chloromethane, dichloromethane, trichloromethane, and tetrachloromethane.

The key factor driving multiple substitutions is the propagation steps in the radical chain mechanism. When a chlorine radical abstracts a hydrogen atom from methane, it forms a reactive methyl radical (CH3•). This methyl radical can continue reacting with additional chlorine molecules present, perpetuating the chain reaction.

The formation of mixed products in the chlorination reaction can be explained by considering how the concentration of reactants and products changes as the reaction progresses. Initially, the mixture contains only chlorine and methane. The primary reaction that occurs is the formation of chloromethane and hydrogen chloride.

As the reaction proceeds, the concentration of chloromethane increases in the mixture. This higher concentration of chloromethane allows for a secondary substitution reaction to take place. Chloromethane reacts with chlorine to produce dichloromethane.

The dichloromethane that is formed can then undergo further reaction, resulting in the formation of trichloromethane. As trichloromethane accumulates in the mixture, it can react with chlorine to produce tetrachloromethane.

Practice Problems

ANSWER: b) is the correct answer since propagation do not couple or create radicals.

ANSWER: c) is the correct answer since we are coupling two radicals