Understanding Free Radical Halogenation: The Initiation, Propagation, and Termination of a Radical Chain Reaction in Organic Chemistry

Free Radical Halogenation

Free radical halogenation is a type of substitution reaction in organic chemistry where a halogen atom is introduced into a molecule by replacing a hydrogen atom

Free radical halogenation is a type of substitution reaction in organic chemistry where a halogen atom is introduced into a molecule by replacing a hydrogen atom. It is a radical chain reaction that involves three main steps: initiation, propagation, and termination.

1. Initiation: The reaction begins with the formation of free radicals, usually by the use of light or heat. For example, in the case of chlorination, chlorine gas (Cl2) is activated by UV light to produce two chlorine radicals (Cl•). This step is called the initiation because it initiates the formation of radicals.

Cl2 + hv → 2 Cl•

2. Propagation: In this step, the radicals react with the organic molecule, leading to the formation of new radicals. These radicals continue to react with new molecules, generating a chain reaction. In the case of chlorination, the chlorine radical (Cl•) abstracts a hydrogen atom from an alkane molecule (RH), forming a hydrogen chloride molecule (HCl) and an alkyl radical (R•).

Cl• + RH → HCl + R•

The alkyl radical (R•) can then react with a chlorine molecule (Cl2) to form a chlorinated alkane (RCl) and a new chlorine radical (Cl•), which continues the chain reaction.

R• + Cl2 → RCl + Cl•

3. Termination: The chain reaction can be terminated by the combination of two radicals or by other reactions that consume the radicals. For example, two chlorine radicals can react to produce a chlorine molecule, terminating the chain reaction.

Cl• + Cl• → Cl2

The selectivity of free radical halogenation reactions depends on the relative strengths of C-H and C-Cl bonds. Hydrogen atoms adjacent to highly substituted carbon atoms (tertiary, secondary) are more reactive and preferentially attacked by radicals. This is due to the stability of the resulting alkyl radicals, which have more hyperconjugative structures. As a result, multiple substitutions may occur in certain cases.

It is important to note that the reaction conditions and reactant concentrations can influence the selectivity and overall outcome of free radical halogenation reactions. Care should also be taken with highly reactive halogens like fluorine, as they tend to be more reactive and can lead to undesired side reactions.

In summary, free radical halogenation is a useful method for introducing halogen atoms into organic molecules. It involves the initiation, propagation, and termination steps of a radical chain reaction. This reaction can be influenced by factors such as reactant concentrations and reaction conditions, and its selectivity depends on the relative strengths of C-H and C-X (halogen) bonds.

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