Radical Substitution Reactions: Mechanism and Stability of Alkanes
The Mechanism of Radical Substitution
- Radical substitution reactions proceed through three key stages: initiation, propagation, and termination.
- Let’s break these down with the example of methane ($CH_4$) reacting with chlorine ($Cl_2$) under UV light.
1. Initiation: Formation of Radicals
- The initiation step generates radicals, which are essential to start the chain reaction.
- For $Cl_2$, UV light provides the energy needed to break the chlorine-chlorine bond via homolytic fission: $$
Cl_2 \xrightarrow{\text{UV light}} Cl• + Cl•
$$ - Each chlorine radical now has one unpaired electron, making it highly reactive.
- Always include the energy source (e.g., UV light) in equations for initiation steps.
- Without it, the reaction cannot proceed.
2. Propagation: The Chain Reaction
- In the propagation stage, radicals react with stable molecules to form new radicals, perpetuating the chain reaction.
- For methane and chlorine, this involves two key steps:
Step 1:
A chlorine radical reacts with a methane molecule, abstracting a hydrogen atom and forming hydrogen chloride ($HCl$) and a methyl radical ($CH_3•$):
$$
Cl• + CH_4 \rightarrow CH_3• + HCl
$$
Step 2:
The methyl radical reacts with another chlorine molecule, forming chloromethane ($CH_3Cl$) and regenerating a chlorine radical:
$$
CH_3• + Cl_2 \rightarrow CH_3Cl + Cl•
$$
- The regenerated chlorine radical can then react with another methane molecule, repeating the cycle.
- This self-sustaining process is why radical substitution is called a chain reaction.
Propagation Steps in Chlorination of Methane
$$Cl• + CH_4 \rightarrow CH_3• + HCl$$
$$CH_3• + Cl_2 \rightarrow CH_3Cl + Cl•$$
