R3.3.3 Substitution reactions with alkanes

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 ( $C H_{4}$ ) reacting with chlorine ( $C l_{2}$ ) under UV light.

1. Initiation: Formation of Radicals

The initiation step generates radicals, which are essential to start the chain reaction.
For $C l_{2}$ , UV light provides the energy needed to break the chlorine-chlorine bond via homolytic fission:

$C l_{2} \overset{UV light}{\to} C l • + C l •$

Each chlorine radical now has one unpaired electron, making it highly reactive.

Tip

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 ( $H C l$ ) and a methyl radical ( $C H_{3} •$ ):

$C l • + C H_{4} \to C H_{3} • + H C l$

Step 2:

The methyl radical reacts with another chlorine molecule, forming chloromethane ( $C H_{3} C l$ ) and regenerating a chlorine radical:

$C H_{3} • + C l_{2} \to C H_{3} C l + C l •$

Note

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.

Example

Propagation Steps in Chlorination of Methane

$C l • + C H_{4} \to C H_{3} • + H C l$
$C H_{3} • + C l_{2} \to C H_{3} C l + C l •$

3. Termination: Stopping the Reaction

The termination step occurs when two radicals combine, forming a stable molecule and effectively removing radicals from the reaction mixture.
In the chlorination of methane, possible termination reactions include:

$C l • + C l • \to C l_{2}$
$C H_{3} • + C l • \to C H_{3} C l$
$C H_{3} • + C H_{3} • \to C_{2} H_{6}$

Termination reduces the concentration of radicals, eventually bringing the reaction to a halt.

Common Mistake

Common Mistake: Forgetting that termination reactions involve two radicals combining. For example, $C H_{4} + C l •$ is not a termination step because $C H_{4}$ is not a radical.

Why Are Alkanes Stable but Reactive in Radical Reactions?

Alkanes, such as methane ( $C H_{4}$ ), are known for their chemical stability. This stability arises from two key factors:

Strong Bonds:
- The carbon-hydrogen ( $C - H$ ) bond has a bond enthalpy of 414 ${kJ mol}^{- 1}$ , and the carbon-carbon ( $C - C$ ) bond has a bond enthalpy of 346 ${kJ mol}^{- 1}$ .
- Breaking these bonds requires significant energy.
Nonpolar Nature:
- Alkanes consist of nonpolar bonds, making them unreactive toward polar reagents.

However, under the right conditions—such as exposure to UV light—alkanes can participate in radical reactions. This is because:

Radical Reactions Are Thermodynamically Favorable:
- Although alkanes are kinetically stable, radical reactions are thermodynamically favorable due to the formation of strong new bonds (e.g., $C - C l$ and $H - C l$ ).
Radicals Are Highly Reactive:
- Once radicals are formed (e.g., $C l •$ ), they can overcome the stability of alkanes by abstracting hydrogen atoms.

Analogy

Think of alkanes as a fortress with strong walls (stable bonds). Radicals act like a battering ram, breaking through the walls when enough force (UV light) is applied.

Tip

Always remember that UV light or heat is required to initiate radical substitution reactions with alkanes. Without these conditions, the strong bonds in alkanes prevent reaction.

Applications and Implications of Radical Substitution

Radical substitution reactions are not just theoretical—they have practical applications in industry and everyday life.

Halogenation of Alkanes:
- Chloromethane ( $C H_{3} C l$ ) and other halogenoalkanes are important intermediates in the production of solvents, refrigerants, and pharmaceuticals.
Environmental Concerns:
- Radical reactions also play a role in the depletion of the ozone layer.
- Chlorofluorocarbons (CFCs) release chlorine radicals in the stratosphere, which catalyze the breakdown of ozone ( $O_{3}$ ).

Reflection

Self review

What are the three stages of a radical substitution reaction?
Write the propagation steps for the reaction between ethane ( $C_{2} H_{6}$ ) and bromine ( $B r_{2}$ ) under UV light.
Why are alkanes kinetically stable but thermodynamically reactive in radical reactions?

Theory of Knowledge

Radical substitution reactions illustrate the interplay between descriptive and explanatory models in science.
For example, curly-arrow mechanisms are not directly observable but provide a powerful way to visualize and predict chemical behavior.
How do such models shape our understanding of chemical processes?

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