Reactivity 3.3.3 — Substitution Reactions Between Alkanes and Halogens: Propagation and Termination Steps

Alkanes are generally unreactive because:

• They are non-polar molecules with low reactivity toward electrophiles and nucleophiles.
• They have strong covalent bonds: \( \text{C–C} \approx 348\, \text{kJ/mol} \), \( \text{C–H} \approx 412\, \text{kJ/mol} \).

However, alkanes undergo free radical substitution reactions with halogens in the presence of ultraviolet (UV) light or heat. These reactions follow a radical chain mechanism with three key stages: initiation, propagation, and termination.

Stepwise Mechanism of Free Radical Substitution:

1. Initiation (Already covered in 3.3.2)

Radical generation through homolytic fission of halogen molecules under UV light:

\( \text{Cl}_2 \xrightarrow{\text{UV}} \cdot\text{Cl} + \cdot\text{Cl} \)

2. Propagation Steps

The chlorine radical reacts with methane (or another alkane), abstracting a hydrogen atom and forming a methyl radical:

\( \cdot\text{Cl} + \text{CH}_4 \rightarrow \text{HCl} + \cdot\text{CH}_3 \)

Then, the methyl radical reacts with another chlorine molecule to form chloromethane and regenerate the chlorine radical:

\( \cdot\text{CH}_3 + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \cdot\text{Cl} \)

These two steps repeat in a chain reaction, continuing until radicals are removed.

3. Termination Steps

Termination occurs when two radicals combine, removing them from the reaction mixture and ending the chain.

Common termination reactions include:

• \( \cdot\text{Cl} + \cdot\text{Cl} \rightarrow \text{Cl}_2 \)
• \( \cdot\text{CH}_3 + \cdot\text{CH}_3 \rightarrow \text{C}_2\text{H}_6 \)
• \( \cdot\text{Cl} + \cdot\text{CH}_3 \rightarrow \text{CH}_3\text{Cl} \)

Mixture of Products:

Due to continued substitution, a mixture of products can form, especially with excess halogen:

• Monosubstitution: \( \text{CH}_3\text{Cl} \)
• Disubstitution: \( \text{CH}_2\text{Cl}_2 \)
• Trisubstitution: \( \text{CHCl}_3 \)
• Tetrasubstitution: \( \text{CCl}_4 \)

Stability of Alkanes and Why UV Light Is Required:

• The high bond enthalpies of the C–H and C–C bonds make them resistant to spontaneous breaking.
• Alkanes are non-polar and lack functional groups, further contributing to their low reactivity.
• UV light is needed to initiate the reaction by supplying energy to break the halogen bond and generate radicals.