Radical Substitution Reactions in Alkanes
Quick Notes:
- Alkanes are saturated hydrocarbons with strong C–C and C–H bonds.
- They are non-polar and kinetically stable, but thermodynamically unstable in the presence of oxygen or radicals.
- React with halogens (e.g. Cl2) via free radical substitution when exposed to UV light.
- The mechanism includes:
- Initiation
- Propagation
- Termination
- Propagation generates new radicals and keeps the reaction going.
- Termination removes radicals by combining them.
Full Notes:
Free-radical substitution has been outlined in more detail here. This page is just what you need to know for IB SL + AHL Chemistry :)
Alkanes can react with halogens in the presence of ultraviolet (UV) light, producing a mixture of products.
Example: Chlorine + methane forming chloromethane.
UV light is required to initiate the reaction. The reaction is an example of free-radical substitution and occurs in several steps which we can show using a mechanism.
Free-Radical Substitution Mechanism
Step 1: Initiation (Radicals Are Formed)
- UV light provides energy to break the Cl–Cl bond by homolytic fission.
- Each chlorine atom ends up with an unpaired electron (•), making it a radical.
Step 2: Propagation (Radicals React and Regenerate)
- Radicals react to form new radicals in a chain reaction.
- Chlorine radical reacts with methane, forming a methyl radical
- Methyl radical reacts with Cl2, forming chloromethane and a new Cl• radical:
The process continues, leading to further substitutions.
Step 3: Termination (Radicals Are Removed)
- Radicals combine to form stable (non-radical) molecules, stopping the reaction.
- There are several possible termination reactions:
Termination stops the chain reaction.
Limitations of Free-Radical Substitution
Free radical substitution is hard to control and a mixture of products usually forms due to further substitution and multiple side reactions.
Why Alkanes Are Generally Unreactive?
Alkanes have strong C–C and C–H bonds that are non-polar due to similar electronegativities of C and H. These factors make them chemically unreactive under normal conditions.
They are kinetically stable (do not react easily due to high activation energy barriers), but thermodynamically unstable (release a lot of energy when they do react, e.g., in combustion).
Summary
- Alkanes undergo substitution with halogens in the presence of UV light.
- The mechanism involves initiation, propagation, and termination steps.
- Reactions are difficult to control and produce mixtures due to further substitution.
- Alkanes are kinetically stable but thermodynamically unstable.