Free-radical substitution
Free-radical substitution
- Alkanes react with chlorine or bromine in ultraviolet light.
- The mechanism is free-radical substitution, in three steps.
- Cracking, meanwhile, turns heavy oil into useful molecules.
Practice
Alkanes react with chlorine by free-radical substitution in the presence of:
UV light starts the reaction by splitting the halogen into radicals.
The three steps
- initiation — UV light splits the halogen into radicals:
$$\text{Cl}_2 \rightarrow 2\,\text{Cl}\cdot$$
- propagation — radicals react and make new radicals:
$$\text{Cl}\cdot + \text{C}_2\text{H}_6 \rightarrow \text{C}_2\text{H}_5\cdot + \text{HCl}$$$$\text{C}_2\text{H}_5\cdot + \text{Cl}_2 \rightarrow \text{C}_2\text{H}_5\text{Cl} + \text{Cl}\cdot$$
- termination — two radicals join, ending the chain:
$$\text{Cl}\cdot + \text{C}_2\text{H}_5\cdot \rightarrow \text{C}_2\text{H}_5\text{Cl}$$
Practice
The initiation step is:
Initiation: Cl₂ → 2Cl·, the homolytic split that starts the chain.
Practice
In a propagation step:
Propagation keeps the chain going: each step uses a radical and produces another.
Practice
Termination happens when:
Two radicals combine, removing radicals and stopping the chain reaction.
Why cracking is useful
- Cracking turns heavy crude-oil fractions into lower-$M_r$ alkanes and alkenes — both more useful.
You've got it
Key idea
- alkanes + halogen react by free-radical substitution in UV light
- initiation (UV splits halogen → radicals) → propagation (chain carries on) → termination (radicals join)
- cracking converts heavy fractions into useful alkanes and alkenes