Free-radical substitution
Introduced| English | Meaning |
|---|---|
| free-radical substitution/friː ˈrædɪkl ˌsʌbstɪˈtjuːʃn/ | A substitution reaction that proceeds through a chain mechanism involving species with unpaired electrons. |
| initiation/ɪˌnɪʃɪˈeɪʃn/ | The first stage of a chain reaction, in which reactive species such as free radicals are produced. |
| propagation/ˌprɒpəˈɡeɪʃn/ | The chain-carrying steps of a free-radical reaction in which a radical is regenerated. |
| termination/ˌtɜːmɪˈneɪʃn/ | The ending stage of a process, such as completion of protein synthesis or the stopping of a reaction chain. |
| fraction/ˈfrækʃn/ | fraction |
Chain reactions in sunlight
- Alkanes react with chlorine or bromine in ultraviolet light.
- The mechanism is free-radical substitution 自由基取代, in three steps.
- Cracking, meanwhile, turns heavy oil into useful molecules.
Alkanes react with chlorine by free-radical substitution in the presence of:
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UV light starts the reaction by splitting the halogen into radicals.
Alkanes react with halogens in UV light by free-radical ______.
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Initiation, propagation and termination steps.
The three steps
- initiation 引发 — UV light splits the halogen into radicals:
$$\text{Cl}_2 \rightarrow 2\,\text{Cl}\cdot$$
- propagation 增长 — 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 终止 — two radicals join, ending the chain:
$$\text{Cl}\cdot + \text{C}_2\text{H}_5\cdot \rightarrow \text{C}_2\text{H}_5\text{Cl}$$
Free-radical substitution: initiation, propagation and termination
Free-radical substitution
Methane reacts with chlorine in UV light by a chain of radical steps.
The initiation step is:
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Initiation: Cl₂ → 2Cl·, the homolytic split that starts the chain.
In a propagation step:
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Propagation keeps the chain going: each step uses a radical and produces another.
Termination happens when:
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Two radicals combine, removing radicals and stopping the chain reaction.
Match each step of free-radical substitution.
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Each item links the term to its correct meaning.
Why cracking is useful
- Cracking turns heavy crude-oil fractions 馏分 into lower-$M_r$ alkanes and alkenes — both more useful.
Complete combustion gives CO2 and water; incomplete also gives CO and soot
Why you get a mixture
- Free-radical substitution is hard to control — it rarely gives one pure product.
- The chlorine can substitute again: CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄.
- Termination also joins radicals in different ways (e.g. longer chains like C₂H₆ from two CH₃·).
- So you get a mixture, which then has to be separated — a real drawback of this reaction.
You've got it
- alkanes + halogen react by free-radical substitution in UV light
- initiation (UV splits halogen → radicals) → propagation (chain carries on) → termination (radicals join)
- it gives a mixture (further substitution + radical combination), so it's hard to get one pure product
- cracking converts heavy fractions into useful alkanes and alkenes