Free-radical substitution · 自由基取代
| English | 中文 | Pinyin · 拼音 |
|---|---|---|
| free-radical substitution/friː ˈrædɪkl ˌsʌbstɪˈtjuːʃn/ | 自由基取代 | zì yóu jī qǔ dài |
| initiation/ɪˌnɪʃɪˈeɪʃn/ | 引发 | yǐn fā |
| propagation/ˌprɒpəˈɡeɪʃn/ | 增长 | zēng zhǎng |
| termination/ˌtɜːmɪˈneɪʃn/ | 终止 | zhōng zhǐ |
| fraction/ˈfrækʃn/ | 馏分 | liú fèn |
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.
阳光下的链反应
- 烷烃在紫外光中与氯或溴反应。
- 机理是自由基取代(free-radical substitution),分三步。
- 与此同时,裂解把重油变成有用的分子。
Alkanes react with chlorine by free-radical substitution in the presence of: · 烷烃在以下哪种存在下通过自由基取代与氯反应:
UV light starts the reaction by splitting the halogen into radicals. · 紫外光通过把卤素分裂成自由基来启动反应。
Alkanes react with halogens in UV light by free-radical ______. · 烷烃在紫外光中通过自由基______与卤素反应。
Initiation, propagation and termination steps. · 引发、链增长和终止步骤。
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
三个步骤
- 引发(initiation)——紫外光把卤素分裂成自由基:
$$\text{Cl}_2 \rightarrow 2\,\text{Cl}\cdot$$
- 链增长(propagation)——自由基反应并制造新自由基:
$$\text{Cl}\cdot + \text{C}_2\text{H}_6 \rightarrow \text{C}_2\text{H}_5\cdot + \text{HCl}$$$$\text{C}_2\text{H}_5\cdot + \text{Cl}_2 \rightarrow \text{C}_2\text{H}_5\text{Cl} + \text{Cl}\cdot$$
- 终止(termination)——两个自由基结合,结束链:
$$\text{Cl}\cdot + \text{C}_2\text{H}_5\cdot \rightarrow \text{C}_2\text{H}_5\text{Cl}$$
自由基取代:引发、链增长和终止
Free-radical substitution · 自由基取代
Methane reacts with chlorine in UV light by a chain of radical steps. · 甲烷在紫外光中通过一连串自由基步骤与氯反应。
The initiation step is: · 引发步骤是:
Initiation: Cl₂ → 2Cl·, the homolytic split that starts the chain. · 引发:Cl₂ → 2Cl·,启动链的均裂。
In a propagation step: · 在一个链增长步骤中:
Propagation keeps the chain going: each step uses a radical and produces another. · 链增长保持链进行:每一步使用一个自由基并产生另一个。
Termination happens when: · 终止发生在当:
Two radicals combine, removing radicals and stopping the chain reaction. · 两个自由基结合,移除自由基并停止链反应。
Match each step of free-radical substitution. · 匹配自由基取代的每个步骤。
Each item links the term to its correct meaning. · 每一项把术语链接到它正确的含义。
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
为什么裂解有用
- 裂解把重的原油馏分(fractions)变成更低 $M_r$ 的烷烃和烯烃——两者都更有用。
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.
为什么你得到一个混合物
- 自由基取代难以控制——它很少给出一个纯产物。
- 氯能再次取代:CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄。
- 终止也以不同的方式结合自由基(例如由两个 CH₃· 形成更长的链如 C₂H₆)。
- 所以你得到一个混合物,然后必须被分离——这个反应的一个真实缺点。
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
你掌握了
- 烷烃 + 卤素通过自由基取代在紫外光中反应
- 引发(紫外光分裂卤素 → 自由基)→ 链增长(链继续)→ 终止(自由基结合)
- 它给出一个混合物(进一步取代 + 自由基结合),所以难以得到一个纯产物
- 裂解把重馏分转化成有用的烷烃和烯烃