Elimination and SN1/SN2 mechanisms · 消除和 SN1/SN2 机理
| English | 中文 | Pinyin · 拼音 |
|---|---|---|
| elimination/ɪˌlɪmɪˈneɪʃn/ | 消去 | xiāo qù |
| transition state/trænˈsɪʃn steɪt/ | 过渡态 | guò dù tài |
| carbocation/ˌkɑːbəˈkeɪʃn/ | 碳正离子 | tàn zhèng lí zi |
| bond energy/bɒnd ˈenədʒi/ | 键能 | jiàn néng |
Elimination 消去 and the SN1/SN2 mechanisms
- The same halogenoalkane can substitute or eliminate.
- Nucleophilic substitution follows one of two mechanisms.
- And the C–X bond strength sets the reactivity.
消除和 SN1/SN2 机理
- 同一个卤代烷能取代或消除。
- 亲核取代遵循两种机理之一。
- 而 C–X 键强度设定反应性。
An SN1 mechanism goes via a carbocation intermediate. · 一个 SN1 机理经一个碳正离子中间体。
SN2 is a one-step concerted mechanism. · SN2 是一个一步协同机理。
Substitution vs elimination
- $\text{NaOH}$ in water → nucleophilic substitution → an alcohol.
- $\text{NaOH}$ in ethanol, heated → elimination → an alkene:
$$\text{C}_2\text{H}_5\text{Br} + \text{NaOH} \rightarrow \text{C}_2\text{H}_4 + \text{NaBr} + \text{H}_2\text{O}$$
SN2 is one step through a transition state 过渡态; SN1 is two steps via a carbocation 碳正离子
取代对比消除
- $\text{NaOH}$ 在水中 → 亲核取代 → 一个醇。
- $\text{NaOH}$ 在乙醇中,加热 → 消除 → 一个烯烃:
$$\text{C}_2\text{H}_5\text{Br} + \text{NaOH} \rightarrow \text{C}_2\text{H}_4 + \text{NaBr} + \text{H}_2\text{O}$$
SN2 是经一个过渡态的一步;SN1 是经一个碳正离子的两步
The SN2 mechanism, step by step · SN2 机理,一步步
Step through nucleophilic substitution. The nucleophile attacks from behind as the halide leaves — all in one smooth step, flipping the molecule inside-out. · 一步步走过亲核取代。亲核试剂在卤离子离开时从背后进攻——全在一个平滑的步骤中,把分子从里到外翻转。
Heating a halogenoalkane with NaOH dissolved in ethanol gives: · 把一个卤代烷与溶在乙醇中的 NaOH 一起加热给出:
NaOH in ethanol with heat favours elimination of HX to form an alkene; NaOH in water favours substitution. · 乙醇中的 NaOH 加热有利于消除 HX 以形成一个烯烃;水中的 NaOH 有利于取代。
Match each mechanism to its description. · 把每个机理匹配到它的描述。
Each item links the term to its correct meaning. · 每一项把术语链接到它正确的含义。
SN1 and SN2
- SN2: one step — the nucleophile attacks as the halogen leaves, through a crowded transition state. Rate depends on both reactants.
- SN1: two steps — the C–X bond breaks to a carbocation first, then the nucleophile attacks. Rate depends only on the halogenoalkane.
- Primary → mostly SN2; tertiary → mostly SN1 (its carbocation is well stabilised); secondary uses both.
The weaker the C-X bond, the faster the halogenoalkane reacts
SN1 和 SN2
- SN2:一步——亲核试剂在卤素离开时进攻,经过一个拥挤的过渡态(transition state)。速率取决于两个反应物。
- SN1:两步——C–X 键先断成一个碳正离子,然后亲核试剂进攻。速率只取决于卤代烷。
- 伯 → 大多 SN2;叔 → 大多 SN1(它的碳正离子被很好地稳定);仲两者都用。
C-X 键越弱,卤代烷反应越快
The SN1 mechanism: · SN1 机理:
SN1: C–X breaks first to a carbocation, then the nucleophile attacks; tertiary halogenoalkanes favour it. · SN1:C–X 先断成一个碳正离子,然后亲核试剂进攻;叔卤代烷有利于它。
The SN2 mechanism: · SN2 机理:
SN2 is a single concerted step; primary halogenoalkanes favour it and its rate depends on both species. · SN2 是一个单一协同步骤;伯卤代烷有利于它,它的速率取决于两个物种。
Reactivity and the C–X bond
- Reactivity depends on the C–X bond strength (bond energy 键能).
- C–I is weakest → iodoalkanes react fastest; C–Cl is strongest → chloroalkanes slowest.
反应性和 C–X 键
- 反应性取决于 C–X 键强度(键能)。
- C–I 最弱 → 碘代烷反应最快;C–Cl 最强 → 氯代烷最慢。
Which halogenoalkane reacts fastest? · 哪个卤代烷反应最快?
The weaker the C–X bond, the faster the reaction, so C–I (weakest) reacts fastest and C–Cl slowest. · C–X 键越弱,反应越快,所以 C–I(最弱)反应最快,C–Cl 最慢。
You've got it
- NaOH in water → substitution (alcohol); NaOH in ethanol + heat → elimination (alkene)
- SN2 = one step, rate depends on both; SN1 = two steps via a carbocation, rate depends on the halogenoalkane only
- primary → SN2, tertiary → SN1; iodoalkanes react fastest (weakest C–X bond)
你掌握了
- NaOH 在水中 → 取代(醇);NaOH 在乙醇中 + 加热 → 消除(烯烃)
- SN2 = 一步,速率取决于两者;SN1 = 经一个碳正离子的两步,速率只取决于卤代烷
- 伯 → SN2,叔 → SN1;碘代烷反应最快(最弱的 C–X 键)