📚 International AS Chemistry: Reaction Mechanisms (Unit 1) | 国际AS化学：反应机理（第一单元）
A reaction mechanism is the detailed step-by-step pathway by which bonds break and form, showing the movement of electrons. For International AS Chemistry Unit 1, mastering the key organic mechanisms — free radical substitution, electrophilic addition, and nucleophilic substitution — is essential for answering exam questions on reaction conditions, curly arrows, intermediates, and product prediction.
反应机理是化学键断裂和形成的详细分步路径，展示电子的转移过程。在国际AS化学第一单元中，掌握自由基取代、亲电加成和亲核取代这些核心有机反应机理，对于解答有关反应条件、弯箭头、中间体和产物预测的考题至关重要。
1. What Is a Reaction Mechanism? | 什么是反应机理？
A reaction mechanism describes the sequence of elementary steps that convert reactants into products. It shows which covalent bonds are broken and formed, and illustrates the flow of electron pairs or single electrons using curly arrows. Understanding mechanisms allows chemists to predict reaction outcomes and to design synthesis routes.
反应机理描述了将反应物转化为产物的一系列基元步骤。它揭示了哪些共价键断裂和生成，并用弯箭头表示电子对或单电子的流动。理解反应机理有助于化学家预测反应结果并设计合成路线。
2. Bond Fission: Homolytic and Heterolytic | 键断裂：均裂与异裂
Homolytic fission is the breaking of a covalent bond where each atom takes one electron from the shared pair, forming two neutral radicals. For example, chlorine under UV light: Cl–Cl → 2 Cl•. Each chlorine atom gets an unpaired electron, shown with a dot. Heterolytic fission occurs when the bond breaks unevenly, with one atom receiving both electrons from the bond, forming a cation and an anion. For example, in the polarisation of HBr: H–Br → H⁺ + Br⁻ (under the influence of an electrophile).
均裂是共价键断裂时每个原子各带走一个共享电子，形成两个中性自由基。例如，氯在紫外光下：Cl–Cl → 2 Cl•。每个氯原子获得一个未成对电子，用点表示。异裂则是键断裂不均匀，一个原子带走两个电子，形成阳离子和阴离子。例如，在 HBr 的极化下：H–Br → H⁺ + Br⁻（受亲电试剂影响）。
3. Curly Arrows and Electron Movement | 弯箭头与电子移动
Curly arrows are used to show the movement of electron pairs during bond-breaking and bond-forming. A full arrow (⇀) represents the movement of an electron pair, either from a bond or from a lone pair. A half-headed arrow, or ‘fish-hook’ arrow, shows the movement of a single electron, used in free radical steps. In exam responses, you must draw curly arrows precisely, starting from the electron source (bond, lone pair, or negative charge) and pointing to the electron-deficient atom or bond.
弯箭头用于表示键断裂和键生成过程中电子对的移动。全箭头（⇀）代表电子对的移动，可以来自一个键或孤对电子。半箭头，即“鱼钩”箭头，表示单电子的移动，用于自由基反应步骤。在考试回答中，必须精确绘制弯箭头，从电子来源（化学键、孤对电子或负电荷）出发，指向缺电子的原子或键。
4. Free Radical Substitution: Overall Reaction | 自由基取代：总反应
Alkanes react with halogens in the presence of UV light to form halogenoalkanes and hydrogen halide. The classic example is methane with chlorine: CH₄ + Cl₂ → CH₃Cl + HCl. This reaction proceeds via a free radical chain mechanism, not by a simple one-step displacement. The mechanism accounts for the formation of multiple substitution products and requires an understanding of initiation, propagation, and termination.
烷烃在紫外光存在下与卤素反应生成卤代烷和卤化氢。典型的例子是甲烷与氯气：CH₄ + Cl₂ → CH₃Cl + HCl。该反应经由自由基链式机理进行，而不是简单的一步取代。这一机理解释了多重取代产物的生成，并要求掌握引发、增长和终止三个阶段。
5. Free Radical Substitution Steps | 自由基取代步骤
Initiation: The Cl–Cl bond undergoes homolytic fission by UV light to give two chlorine radicals. Cl–Cl → 2 Cl•. Propagation: (i) A chlorine radical abstracts a hydrogen atom from methane, forming HCl and a methyl radical: CH₄ + Cl• → •CH₃ + HCl. (ii) The methyl radical reacts with a Cl₂ molecule, generating chloromethane and a new chlorine radical: •CH₃ + Cl₂ → CH₃Cl + Cl•. These two steps repeat in a chain reaction. Termination: Two radicals combine to form a stable molecule, stopping the chain: Cl• + Cl• → Cl₂, •CH₃ + Cl• → CH₃Cl, or •CH₃ + •CH₃ → C₂H₆.
引发：紫外光使 Cl–Cl 键发生均裂，生成两个氯自由基。Cl–Cl → 2 Cl•。增长：（i）一个氯自由基从甲烷中夺取一个氢原子，生成 HCl 和甲基自由基：CH₄ + Cl• → •CH₃ + HCl。（ii）甲基自由基与 Cl₂ 分子反应，生成氯甲烷和新的氯自由基：•CH₃ + Cl₂ → CH₃Cl + Cl•。这两步循环进行，构成链式反应。终止：两个自由基结合形成稳定分子，终止链反应：如 Cl• + Cl• → Cl₂，•CH₃ + Cl• → CH₃Cl，或 •CH₃ + •CH₃ → C₂H₆。
6. Electrophilic Addition: Mechanism Overview | 亲电加成：机理概述
Alkenes contain an electron-rich C=C double bond that can attack electrophiles (electron-deficient species). The general electrophilic addition mechanism involves the π bond acting as a nucleophile, attacking the electrophilic part of the reagent, leading to the formation of a carbocation intermediate or a cyclic intermediate, followed by rapid attack by a nucleophile to give the saturated addition product.
烯烃含有富电子的 C=C 双键，能够进攻亲电试剂（缺电子物种）。一般的亲电加成机理是：π键作为亲核体，进攻试剂中亲电的部分，首先生成碳正离子中间体或环状中间体，然后亲核试剂迅速进攻，得到饱和的加成产物。
7. Electrophilic Addition with Hydrogen Halides | 与卤化氢的亲电加成
When ethene reacts with HBr, the polar H–Br bond is the electrophile. The mechanism: the π electrons attack the hydrogen, leading to heterolytic fission of H–Br and formation of a carbocation (C₂H₅⁺) and Br⁻. The bromide ion then donates its lone pair to the carbocation, forming bromoethane. For unsymmetrical alkenes like propene, Markovnikov’s rule applies: the major product arises from the more stable carbocation intermediate (tertiary > secondary > primary).
当乙烯与溴化氢反应时，极性的 H–Br 键是亲电试剂。机理：π电子进攻氢，导致 H–Br 发生异裂，生成碳正离子（C₂H₅⁺）和 Br⁻。然后溴离子将孤对电子给予碳正离子，形成溴乙烷。对于不对称烯烃如丙烯，适用马尔科夫尼科夫规则：主要产物来自更稳定的碳正离子中间体（叔碳正离子 > 仲碳正离子 > 伯碳正离子）。
8. Electrophilic Addition with Bromine | 与溴的亲电加成
Ethene decolourises bromine water because of electrophilic addition. The mechanism differs slightly from HBr because Br₂ is non-polar but becomes polarised when approaching the double bond. The π electrons attack one bromine atom, displacing the other as Br⁻. A cyclic bromonium ion (C₂H₄Br⁺) is formed, in which the positive charge is delocalised over a three-membered ring. The bromide ion then attacks from the backside, opening the ring and giving 1,2-dibromoethane. This explains the trans addition stereochemistry.
乙烯能使溴水褪色，原因是发生了亲电加成。其机理与 HBr 略有不同，因为 Br₂ 是非极性分子，但靠近双键时会被极化。π电子进攻一个溴原子，将另一个以 Br⁻ 的形式推出。形成一个环状溴鎓离子（C₂H₄Br⁺），正电荷离域在一个三元环上。然后溴离子从背面进攻，开环生成 1,2-二溴乙烷。这解释了反式加成的立体化学。
9. Nucleophilic Substitution: SN2 Mechanism | 亲核取代：SN2机理
SN2 stands for bimolecular nucleophilic substitution. It occurs in one step: the nucleophile attacks the carbon attached to the halogen from the opposite side, forming a transition state in which the carbon is partially bonded to both the nucleophile and the leaving group. The halogen then leaves, and the product undergoes inversion of configuration. Rate = k[halogenoalkane][nucleophile]. Primary halogenoalkanes favour SN2 due to minimal steric hindrance.
SN2 代表双分子亲核取代。它是一步反应：亲核试剂从背面进攻与卤素相连的碳，形成一个过渡态，其中碳同时与亲核试剂和离去基团部分成键。随后卤素离去，产物发生构型翻转。速率方程为 Rate = k[卤代烷][亲核试剂]。伯卤代烷因位阻小而倾向于SN2机理。
10. Nucleophilic Substitution: SN1 Mechanism | 亲核取代：SN1机理
SN1 stands for unimolecular nucleophilic substitution. It involves two steps: (1) slow heterolytic fission of the C–Hal bond to form a planar carbocation and a halide ion; (2) fast attack by the nucleophile on either face of the carbocation, leading to a racemic mixture if the carbon is chiral. Rate = k[halogenoalkane], independent of nucleophile concentration. Tertiary halogenoalkanes favour SN1 because the carbocation is stabilised by alkyl groups.
SN1 代表单分子亲核取代。它分两步进行：（1）C–Hal 键发生缓慢的异裂，形成平面碳正离子和卤离子；（2）亲核试剂快速从碳正离子的任一面进攻，如果碳是手性的，则得到外消旋混合物。速率方程为 Rate = k[卤代烷]，与亲核试剂浓度无关。叔卤代烷倾向于SN1，因为碳正离子被烷基稳定。
11. Comparing SN1 and SN2 and Key Factors | SN1与SN2的对比和关键因素
Substrate structure: SN2 prefers primary > secondary > tertiary (steric hindrance slows down backside attack). SN1 prefers tertiary > secondary > primary (carbocation stability). Nucleophile strength: Strong nucleophiles favour SN2; weak nucleophiles favour SN1. Solvent: Polar protic solvents stabilise carbocations and favour SN1; polar aprotic solvents enhance nucleophile strength and favour SN2. Leaving group ability: Good leaving groups (weak bases like I⁻, Br⁻, H₂O) are required for both mechanisms. Understanding these trends allows prediction of the dominant mechanism.
底物结构：SN2 偏好 伯 > 仲 > 叔（位阻会阻碍背面进攻）。SN1 偏好 叔 > 仲 > 伯（碳正离子稳定性）。亲核试剂强度：强亲核试剂有利于SN2；弱亲核试剂有利于SN1。溶剂：极性质子溶剂稳定碳正离子，有利于SN1；极性非质子溶剂增强亲核试剂强度，有利于SN2。离去基团能力：两种机理都需要好的离去基团（弱碱，如 I⁻、Br⁻、H₂O）。掌握这些趋势可以预测主导的机理。
12. Exam Tips and Common Pitfalls | 考试技巧与常见误区
Always show curly arrows from the electron pair (bond or lone pair) to the atom or bond accepting electrons. For radical mechanisms, use half arrows for single-electron movement. Never mix-up homolytic and heterolytic fission in the wrong context. Specify the necessary condition, such as UV light for free radical substitution. For electrophilic addition to unsymmetrical alkenes, justify the major product by stating the stability of the carbocation intermediate (tertiary > secondary > primary). For SN2, indicate inversion; for SN1, state possible racemisation. Label the rate-determining step in SN1 as the carbocation formation.
始终从电子对（键或孤对电子）出发画弯箭头指向接受电子的原子或键。对自由基机理使用半箭头表示单电子移动。切勿在错误的上下文中混淆均裂和异裂。明确所需条件，例如自由基取代需要紫外光。对于不对称烯烃的亲电加成，用碳正离子稳定性顺序（叔 > 仲 > 伯）说明为何主要产物是那个。SN2 要指明翻转；SN1 要说明可能发生外消旋化。将 SN1 中的决速步标为碳正离子的生成。
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