📚 Reaction Mechanisms | 反应机理
A reaction mechanism is the step-by-step sequence of elementary reactions by which an overall chemical change occurs. Understanding mechanisms allows chemists to predict products, control reaction conditions, and design new synthetic pathways. In A-level Chemistry, you will encounter three fundamental organic reaction mechanisms: free radical substitution, electrophilic addition, and nucleophilic substitution. Each mechanism is governed by the movement of electrons, shown by curly arrows, and the relative stability of intermediates such as radicals, carbocations, and transition states.
反应机理是总化学反应所经历的一系列基元步骤的详细顺序。理解机理能让化学家预测产物、控制反应条件并设计新的合成路线。在 A-level 化学中，你会遇到三种基本的有机反应机理：自由基取代、亲电加成和亲核取代。每种机理都由电子对的运动（用弯箭头表示）以及中间体（如自由基、碳正离子和过渡态）的相对稳定性所决定。
1. Curly Arrows and Bond Breaking | 弯箭头与键的断裂
Curly arrows are used to show the movement of electron pairs during bond breaking and bond making. A full curly arrow (↷) represents the movement of an electron pair, while a half-headed arrow (⌬) shows the movement of a single electron – this is used only in free radical mechanisms. Bonds can break in two ways: homolytic fission, where each atom takes one electron from the shared pair, forming two radicals; and heterolytic fission, where one atom takes both electrons, forming a cation and an anion. Heterolytic fission is typical in polar reactions, and the curly arrow starts from a lone pair or a bond and points towards the atom that accepts the electrons.
弯箭头用来表示成键和断键过程中电子对的移动。全箭头（↷）表示一对电子的移动，而半箭头（⌬）表示单个电子的移动——仅用于自由基机理。键的断裂有两种方式：均裂，每个原子从共用电子对中各取走一个电子，形成两个自由基；异裂，一个原子取走一对电子，形成一个阳离子和一个阴离子。异裂常见于极性反应中，弯箭头从孤对电子或键出发，指向接受电子的原子。
2. Free Radical Substitution | 自由基取代反应
Free radical substitution occurs when alkanes react with halogens in the presence of ultraviolet (UV) light. The mechanism proceeds through three stages: initiation, propagation, and termination. In initiation, UV light provides the energy to break the halogen molecule homolytically: Cl2 → 2Cl•. During propagation, chlorine radicals abstract a hydrogen atom from the alkane to form H–Cl and an alkyl radical (e.g., CH3•), which then reacts with another Cl2 molecule to produce the halogenoalkane and regenerate a Cl• radical. Termination occurs when two radicals combine, ending the chain reaction. A mixture of mono-, di-, and polysubstituted products is often formed because the radical substitutions are not selective.
自由基取代反应发生在烷烃与卤素在紫外光照射下。反应机理包括三个阶段：引发、增长和终止。引发阶段，紫外光提供能量使卤素分子均裂：Cl2 → 2Cl•。在增长阶段，氯自由基从烷烃中夺取一个氢原子生成 HCl 和一个烷基自由基（如 CH3•），然后该烷基自由基再与另一个 Cl2 分子反应生成卤代烷并再生 Cl• 自由基。终止阶段发生在两个自由基结合时，链反应结束。由于自由基取代没有选择性，通常会得到一、二和多取代产物的混合物。
3. Electrophilic Addition to Alkenes: Overview | 烯烃的亲电加成概述
Alkenes contain a carbon–carbon double bond, which is an area of high electron density. This makes alkenes susceptible to attack by electrophiles – species that are electron-deficient and can accept a pair of electrons. The typical mechanism involves two steps: first, the electrophile accepts a pair of electrons from the π bond, forming a carbocation intermediate and a negatively charged species. In the second step, a nucleophile (often the negatively charged species produced in step one) donates a pair of electrons to the carbocation to form the addition product. The key intermediate is the carbocation, whose stability determines the regioselectivity of the reaction.
烯烃含有碳碳双键，该区域电子密度很高。这使得烯烃容易受到亲电试剂的进攻——亲电试剂是缺电子、能接受一对电子的物种。典型的机理包括两步：首先，亲电试剂从 π 键中接受一对电子，形成碳正离子中间体和带负电的物种。第二步，亲核试剂（通常是第一步生成的带负电物种）将一对电子给予碳正离子，生成加成产物。关键中间体是碳正离子，其稳定性决定了反应的区域选择性。
4. Addition of Hydrogen Halides and Markovnikov's Rule | 卤化氢加成与马氏规则
When a hydrogen halide (H–X) adds to an unsymmetrical alkene, the mechanism still involves electrophilic attack of H⁺ on the double bond, forming the most stable carbocation intermediate. The carbocation then rapidly combines with the halide ion. Markovnikov's rule states that the hydrogen atom attaches to the carbon that already has more hydrogen atoms (the less substituted carbon), while the halide attaches to the carbon that is more substituted. This outcome arises because the more stable carbocation is formed preferentially. A tertiary carbocation (3°) is more stable than a secondary (2°), which is more stable than a primary (1°), due to the positive inductive effect and hyperconjugation of alkyl groups.
当卤化氢（H–X）与不对称烯烃加成时，机理依然是 H⁺ 亲电进攻双键，生成最稳定的碳正离子中间体。碳正离子随后快速与卤离子结合。马氏规则指出，氢原子加在原本含氢较多的碳上（取代度较低的碳），而卤素加在取代度较高的碳上。这样的结果是因为首先生成了更稳定的碳正离子。叔碳正离子（3°）比仲碳正离子（2°）稳定，更比伯碳正离子（1°）稳定，这归因于烷基的正诱导效应和超共轭作用。
5. Addition of Halogens to Alkenes | 卤素与烯烃的加成
When a halogen such as Br2 or Cl2 adds to an alkene, the reaction proceeds via a similar electrophilic addition pathway, but with a subtlety: the first step involves the polarisation of the halogen molecule as it approaches the double bond, forming a cyclic ‘halonium’ ion (e.g., bromonium ion) rather than an open carbocation. The second step involves attack of the halide ion from the opposite face of the halonium ring, leading to anti addition. This stereospecificity can be observed in the formation of trans products from cyclic alkenes, and is often tested using the decolourisation of bromine water as a qualitative test for unsaturation.
当溴（Br2）或氯（Cl2）等卤素与烯烃加成时，反应经由类似的亲电加成路径，但有一个细微之处：第一步中，卤素分子靠近双键时发生极化，形成一个环状的“卤鎓”离子（如溴鎓离子），而不是开放的碳正离子。第二步是卤离子从卤鎓环的背面进攻，导致反式加成。这种立体专一性体现在环状烯烃可生成反式产物，并且常用于利用溴水褪色作为不饱和性的定性检验。
6. Nucleophilic Substitution: SN1 and SN2 Pathways | 亲核取代：SN1 和 SN2 路径
Halogenoalkanes undergo nucleophilic substitution reactions because the polar C– r nucleophilic substitution) and SN1 (unimolecular nucleophilic substitution). In an SN2 reaction, the nucleophile attacks the carbon from the opposite side of the leaving group, resulting in a single concerted step with inversion of configuration. The rate depends on both the halogenoalkane and the nucleophile: rate = k[RX][Nu]. In contrast, the SN1 mechanism proceeds via a two-step process: slow formation of a planar carbocation intermediate, followed by fast attack of the nucleophile from either side, leading to racemisation. The rate depends only on the halogenoalkane: rate = k[RX].
卤代烷能发生亲核取代反应，是因为极性的 C–X 键使碳原子上具有部分正电荷，成为亲电中心。有两种截然不同的机理：SN2（双分子亲核取代）和 SN1（单分子亲核取代）。在 SN2 反应中，亲核试剂从离去基团的背面进攻碳原子，一步协同完成，构型发生翻转。速率同时取决于卤代烷和亲核试剂：速率 = k[RX][Nu]。相反，SN1 机理通过两步进行：先是缓慢形成平面碳正离子中间体，然后亲核试剂从任意一侧快速进攻，导致外消旋化。速率仅取决于卤代烷：速率 = k[RX]。
7. Comparing SN1 and SN2 Mechanisms | 比较 SN1 和 SN2 机理
| Feature | SN1 | SN2 |
|---|---|---|
| Steps | Two (carbocation intermediate) | One (concerted) |
| Rate law | Rate = k[RX] | Rate = k[RX][Nu] |
| Substrate preference | 3° > 2° > 1° (carbocation stability) | 1° > 2° > 3° (steric hindrance) |
| Stereochemistry | Racemisation (planar intermediate) | Inversion (backside attack) |
| Leaving group effect | Good leaving group required | Good leaving group required |
| Solvent | Polar protic favours (stabilises ions) | Polar aprotic favours |
The choice between SN1 and SN2 depends mainly on the structure of the halogenoalkane, the strength and steric bulk of the nucleophile, the nature of the solvent, and the leaving group ability. Tertiary halogenoalkanes undergo SN1 almost exclusively because the tertiary carbocation is highly stable, while primary halogenoalkanes prefer SN2 due to less steric hindrance. Secondary halogenoalkanes can follow either pathway depending on conditions.
SN1 和 SN2 的选择主要取决于卤代烷的结构、亲核试剂的强度和体积、溶剂的性质以及离去基团的能力。叔卤代烷几乎只进行 SN1 反应，因为叔碳正离子非常稳定；而伯卤代烷由于位阻较小，倾向于 SN2 反应。仲卤代烷则可依据条件走其中任一路径。
8. Hydrolysis of Halogenoalkanes | 卤代烷的水解反应
The hydrolysis of halogenoalkanes with aqueous alkali (e.g., NaOH(aq)) proceeds via nucleophilic substitution, where OH⁻ acts as the nucleophile. For primary halogenoalkanes, the reaction follows an SN2 mechanism, producing the corresponding alcohol with inversion. For tertiary halogenoalkanes, an SN1 pathway dominates, and the planar carbocation leads to a mixture of stereoisomers if the carbon is chiral. The rate of hydrolysis can be investigated by adding silver nitrate solution and ethanol; the appearance of a silver halide precipitate indicates halide ion release. The reactivity trend is: iodoalkanes > bromoalkanes > chloroalkanes, dictated by the weakening C–X bond strength down the group.
卤代烷在氢氧化钠水溶液中的水解经由亲核取代进行，OH⁻ 作为亲核试剂。对于伯卤代烷，反应遵循 SN2 机理，生成构型翻转的醇。对于叔卤代烷，SN1 路径占主导，由于碳正离子为平面构型，如果碳为手性中心，会得到立体异构体的混合物。水解速率可通过加入硝酸银乙醇溶液来探究；卤化银沉淀的出现指示卤离子的释放。活性趋势为：碘代烷 > 溴代烷 > 氯代烷，这取决于碳-卤键强度从上至下逐渐减弱。
9. Reaction with Cyanide and Ammonia | 与氰化物和氨的反应
Nucleophilic substitution with cyanide ions (CN⁻) is a key method for extending carbon chains. Primary halogenoalkanes react with KCN in ethanol via SN2 to form nitriles (R–CN), which can be hydrolysed to carboxylic acids. This reaction increases the carbon chain length by one. With ammonia, primary halogenoalkanes undergo SN2 to form primary amines. However, because the product amine is itself nucleophilic, further substitutions may occur to give secondary, tertiary amines, and quaternary ammonium salts unless an excess of ammonia is used. These reactions illustrate the synthetic utility of halogenoalkanes in building more complex molecules.
氰离子（CN⁻）的亲核取代是延长碳链的关键方法。伯卤代烷与 KCN 的乙醇溶液通过 SN2 反应生成腈（R–CN），腈可进一步水解成羧酸。该反应使碳链延长一个碳原子。与氨反应时，伯卤代烷经 SN2 生成伯胺。但由于产物胺本身具有亲核性，后续可继续发生取代，生成仲胺、叔胺和季铵盐，除非使用过量的氨。这些反应展示了卤代烷在构建更复杂分子中的合成用途。
10. Factors Affecting Nucleophilic Substitution | 影响亲核取代的因素
The competition between SN1 and SN2 is influenced by several factors. (1) Structure of the substrate: tertiary alkyl halides favour SN1 because the carbocation is stabilised by three alkyl groups; primary substrates favour SN2 because the backside attack is sterically accessible. (2) Nucleophile: strong, concentrated, small nucleophiles (e.g., OH⁻, CN⁻) promote SN2; weak nucleophiles (e.g., H2O) often lead to SN1. (3) Leaving group: a good leaving group such as iodide (I⁻) stabilises the transition state or carbocation and accelerates both mechanisms. (4) Solvent: polar protic solvents (water, alcohols) solvate ions and stabilise the carbocation, thus favouring SN1; polar aprotic solvents (propanone, DMSO) solvate cations but leave the nucleophile unsolvated, enhancing SN2 rates. Understanding these factors allows chemists to steer a reaction towards the desired product.
SN1 与 SN2 的竞争受多种因素影响。（1）底物结构：叔卤代烷利于 SN1，因三烷基稳定碳正离子；伯卤代烷利于 SN2，因位阻小有利于背面进攻。（2）亲核试剂：强、浓、体积小的亲核试剂（如 OH⁻、CN⁻）促进 SN2；弱亲核试剂（如 H2O）常导致 SN1。（3）离去基团：好的离去基团如碘离子（I⁻）能稳定过渡态或碳正离子，加速两种机理。（4）溶剂：极性质子溶剂（水、醇）溶剂化离子并稳定碳正离子，因此有利于 SN1；极性非质子溶剂（丙酮、DMSO）溶剂化阳离子而使亲核试剂裸露，提高 SN2 速率。掌握这些因素可让化学家将反应导向期望的产物。
11. Summary and Exam Technique | 总结与考试技巧
When writing reaction mechanisms in exams, always draw full structural formulae, show all charges on intermediates, and use the correct curly arrows. The arrow must start from the electron source (a lone pair or a bond) and point exactly to the atom that receives the electrons. For free radical substitution, half arrows are essential. For electrophilic addition, show the intermediate carbocation or halonium ion clearly. In nucleophilic substitution, highlight the attack, transition state, and departure of the leaving group. Be mindful of stereochemistry: SN2 leads to inversion, while SN1 leads to racemisation if a chiral centre is formed. Practice by drawing mechanisms for a variety of examples, and consult the mark schemes from past papers for precise drawing conventions.
在考试中书写反应机理时，务必画出完整结构式、标出中间体上的所有电荷，并使用正确的弯箭头。箭头必须从电子来源（孤对电子或键）出发，精确指向接受电子的原子。自由基取代必须使用半箭头。亲电加成要清晰地画出碳正离子或卤鎓离子中间体。亲核取代中，要突出进攻、过渡态和离去基团的离去。务必留意立体化学：SN2 导致构型翻转，而如果生成手性中心，SN1 导致外消旋化。通过绘制多种实例的机理进行练习，并参考往年试题的评分标准来掌握精确的画法惯例。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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