📚 Reaction Mechanisms for IB AQA Chemistry | IB AQA 化学：反应机理考点精讲
A reaction mechanism is the step-by-step sequence of elementary steps by which a chemical change occurs. Understanding mechanisms enables chemists to predict products, design syntheses and explain why particular conditions favour certain pathways. For IB and AQA chemistry, mastering curly arrows, homolytic versus heterolytic fission, and the distinction between substitution, addition and elimination mechanisms is essential.
反应机理是化学变化发生的逐步基元步骤序列。理解机理能让化学家预测产物、设计合成路线并解释为何特定条件有利某些路径。对 IB 和 AQA 化学来说，掌握卷曲箭头、均裂与异裂的区别，以及取代、加成与消除机理的区分至关重要。
1. What Are Reaction Mechanisms? | 什么是反应机理
A reaction mechanism breaks down the overall equation into individual elementary processes involving bond making and breaking. Each step shows the movement of electrons using curly arrows, with intermediates forming and being consumed. The slowest step, the rate-determining step, governs the overall kinetics.
反应机理将总反应方程式分解为涉及键的生成与断裂的独立基元过程。每一步用卷曲箭头表示电子移动，中间体形成并被消耗。最慢的一步，即决速步，决定着总反应动力学。
Key point: A mechanism must be consistent with the rate law, stereochemistry and the presence of intermediates detected experimentally.
关键点：机理必须与速率方程、立体化学以及实验中检测到的中间体相一致。
2. Curly Arrows and Electron Movement | 卷曲箭头与电子转移
Curly arrows depict the movement of an electron pair. Full arrows show two-electron shifts, while ‘fish-hook’ (half) arrows represent single-electron movements in radical processes. The tail starts where the electrons are originally located (a lone pair, a bond or a negative charge) and the head points to the atom or bond that receives them.
卷曲箭头描述电子对的移动。全箭头表示双电子转移，而“鱼钩”（半）箭头代表自由基过程中的单电子移动。箭尾始于电子原来所在位置（孤对电子、键或负电荷），箭头指向接受电子的原子或键。
Common mistakes include drawing arrows from a positive charge or starting from an atom that is already electron‑deficient. Always ensure the arrow flow respects electronegativity and formal charge stability.
常见错误包括从正电荷出发画箭头或从缺电子原子开始画。务必确保箭头流向符合电负性与形式电荷稳定性。
3. Homolytic vs Heterolytic Bond Fission | 均裂与异裂
In homolytic fission, a bond breaks evenly, each atom taking one electron, generating two radicals. This requires energy (UV light or high temperature) and occurs in non-polar solvents. Example: Cl–Cl → 2 Cl•.
均裂中，键均匀断裂，每个原子各得一个电子，产生两个自由基。此过程需要能量（紫外光或高温）并在非极性溶剂中进行。例如：Cl–Cl → 2 Cl•。
Heterolytic fission produces a cation and an anion because both electrons go to the more electronegative atom. Example: H–Br → H⁺ + Br⁻. This is common in polar solvents that stabilise ions.
异裂产生阳离子和阴离子，因为两个电子都转移到电负性更强的原子上。例如：H–Br → H⁺ + Br⁻。这在能稳定离子的极性溶剂中很常见。
| Feature | Homolytic | Heterolytic |
|---|---|---|
| Electron distribution | One electron each | Both to one atom |
| Products | Radicals | Cation + anion |
| Arrow type | Half arrows (fish-hook) | Full curly arrows |
4. Free Radical Substitution | 自由基取代反应
The free radical substitution of alkanes with halogens occurs in three stages: initiation, propagation and termination. Initiation: Cl₂ → 2 Cl• (UV light). Propagation: CH₄ + Cl• → •CH₃ + HCl; then •CH₃ + Cl₂ → CH₃Cl + Cl•. Termination: two radicals combine, e.g. Cl• + Cl• → Cl₂.
烷烃与卤素的自由基取代分三个阶段进行：引发、链增长与终止。引发：Cl₂ → 2 Cl•（紫外光）。链增长：CH₄ + Cl• → •CH₃ + HCl；接着 •CH₃ + Cl₂ → CH₃Cl + Cl•。终止：两个自由基结合，例如 Cl• + Cl• → Cl₂。
Mixtures can form due to further substitution. To favour monosubstitution, use an excess of the alkane. For IB/AQA, you must be able to write equations for each step with correct fish-hook arrows and identify initiation/propagation/termination steps.
由于可能发生进一步取代，产物常为混合物。为利于单取代，使用过量烷烃。对于 IB/AQA，你必须能用正确的鱼钩箭头写出各步方程式，并识别引发、增长与终止步骤。
5. Electrophilic Addition in Alkenes | 烯烃的亲电加成
Alkenes undergo electrophilic addition because the π bond is an electron‑rich region that attracts electrophiles. Mechanism: the electrophile (e.g. H⁺ from HBr) accepts the π electrons, forming a carbocation intermediate. The carbocation rapidly combines with the nucleophile (Br⁻) to give the addition product.
烯烃因 π 键电子云丰富会吸引亲电试剂而发生亲电加成。机理：亲电试剂（例如 HBr 中的 H⁺）接受 π 电子，形成碳正离子中间体。碳正离子迅速与亲核试剂（Br⁻）结合得到加成产物。
For unsymmetrical alkenes, Markovnikov’s rule predicts that the more stable carbocation (tertiary > secondary > primary) forms preferentially. This leads to the major product where the hydrogen attaches to the carbon with more hydrogens initially. The rule arises from carbocation stability: alkyl groups donate electron density, stabilising the positive charge.
对于不对称烯烃，马尔科夫尼科夫规则预测更稳定的碳正离子（叔碳 > 仲碳 > 伯碳）优先形成，从而得到主要产物，其中氢加到初始氢较多的碳上。该规则源于碳正离子稳定性：烷基供电子，稳定正电荷。
At AQA, you will also see addition reactions with Br₂ (bromine water decolourises), H₂SO₄ and interhalogens. Always draw the intermediate carbocation and use curly arrows to show the breaking of the π bond and formation of the new C–X bond.
在 AQA 中，你还会看到与 Br₂（溴水褪色）、H₂SO₄ 及卤间化合物的加成反应。始终画出中间体碳正离子，并用卷曲箭头表示 π 键断裂和新 C–X 键的形成。
6. Nucleophilic Substitution (SN1 and SN2) | 亲核取代（SN1 与 SN2）
Nucleophilic substitution replaces a leaving group on a saturated carbon with a nucleophile. Two distinct mechanisms exist:
亲核取代是用亲核试剂替换饱和碳上的离去基团。存在两种截然不同的机理：
SN2 (bimolecular nucleophilic substitution): The nucleophile attacks from the back side of the C–L bond in a single, concerted step. This leads to inversion of configuration (Walden inversion). Rate = k [RX][Nu]. It is favoured by primary haloalkanes, strong nucleophiles and aprotic solvents.
SN2（双分子亲核取代）： 亲核试剂从 C–L 键背面进攻，一步协同完成，导致构型翻转（瓦尔登翻转）。速率 = k [RX][Nu]。伯卤代烷、强亲核试剂与非质子溶剂有利于 SN2。
SN1 (unimolecular nucleophilic substitution): The leaving group departs first, forming a planar carbocation intermediate. The nucleophile then attacks from either side, leading to racemisation. Rate = k [RX] only. Favoured by tertiary haloalkanes, weak nucleophiles and protic solvents.
SN1（单分子亲核取代）： 离去基团先离去，形成平面碳正离子中间体，然后亲核试剂可从任一侧进攻，导致外消旋化。速率 = k [RX]。叔卤代烷、弱亲核试剂与质子溶剂有利于 SN1。
IB expects you to draw the transition state for SN2 (dashed lines for partially formed/broken bonds) and the carbocation intermediate for SN1 with an empty p orbital.
IB 要求画出 SN2 的过渡态（用虚线表示部分形成/断裂的键）以及 SN1 中带有空 p 轨道的碳正离子中间体。
7. Factors Affecting SN1 vs SN2 | 影响 SN1 与 SN2 的因素
Substrate structure: primary alkyl halides strongly favour SN2; tertiary exclusively SN1; secondary can go either way, depending on nucleophile and solvent.
底物结构：伯卤代烷强烈倾向 SN2；叔卤代烷仅发生 SN1；仲卤代烷视亲核试剂与溶剂而定。
Nucleophile strength: strong, charged nucleophiles (e.g. OH⁻, CN⁻) promote SN2; weak neutral nucleophiles (e.g. H₂O) make SN1 more likely.
亲核试剂强度：强带电荷亲核试剂（如 OH⁻、CN⁻）促进 SN2；弱中性亲核试剂（如 H₂O）使 SN1 更可能发生。
Leaving group ability: good leaving groups (weak bases like I⁻, Br⁻, tosylate) stabilise the transition state for both mechanisms, but especially help SN1.
离去基团能力：好的离去基团（弱碱如 I⁻、Br⁻、对甲苯磺酸根）能稳定两种机理的过渡态，但尤其有助于 SN1。
Solvent: polar aprotic solvents (propanone, DMSO) solvate cations well, leaving nucleophiles ‘naked’ and highly reactive for SN2. Polar protic solvents (water, alcohols) stabilise carbocations and favour SN1.
溶剂：极性非质子溶剂（丙酮、DMSO）能很好地溶剂化阳离子，使亲核试剂“裸露”并具有高反应性，利于 SN2。极性质子溶剂（水、醇）稳定碳正离子，利于 SN1。
8. Elimination Reactions (E1 and E2) | 消除反应（E1 与 E2）
Elimination removes a leaving group and a β‑hydrogen to form an alkene. E2 is a concerted, bimolecular process where a strong base abstracts a proton while the leaving group departs, and the double bond forms. Rate = k [RX][base]. Anti-periplanar geometry (H and leaving group at 180°) is required for efficient orbital overlap.
消除反应移除离去基团和一个 β-氢形成烯烃。E2 是协同的双分子过程，强碱夺取质子同时离去基团离去，双键形成。速率 = k [RX][碱]。需要反式共平面构象（氢与离去基团呈 180°）以实现有效的轨道重叠。
E1 proceeds via a carbocation intermediate just like SN1, followed by loss of a β‑proton. Rate = k [RX]. It gives mixtures with SN1 products and favours tertiary substrates.
E1 类似 SN1，经由碳正离子中间体进行，随后失去 β-质子。速率 = k [RX]。它与 SN1 产物形成混合物，并有利于叔底物。
Zaitsev’s rule states that the more substituted, more stable alkene is the major product in elimination, unless the base is sterically hindered (e.g. t‑butoxide favours Hofmann product).
扎伊采夫规则指出，更取代、更稳定的烯烃是消除反应的主要产物，除非碱受到空间位阻（例如叔丁氧基则倾向于霍夫曼产物）。
9. Electrophilic Substitution in Benzene | 苯的亲电取代
Benzene’s delocalised π‑system makes it resistant to addition; instead it undergoes electrophilic substitution. A reactive electrophile (e.g. NO₂⁺ from HNO₃/H₂SO₄) attacks the ring, forming a positively charged intermediate (Wheland complex). The intermediate then loses a proton to restore aromaticity.
苯的离域 π 体系使其难发生加成反应，而进行亲电取代。活性亲电试剂（例如由 HNO₃/H₂SO₄ 产生的 NO₂⁺）进攻苯环，形成带正电的中间体（惠兰德络合物）。然后中间体失去一个质子恢复芳香性。
Common reactions include nitration, halogenation (with FeBr₃ or AlCl₃ catalyst), Friedel–Crafts alkylation and acylation. For AQA/IB, you must draw the mechanism with curly arrows showing the electrophile attacking the π‑cloud and the loss of H⁺. Remember that the catalyst regenerates.
常见反应包括硝化、卤化（用 FeBr₃ 或 AlCl₃ 催化剂）、傅克烷基化与酰基化。对 AQA/IB，你必须画出用卷曲箭头表示亲电试剂进攻 π 电子云并失去 H⁺ 的机理。注意催化剂会再生。
10. Reaction Profiles and Energy Diagrams | 反应进程与能量图
Energy profile diagrams depict the enthalpy changes and activation energies of a mechanism. For a two‑step SN1 mechanism, there are two energy barriers separated by a valley where the carbocation intermediate sits. The step with the highest activation energy is the rate‑determining step.
能量曲线图描绘了机理的焓变与活化能。对于两步 SN1 机理，有两个能垒被一个谷形区域隔开，碳正离子中间体就位于谷底。活化能最高的一步是决速步。
For concerted mechanisms (SN2, E2), there is a single transition state and a single energy peak. Transition state structures have partially formed and broken bonds, often denoted with dotted lines. They cannot be isolated, unlike intermediates.
对于协同机理（SN2、E2），只有一个过渡态和一个能量峰。过渡态结构中键部分形成、部分断裂，通常用虚线表示。它们不能被分离，不同于中间体。
Knowing how to sketch these diagrams and label ΔH, Eₐ, intermediates and transition states is crucial for exam success.
掌握如何画出这些图并标注 ΔH、Eₐ、中间体与过渡态对考试成功至关重要。
11. Intermediates vs Transition States | 中间体与过渡态
An intermediate is a stable species residing in a local energy minimum on the reaction coordinate; it has a finite lifetime and can sometimes be detected spectroscopically. Carbocations, radicals and the Wheland complex are intermediates.
中间体是位于反应坐标上局部能量最低点的稳定物种；它具有有限的寿命，有时可通过光谱检测。碳正离子、自由基和惠兰德络合物都是中间体。
A transition state is the highest‑energy structure along the reaction coordinate, existing only at the peak of an energy barrier. It represents an unstable arrangement of atoms where bonds are partially broken and formed. Transition states have no measurable lifetime.
过渡态是沿反应坐标能量最高的结构，仅存在于能垒顶峰。它代表原子排布不稳定的状态，键部分断裂和部分形成。过渡态没有可测量的寿命。
| Property | Intermediate | Transition State |
|---|---|---|
| Energy level | Local minimum | Maximum |
| Lifetime | Can be isolated/detected | Cannot be isolated |
| Bonds | Complete bonds | Partially formed/broken |
12. Common Pitfalls and Exam Tips | 常见错误与应试技巧
Arrow direction: Always draw full curly arrows from an electron‑rich site (lone pair, π bond, negative charge) towards an electron‑deficient atom. Never start from a positive charge.
箭头方向： 始终从富电子位置（孤对电子、π 键、负电荷）向缺电子原子画全卷曲箭头。切勿从正电荷出发画箭头。
SN1/SN2 mix‑up: Check substrate classification carefully. Primary = SN2, tertiary = SN1/SN2 impossible. For secondary, consider the nucleophile and solvent.
SN1/SN2 混淆： 仔细检查底物分类。伯碳 = SN2，叔碳 = 不可能 SN2 只能 SN1。仲碳需考虑亲核试剂与溶剂。
Radical arrows: Use half‑headed fish‑hook arrows for single‑electron movements in radical mechanisms. A full arrow in a radical context will lose marks.
自由基箭头： 在自由基机理中使用半箭头（鱼钩箭头）表示单电子移动。在自由基环境下使用全箭头会失分。
Benzene mechanism: Never draw addition products for electrophilic substitution. The product is always a substituted benzene, not a cyclohexadiene.
苯的反应机理： 绝对不要为亲电取代画出加成产物。产物始终是取代苯，而非环己二烯。
Markovnikov vs anti‑Markovnikov: Apply Markovnikov to electrophilic addition of H–X to alkenes; for HBr with peroxides, the anti‑Markovnikov radical addition product forms – know the difference.
马氏规则与反马氏规则： 对 H–X 与烯烃的亲电加成应用马氏规则；对于过氧化物存在下的 HBr，则生成反马氏自由基加成产物——务必区分两者。
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