Electrophilic Addition in IB Chemistry | IB化学亲电加成考点精讲

📚 Electrophilic Addition in IB Chemistry | IB化学亲电加成考点精讲

Electrophilic addition is the characteristic reaction of alkenes and other unsaturated hydrocarbons. In IB Chemistry, understanding the stepwise mechanism, the role of the electrophile, Markovnikov’s rule, and carbocation stability is essential for both Paper 1 and Paper 2. This article breaks down every key aspect of electrophilic addition with clear explanations, reaction schemes, and strategic tips to help you master the topic.

亲电加成是烯烃和其他不饱和烃的特征反应。在 IB 化学中，理解分步机理、亲电试剂的角色、马氏规则以及碳正离子稳定性，对于 Paper 1 和 Paper 2 都至关重要。本文通过清晰的解释、反应通式和备考策略，逐一剖析亲电加成的每个核心要点，助你彻底掌握该考点。

1. What is Electrophilic Addition? | 什么是亲电加成？

Electrophilic addition is a reaction in which a pi bond in an alkene (or alkyne) is broken and two new sigma bonds are formed by the addition of an electrophile and a nucleophile across the double bond. The alkene acts as a nucleophile because of the high electron density in the carbon-carbon double bond. The reaction is spontaneous and exothermic as one pi bond and one sigma bond of the reagent are replaced by two stronger sigma bonds.

亲电加成是指烯烃（或炔烃）中的π键断裂，亲电试剂与亲核试剂加合到双键两端，生成两个新的σ键的反应。烯烃由于碳碳双键的高电子密度，起到亲核试剂的作用。该反应通常为放热反应，因为一个π键和一个试剂中的σ键被两个更强的σ键所取代，整体能量降低。

General reaction:

C=C + X-Y → X-C-C-Y

. X-Y represents the adding molecule where X is the electrophilic part and Y the nucleophilic part.

通式：

C=C + X-Y → X-C-C-Y

，X-Y 代表加成分子，X 为亲电部分，Y 为亲核部分。

2. The Stepwise Mechanism | 分步反应机理

The mechanism proceeds in two main steps. First, the pi electrons attack the electrophile, forming a carbocation intermediate and releasing the leaving group (often a halide ion or the conjugate base). This is the slow, rate-determining step. Second, the nucleophile rapidly attacks the carbocation to form the final saturated product. Understanding this order is crucial because it explains both regioselectivity (Markovnikov’s rule) and stereochemical outcomes.

反应机理主要分为两步。第一步，π电子进攻亲电试剂，生成碳正离子中间体，同时离去基团（常见为卤离子或共轭碱）离去。这是慢反应，是速率控制步骤。第二步，亲核试剂迅速进攻碳正离子，形成最终的饱和产物。理解这一顺序至关重要，因为它既能解释区域选择性（马氏规则），也能解释立体化学结果。

For addition of HBr to ethene: Step 1: C₂H₄ + H⁺ → C₂H₅⁺ (carbocation). Step 2: C₂H₅⁺ + Br⁻ → C₂H₅Br. The H⁺ is the electrophile; Br⁻ is the nucleophile.

以乙烯与 HBr 加成为例：第一步：C₂H₄ + H⁺ → C₂H₅⁺（碳正离子）；第二步：C₂H₅⁺ + Br⁻ → C₂H₅Br。H⁺ 为亲电试剂，Br⁻ 为亲核试剂。

3. Electrophiles in Common Addition Reactions | 常见亲电加成反应中的亲电试剂

In IB, you must identify the electrophile in each reaction. For hydrogen halides (HX) it is H⁺; for halogen addition it is the polarised halogen molecule where the δ⁺ end acts as the electrophile; for hydration with acid catalyst it is again H⁺. Sometimes the electrophile is generated in situ, such as in addition of HOBr (Br₂ + H₂O) where Br⁺ is the effective electrophile.

在 IB 考试中，你必须能识别每种反应中的亲电试剂。对氢卤酸 (HX) 加成，亲电试剂是 H⁺；对卤素加成，卤素分子极化后 δ⁺ 端充当亲电试剂；对酸催化水合反应，亲电试剂同样是 H⁺。有时亲电试剂是现场生成的，例如在 HOBr 加成反应中（Br₂ + H₂O），实际亲电试剂为 Br⁺。

Reaction	Electrophile	Typical Conditions
Addition of HX (X = Cl, Br, I)	H⁺	Room temp, gas or conc. aq.
Hydration (H₂O / H⁺)	H⁺	H₃PO₄ cat, 300 °C, 6 MPa
Halogenation (Br₂, Cl₂)	δ⁺ Br (polarised Br₂)	Inert solvent (CCl₄), dark
Halohydrin (Br₂ + H₂O)	Br⁺ (from Br₂)	Aqueous, cold

Recognising the electrophile helps you predict the first step and the regiochemistry. In halogens, the non-polar Br–Br bond becomes polarised as it approaches the electron-rich alkene, with the bromine nearer the pi cloud becoming electron-deficient.

识别亲电试剂有助于预测反应的第一步和区域选择性。对于卤素，非极性的 Br–Br 键在靠近富电子的烯烃时发生极化，更靠近π电子云的溴原子变得缺电子，成为亲电中心。

4. Addition of Hydrogen Halides (HX) | 氢卤酸 (HX) 的加成

Hydrogen halides add across the double bond to give haloalkanes. For symmetrical alkenes like ethene, only one product is possible. For unsymmetrical alkenes like propene, two regioisomers can form, but one predominates according to Markovnikov’s rule.

氢卤酸与双键加成生成卤代烷。对称烯烃如乙烯只有一种产物；不对称烯烃如丙烯可能生成两种区域异构体，但根据马氏规则其中一种为主产物。

Example: propene + HBr → 2-bromopropane (major) + 1-bromopropane (minor). The H⁺ adds to the less substituted carbon of the double bond to give the more stable secondary carbocation, which then captures Br⁻.

例：丙烯 + HBr → 2-溴丙烷（主产物） + 1-溴丙烷（次产物）。H⁺ 加合到双键上取代较少的碳上，生成更稳定的仲碳正离子，随后与 Br⁻ 结合。

Rate of reaction: HI > HBr > HCl > HF. This order follows the strength of the hydrogen-halogen bond; weaker bonds react faster. IB may ask you to compare reactivity using bond enthalpy.

反应速率：HI > HBr > HCl > HF。这一顺序与氢-卤键键能相关，键越弱反应越快。IB 可能要求你运用键焓比较反应活性。

5. Markovnikov’s Rule Explained | 马氏规则解析

Markovnikov’s rule states that in the addition of HX to an unsymmetrical alkene, the hydrogen atom adds to the carbon that already has more hydrogen atoms, and the halide adds to the carbon with fewer hydrogen atoms. Put simply: “the rich get richer” – the carbon richer in hydrogen gets another hydrogen.

马氏规则指出，在 HX 与不对称烯烃的加成中，氢原子加合到含氢较多的碳原子上，卤素加到含氢较少的碳原子上。简言之，“富者愈富”——氢越多的碳越容易再加一个氢。

The modern mechanistic interpretation: the regioselectivity is controlled by carbocation stability. The reaction proceeds via the most stable carbocation intermediate. The order of carbocation stability is tertiary (3°) > secondary (2°) > primary (1°) > methyl. Therefore, the hydrogen adds to the carbon that leads to the more stable carbocation, which is usually the less substituted carbon.

现代机理解释：区域选择性受碳正离子稳定性控制。反应途经最稳定的碳正离子中间体。碳正离子稳定性次序为：叔碳正离子 (3°) > 仲碳正离子 (2°) > 伯碳正离子 (1°) > 甲基碳正离子。因此，氢加合到能生成更稳定碳正离子的碳上，通常是取代较少的碳。

Inductive effect: alkyl groups are electron-donating, which spreads the positive charge and stabilises the carbocation. More alkyl groups attached to the positively charged carbon mean greater stability.

诱导效应：烷基是给电子基团，能够分散正电荷，从而稳定碳正离子。连接在带正电碳上的烷基越多，碳正离子越稳定。

6. Carbocation Rearrangements | 碳正离子重排

In some reactions, the initially formed carbocation may not be the most stable one, and a rearrangement can occur via a 1,2-hydride shift or a 1,2-alkyl shift. This produces unexpected products. IB questions sometimes ask you to predict the product when a rearrangement is possible, e.g. addition of HBr to 3-methyl-1-butene. The initial secondary carbocation can rearrange to a more stable tertiary carbocation before the nucleophile attacks.

在某些反应中，最初生成的碳正离子可能不是最稳定的，可能会经由 1,2-氢迁移或 1,2-烷基迁移发生重排，从而生成意料之外的产物。IB 有时会考查重排产物的预测，如 HBr 与 3-甲基-1-丁烯加成，初始的仲碳正离子会重排成更稳定的叔碳正离子，再与亲核试剂反应。

Example: 3-methyl-1-butene + HCl → 2-chloro-2-methylbutane (major, via tertiary cation) rather than 2-chloro-3-methylbutane. You should be able to draw the curved arrow mechanism for the hydride shift.

例：3-甲基-1-丁烯 + HCl → 2-氯-2-甲基丁烷（主产物，经由叔碳正离子），而非 2-氯-3-甲基丁烷。你需要会画氢迁移的弯箭头机理。

Rearrangements only occur if they lead to a more stable carbocation. If the initial carbocation is already tertiary, no rearrangement is expected.

只有当重排能生成更稳定的碳正离子时，重排才会发生。如果初始碳正离子已经是叔碳正离子，则通常不会重排。

7. Acid-Catalysed Hydration of Alkenes | 烯烃的酸催化水合反应

Water can add across the double bond in the presence of an acid catalyst (usually concentrated phosphoric acid, H₃PO₄) to give alcohols. This is an industrial method for producing ethanol from ethene. The mechanism again follows Markovnikov addition: H⁺ adds to the less substituted carbon to give the more stable carbocation, then water attacks as a nucleophile, and finally deprotonation yields the alcohol.

在酸催化（通常用浓磷酸 H₃PO₄）下，水可以加成到双键上生成醇。这是乙烯工业生产乙醇的方法。机理同样遵守马氏加成规则：H⁺ 首先加到取代较少的碳上生成较稳定的碳正离子，然后水作为亲核试剂进攻，最后去质子化得到醇。

Conditions: 300 °C, 6 MPa, conc. H₃PO₄ on a solid support. For asymmetric alkenes, the -OH group attaches to the more substituted carbon. Example: propene → propan-2-ol (major), not propan-1-ol.

反应条件：300 °C、6 MPa，浓磷酸负载在固态载体上。对于不对称烯烃，-OH 基加合到取代较多的碳上。例如丙烯水合主要生成丙-2-醇，而非丙-1-醇。

This reaction is reversible; the reverse is alcohol dehydration to alkenes, which proceeds via E1 mechanism. Exam tip: pay attention to arrows in the mechanism – protonation of the alcohol product is not the final step, deprotonation regenerates the catalyst.

该反应是可逆的，逆反应为醇脱水生成烯烃，通过 E1 机理进行。备考提示：机理箭头中注意，最终步不是醇产物的质子化，而是去质子化再生催化剂。

8. Halogen Addition: Br₂ and Cl₂ | 卤素加成：Br₂ 与 Cl₂

Addition of bromine or chlorine to an alkene is a classic test for unsaturation. The orange-brown bromine water (or bromine in CCl₄) is decolourised as the bromine is consumed. The mechanism goes through a cyclic bromonium ion (or chloronium ion), not a free carbocation. This three-membered ring cation is formed by a backside attack of the other halogen atom. The resulting anti-addition gives trans stereochemistry.

烯烃与溴或氯的加成是检验不饱和键的经典方法。橙棕色的溴水（或溴的四氯化碳溶液）会因溴被消耗而褪色。该反应机理经过环状溴鎓离子（或氯鎓离子），而非游离碳正离子。这种三元环阳离子由另一个卤原子从背面进攻形成。最终发生反式加成，给出特定的立体化学产物。

For example, cyclopentene + Br₂ → trans-1,2-dibromocyclopentane only, not the cis isomer. You must be able to draw the bromonium ion intermediate and show the backside attack of Br⁻ in the second step.

例如，环戊烯 + Br₂ 只生成反-1,2-二溴环戊烷，得不到顺式异构体。你需要能画出溴鎓离子中间体，并展示第二步中 Br⁻ 的背面进攻。

The cyclic ion explains why halogen addition is stereospecific (anti). This contrasts with HX addition which can give mixtures if the carbocation is planar and attack occurs from both faces.

环状离子解释了卤素加成为何具有立体专一性（反式加成）。这与 HX 加成不同，后者碳正离子平面构型，可从两面进攻得到混合构型产物。

9. Halohydrin Formation | 卤代醇的生成

When bromine water is used instead of pure Br₂, water competes as a nucleophile, leading to a bromohydrin (halohydrin). The electrophile is still Br⁺, forming the bromonium ion. Then water attacks the more substituted carbon of the three-membered ring, where the positive charge is better stabilised. This is an example of regioselectivity: the nucleophile (H₂O) goes to the carbon that can better bear a partial positive charge.

当使用溴水而不是纯溴时，水作为亲核试剂参与竞争，产物为溴代醇（卤代醇）。亲电试剂仍是 Br⁺，形成溴鎓离子。然后水进攻三元环中取代较多的碳，因为该位置正电荷更稳定。这是区域选择性的一个例子：亲核试剂 (H₂O) 进攻能更好地承载部分正电荷的碳。

Example: propene + Br₂/H₂O → 1-bromopropan-2-ol (major), not 2-bromopropan-1-ol. The OH attaches to the more substituted carbon. This still follows Markovnikov-type orientation because the bromonium ring opens at the more substituted C due to greater partial positive character.

例：丙烯 + Br₂/H₂O → 1-溴丙-2-醇（主要产物），而非 2-溴丙-1-醇。OH 连接在取代较多的碳上。这依然遵循类马氏规则的取向，因为溴鎓环在取代较多的碳上开环，该处部分正电荷更强。

10. Catalytic Hydrogenation | 催化氢化

Hydrogenation is the addition of H₂ across a double bond in the presence of a metal catalyst (Ni, Pt, or Pd). This is technically an addition reaction, but it is not electrophilic addition by the same polar mechanism. It involves adsorption of both H₂ and the alkene onto the catalyst surface, weakening the H-H bond and the pi bond, leading to syn addition of both hydrogen atoms from the same side. IB expects you to know the conditions and that it is used to convert unsaturated oils to saturated fats.

催化氢化是在金属催化剂（Ni、Pt 或 Pd）存在下，H₂ 加合到双键上的反应。虽然这也是加成反应，但并非通过相同的极性亲电机理进行。氢化涉及 H₂ 和烯烃同时吸附在催化剂表面，削弱 H-H 键和π键，导致两个氢原子从同一侧同面加成。IB 要求你了解其反应条件，以及用于将不饱和油转变为饱和脂肪的应用。

Stereochemistry: hydrogenation gives syn addition, meaning both H atoms are added to the same face. This becomes important with cyclic alkenes or chiral products.

立体化学：氢化反应为同面加成，即两个氢原子加在同一面。这在环状烯烃或手性产物中变得重要。

11. Stereochemistry of Electrophilic Addition | 亲电加成的立体化学

The stereochemical outcome depends on the nature of the intermediate. For HX addition via planar carbocation, attack can occur from either face, leading to a racemic mixture if a chiral centre is formed. For halogen addition, the cyclic ion forces anti addition, so trans products are obtained from cyclic alkenes. For hydrogenation, syn addition dominates.

立体化学结果取决于中间体的性质。对于通过平面碳正离子的 HX 加成，亲核试剂可以从两面进攻，若生成手性中心则得到外消旋混合物。对于卤素加成，环状离子迫使反式加成，环状烯烃得到反式产物。对于氢化，则以同面加成为主。

IB questions often show a substituted alkene and ask you to draw the product(s) with correct stereochemistry. For example, (Z)-but-2-ene + Br₂ gives a mixture of enantiomers of 2,3-dibromobutane, both with threo configuration.

IB 试题常要求画出取代烯烃的产物并标注正确立体化学。例如，(Z)-丁-2-烯与 Br₂ 加成会得到 2,3-二溴丁烷的一对对映体混合物，均为苏式构型。

Pay attention to E/Z or cis/trans nomenclature in the starting material and use wedge/dash bonds to represent 3D structure in your answer.

注意起始物的 E/Z 或顺/反标记，并用楔形/虚线键表示三维结构。

12. Exam Tips and Common Pitfalls | 考试技巧与常见失分点

When writing mechanisms, always use full, single-barbed curly arrows for electron movement. Show the exact source and sink of electrons. For addition of HX, the arrow goes from the pi bond to the H atom, and another arrow from the H-X bond to the X to show bond breaking. Do not forget formal charges on the carbocation. In halogen addition, draw the bridged intermediate with the positive charge on the halogen. Use partial charges to show bond polarisation when necessary.

书写机理时，务必用完整的单钩弯箭头表示电子移动，明确显示电子的来源和去向。HX 加成中，箭头从π键出发进攻 H，另一个箭头从 H-X 键指向 X 表示断键。不要忘记在碳正离子上标注形式电荷。卤素加成中，画出桥环中间体，正电荷标在卤素上。必要时用部分电荷表示键的极化。

Common mistake: applying Markovnikov’s rule to halogen addition without considering the cyclic ion. Anti-Markovnikov addition of HBr can occur in the presence of peroxides (free radical mechanism), which is a different pathway – know the conditions and the radical initiator effect.

常见错误：在不考虑环状离子的情况下，直接对卤素加成套用马氏规则。有过氧化物存在时，HBr 可发生反马氏加成（自由基机理），这属于不同路径——需要了解其反应条件及自由基引发剂的影响。

Revise all conditions: H₂ + Ni/Pd/Pt for hydrogenation; HBr in the dark for ionic addition; Br₂ in CCl₄ or water for testing unsaturation; H₂O with H₃PO₄ at high T and P for hydration. Correlate rate data with carbocation stability to justify product distribution.

复习所有反应条件：氢化用 H₂ + Ni/Pd/Pt；离子型加成用 HBr 避光；检验不饱和键用 Br₂/CCl₄ 或溴水；水合反应需要 H₃PO₄ 在高温高压下进行。将速率数据与碳正离子稳定性相联系，以合理解释产物分布。

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Electrophilic Addition in IB Chemistry | IB化学 亲电加成 考点精讲