Electrophilic Addition Exam Focus | A-Level WJEC 化学：亲电加成考点精讲

📚 Electrophilic Addition Exam Focus | A-Level WJEC 化学：亲电加成考点精讲

Electrophilic addition is the characteristic reaction of alkenes, underpinning much of organic synthesis and testing your understanding of reaction mechanisms, carbocation stability, and regioselectivity. In the WJEC A-Level Chemistry specification, you must be able to describe the mechanism for the addition of hydrogen halides, halogens, sulfuric acid, and water, and apply Markovnikov’s rule to predict the major product from an unsymmetrical alkene. This article provides a comprehensive breakdown of the key concepts, common pitfalls, and exam-ready explanations, all presented in bilingual English–Chinese format to strengthen both your knowledge and your ability to express it precisely.

亲电加成是烯烃的特征反应，构成了大量有机合成的基础，并考查你对反应机理、碳正离子稳定性及区域选择性的理解。在 WJEC A-Level 化学大纲中，你必须能够描述卤化氢、卤素、硫酸和水的加成机理，并运用马氏规则预测不对称烯烃的主要产物。本文全面梳理核心概念、常见雷区与应试要点，用中英双语呈现，帮助你既巩固知识，又能精准表达。

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

Electrophilic addition is a reaction in which an electron‑deficient species (an electrophile) is attracted to an electron‑rich multiple bond, typically the C=C double bond of an alkene. The π‑bond breaks, and two new σ‑bonds are formed as the electrophile and a nucleophile add to the carbon atoms. This reaction type is favoured by the high electron density of the π‑bond above and below the plane of the molecule.

亲电加成是指缺电子的物种（亲电试剂）被富电子的重键所吸引，通常为烯烃的 C=C 双键。π 键断裂，亲电试剂与亲核试剂分别加到两个碳原子上，形成两个新的 σ 键。由于 π 键在分子平面上方和下方具有高电子密度，该反应类型容易发生。

Alkenes undergo electrophilic addition because the π‑electrons are exposed and loosely held, making them susceptible to attack by species that seek electrons. The general outcome is the conversion of an unsaturated molecule into a saturated one, often introducing functional groups such as halo, hydroxyl, or sulfate groups.

烯烃能够发生亲电加成，是因为 π 电子裸露且束缚较弱，容易受到亲电试剂的进攻。通常结果是将不饱和分子转变为饱和分子，并引入卤素、羟基或硫酸酯基等官能团。

2. The Electrophilic Addition Mechanism | 亲电加成机理

The mechanism proceeds in two steps. First, the electrophile (e.g. H⁺ from HBr) accepts a pair of π‑electrons and forms a covalent bond to one of the alkene carbons. This creates a carbocation on the adjacent carbon – a positively charged intermediate. The second step is a fast reaction between the carbocation and a nucleophile (e.g. Br⁻) to give the final addition product.

该机理分为两步。首先，亲电试剂（如 HBr 中的 H⁺）接受一对 π 电子，并与其中一个烯烃碳原子形成共价键，从而使相邻碳上生成碳正离子——一个带正电的中间体。第二步是碳正离子与亲核试剂（如 Br⁻）之间的快速反应，生成最终的加成产物。

In the WJEC exam, you must draw curly arrows correctly: a curly arrow from the C=C bond to the electrophile (e.g. H of HBr), then a curly arrow from the H–Br bond to the Br to show heterolytic fission, producing Br⁻. Then, a curly arrow from the Br⁻ lone pair to the positively charged carbon. It is essential to show the positive charge on the carbocation and the movement of electron pairs.

在 WJEC 考试中，你必须正确画出弯箭头：从 C=C 键指向亲电试剂（如 HBr 的 H）的弯箭头，然后从 H–Br 键指向 Br 的弯箭头表示异裂，生成 Br⁻。接着，从 Br⁻ 的孤对电子指向带正电的碳原子。务必标出碳正离子的正电荷以及电子对的移动。

The rate‑determining step is the formation of the carbocation, which is slow because it requires heterolytic bond breaking and charge separation. The subsequent attack by the nucleophile is rapid.

决速步是碳正离子的生成，这一步因需要异裂和电荷分离而较慢。随后的亲核进攻则是快速步骤。

3. Addition of Hydrogen Halides (HX) | 卤化氢（HX）的加成

Hydrogen halides (HCl, HBr, HI) add across the C=C double bond to form a haloalkane. The reaction is performed at room temperature with the alkene gas bubbled into a solution of the hydrogen halide. The H–X bond undergoes heterolytic fission, generating H⁺ as the electrophile and X⁻ as the nucleophile.

卤化氢（HCl，HBr，HI）可加成到 C=C 双键上，生成卤代烷烃。反应在室温下进行，将烯烃气体通入卤化氢溶液中。H–X 键发生异裂，产生亲电的 H⁺ 与亲核的 X⁻。

With symmetrical alkenes such as ethene, only one product is possible: CH₂=CH₂ + HBr → CH₃CH₂Br. However, with unsymmetrical alkenes like propene, two possible products can arise, and regioselectivity is determined by carbocation stability – a concept governed by Markovnikov’s rule.

对于乙烯等对称烯烃，只能得到一种产物：CH₂=CH₂ + HBr → CH₃CH₂Br。但对于丙烯等不对称烯烃，可能生成两种产物，区域选择性由碳正离子稳定性决定——这一概念由马氏规则支配。

4. Markovnikov’s Rule | 马氏规则

Markovnikov’s rule states that in the addition of HX to an unsymmetrical alkene, the hydrogen atom attaches to the carbon that already has the greater number of hydrogen atoms, while the halogen attaches to the more substituted carbon (the one with more C–C bonds). This leads to the more stable carbocation intermediate being formed preferentially.

马氏规则指出：在 HX 与不对称烯烃的加成中，氢原子加在原本含氢较多的碳上，而卤素则加在取代程度更高（C–C 键更多）的碳上。这使得优先生成更稳定的碳正离子中间体。

The rule is a consequence of carbocation stability: tertiary (3°) carbocations are more stable than secondary (2°), which are more stable than primary (1°). Therefore, the transition state leading to the more stable carbocation is lower in energy, and that product predominates. The major product is known as the Markovnikov product.

该规则是碳正离子稳定性的结果：叔（3°）碳正离子比仲（2°）稳定，仲碳正离子又比伯（1°）稳定。因此，通向更稳定碳正离子的过渡态能量更低，该产物占优势，称为马氏产物。

Example: propene + HBr → 2‑bromopropane (major) via a 2° carbocation, not 1‑bromopropane (minor) via a 1° carbocation.

示例：丙烯 + HBr → 2‑溴丙烷（主要），经由 2° 碳正离子；而非经由 1° 碳正离子的 1‑溴丙烷（次要）。

5. Carbocation Stability | 碳正离子的稳定性

Carbocations are classified by the number of carbon atoms directly bonded to the charged carbon: primary (1°) has one C–C bond, secondary (2°) has two, and tertiary (3°) has three. Stability increases with substitution because alkyl groups release electron density through the inductive effect and hyperconjugation, helping to disperse the positive charge.

碳正离子按直接连接在带电碳上的碳原子数分类：伯（1°）有一个 C–C 键，仲（2°）有两个，叔（3°）有三个。稳定性随取代程度增加而升高，因为烷基可通过诱导效应和超共轭效应释放电子密度，帮助分散正电荷。

The order of stability is: (CH₃)₃C⁺ > (CH₃)₂CH⁺ > CH₃CH₂⁺ > CH₃⁺. In an exam, you must be able to identify the type of carbocation that would form from a given alkene and predict which pathway will dominate based on relative stability.

稳定性顺序为：(CH₃)₃C⁺ > (CH₃)₂CH⁺ > CH₃CH₂⁺ > CH₃⁺。在考试中，你必须能识别给定烯烃可能生成的碳正离子类型，并根据相对稳定性预测哪种途径占主导。

6. Inductive Effects and Hyperconjugation | 诱导效应与超共轭

The inductive effect arises because alkyl groups are slightly electron‑releasing compared to hydrogen. They push electron density towards the positively charged carbon, thereby reducing the density of positive charge and stabilising the ion. More alkyl groups lead to a greater stabilising effect.

诱导效应源于烷基相对于氢具有轻微给电子性，它们将电子密度推向带正电的碳，从而降低正电荷密度，使离子稳定。烷基越多，稳定效应越强。

Hyperconjugation involves the delocalisation of electrons from the σ‑bonds of adjacent C–H or C–C bonds into the empty p‑orbital of the carbocation. This overlap allows the positive charge to be spread over several atoms, further lowering the energy of the carbocation. This effect is particularly significant in tertiary carbocations with nine potential C–H σ‑bonds for hyperconjugation.

超共轭涉及相邻 C–H 或 C–C 键的 σ 电子离域到碳正离子的空 p 轨道中。这种重叠使正电荷分布到多个原子上，进一步降低碳正离子的能量。对于有九个潜在 C–H σ 键的叔碳正离子，该效应尤为显著。

In WJEC answers, you can state: ‘The tertiary carbocation is more stable than the secondary due to the greater inductive effect and more hyperconjugation from the surrounding alkyl groups.’

在 WJEC 答案中，你可以表述：“叔碳正离子比仲碳正离子更稳定，因为周围烷基的诱导效应更强，且超共轭作用更多。”

7. Addition of Halogens (Br₂, Cl₂) | 卤素（Br₂, Cl₂）的加成

When bromine or chlorine is added to an alkene at room temperature, the halogen molecule acts as the electrophile. The π‑electrons of the alkene induce a dipole in the approaching halogen molecule, causing the nearer halogen atom to acquire a partial positive charge. The reaction proceeds through a cyclic bromonium (or chloronium) ion intermediate, not a free carbocation.

在室温下将溴或氯加到烯烃中时，卤素分子作为亲电试剂。烯烃的 π 电子诱导接近的卤素分子产生偶极，使较近的卤素原子带上部分正电荷。反应通过一个环状的溴鎓（或氯鎓）离子中间体进行，而非生成游离的碳正离子。

The mechanism: A curly arrow from C=C to the partially positive halogen, and from the halogen–halogen bond to the other halogen to break it. This forms a three‑membered ring with a positive charge on the halogen. Then, the halide ion attacks from the opposite side (anti‑addition), opening the ring and giving the dihaloalkane with trans stereochemistry.

机理：从 C=C 到部分带正电的卤素的弯箭头，以及从卤素‑卤素键指向另一卤素原子使其断裂的弯箭头，形成含正电荷的三元环。然后卤离子从相反一侧进攻（反式加成），开环得到立体化学为反式的二卤代烷。

This anti‑addition is an important stereochemical outcome. If the alkene is cyclic, the two halogen atoms will be added to opposite faces of the ring, producing the trans isomer exclusively.

这种反式加成是重要的立体化学结果。如果烯烃是环状的，两个卤素原子将加在环的异侧，专一地生成反式异构体。

8. Addition of Water – Acid‑Catalysed Hydration | 水的加成——酸催化水合

Alkenes can undergo hydration in the presence of a strong acid catalyst (typically concentrated sulfuric acid followed by water, or directly with steam and a phosphoric acid catalyst in industrial ethanol production). The mechanism is electrophilic addition, with the proton H⁺ acting as the electrophile.

烯烃在强酸催化下可发生水合（工业上常用浓硫酸后加水，或直接用蒸汽与磷酸催化剂制乙醇）。机理是亲电加成，质子 H⁺ 充当亲电试剂。

Step 1: Protonation of the alkene gives the most stable carbocation. Step 2: Water acts as a nucleophile, attacking the carbocation and forming a protonated alcohol. Step 3: Loss of H⁺ reforms the acid catalyst and yields the alcohol. The overall addition follows Markovnikov’s rule, so unsymmetrical alkenes give the more substituted alcohol.

第一步：烯烃的质子化生成最稳定的碳正离子。第二步：水作为亲核试剂进攻碳正离子，形成质子化的醇。第三步：脱去 H⁺ 再生酸催化剂，同时得到醇。总加成遵循马氏规则，因而不对称烯烃生成取代程度更高的醇。

For example, propene + H₂O (H⁺) → propan‑2‑ol (major), not propan‑1‑ol. This is a key distinction that builds on your understanding of carbocation stability.

例如，丙烯 + H₂O (H⁺) → 丙‑2‑醇（主要），而非丙‑1‑醇。这是基于碳正离子稳定性理解上的一个关键鉴别点。

9. Reaction with Sulfuric Acid | 与硫酸的反应

Cold concentrated sulfuric acid reacts with alkenes by electrophilic addition to form alkyl hydrogen sulfates. The electrophile is generated from H₂SO₄: an O–H bond breaks heterolytically to give H⁺ and HSO₄⁻. The proton adds to the alkene forming a carbocation, which is then attacked by the HSO₄⁻ ion.

冷的浓硫酸与烯烃发生亲电加成，生成硫酸氢烷基酯。亲电试剂由 H₂SO₄ 异裂产生：一个 O–H 键断裂产生 H⁺ 和 HSO₄⁻。质子加到烯烃上形成碳正离子，随后被 HSO₄⁻ 离子进攻。

The product is an alkyl hydrogen sulfate, e.g. ethene + H₂SO₄ → ethyl hydrogen sulfate (CH₃CH₂OSO₃H). This reaction is important because subsequent hydrolysis with water and heating produces the corresponding alcohol, making it a two‑step route for hydration.

产物为硫酸氢烷基酯，如乙烯 + H₂SO₄ → 硫酸氢乙酯 (CH₃CH₂OSO₃H)。该反应很重要，因为随后加水加热水解可得到相应的醇，这构成了一种两步水合路线。

With unsymmetrical alkenes, Markovnikov’s rule again applies: the hydrogen adds to the carbon with more hydrogens, and the HSO₄⁻ group attaches to the more substituted carbon. This intermediate can then be hydrolysed to give the Markovnikov alcohol.

对于不对称烯烃，马氏规则同样适用：氢加在含氢较多的碳上，HSO₄⁻ 基团连在取代程度更高的碳上。该中间体随后可水解脱去，得到马氏醇。

10. Stereochemical Considerations | 立体化学考量

Electrophilic addition of HX can produce a racemic mixture if the carbocation intermediate is planar and the nucleophile can attack from either side with equal probability. In the case of unsymmetrical alkenes where the product contains a chiral centre, a 50:50 mixture of enantiomers is obtained. This is because the trigonal planar carbocation can be attacked from above or below with no preference.

HX 的亲电加成若涉及平面型碳正离子中间体，且亲核试剂可从两侧等概率进攻，则可能得到外消旋混合物。对于产物含有手性中心的不对称烯烃，会得到对映异构体的 50:50 混合物，这是因为平面三角形的碳正离子可以从上方或下方不加区分地受到进攻。

For halogen addition (Br₂), the anti‑addition gives rise to specific stereochemistry. For example, addition to cyclopentene produces trans‑1,2‑dibromocyclopentane exclusively. With acyclic alkenes, such as trans‑2‑butene, the addition of Br₂ gives (2R,3S)‑2,3‑dibromobutane, a meso‑compound. This type of reasoning is common in WJEC structural and stereochemical questions.

卤素（Br₂）加成的反式加成导致特定的立体化学。例如，对环戊烯加成专一生成反式‑1,2‑二溴环戊烷。对于开链烯烃，如反式‑2‑丁烯，Br₂ 加成得到 (2R,3S)‑2,3‑二溴丁烷，即内消旋化合物。这类推理在 WJEC 的结构与立体化学试题中很常见。

11. Common Mistakes and How to Avoid Them | 常见错误与避免方法

One of the most frequent errors is misapplying Markovnikov’s rule by placing the H on the carbon with more alkyl groups. Remember: H goes to the carbon that already has more H atoms, i.e. the less substituted carbon. Another error is forgetting to show heterolytic fission of the electrophile in the mechanism diagram – always draw the curly arrow breaking the H–X or X–X bond away from the atom that becomes the nucleophile.

最常见的错误之一是错误地运用马氏规则，把 H 加在烷基更多的碳上。记住：H 加在原本含 H 较多的碳上，即取代度较低的碳。另一错误是在机理图中忘记画出亲电试剂的异裂——一定要画出从 H–X 或 X–X 键断裂指向要成为亲核试剂的原子的弯箭头。

When dealing with carbocations, never draw a primary carbocation as the major intermediate if there is a possibility of rearrangement to a more stable secondary or tertiary carbocation. However, in simple addition of HX, the carbocation forms where the alkene is protonated – WJEC at A‑level does not typically require carbocation rearrangements unless explicitly indicated, but it is good to be aware.

在涉及碳正离子时，如果可能重排为更稳定的仲或叔碳正离子，千万不要把伯碳正离子作为主要中间体画出。然而，在简单的 HX 加成中，碳正离子在烯烃质子化处生成——WJEC 在 A‑level 阶段一般不要求碳正离子重排，除非明确提示，但了解这一点有益无害。

Finally, in halogen addition, do not draw a free carbocation. The intermediate is a halonium ion, and you must show the ring structure and anti‑addition stereochemistry when asked. Practising curly‑arrow drawings under timed conditions is essential for fluency.

最后，在卤素加成中，不要画出游离的碳正离子。中间体是卤鎓离子，必须展示环状结构，并在要求时表达反式加成立体化学。在限时条件下练习弯箭头画法对熟练度至关重要。

12. Summary of Key Points | 考点总结

Electrophilic addition of alkenes involves the initial attack of an electrophile on the π‑bond, formation of a carbocation or halonium ion, and subsequent nucleophilic attack. Markovnikov’s rule allows prediction of the major product based on carbocation stability: tertiary > secondary > primary. The mechanisms for HX, H₂O (H⁺), H₂SO₄, and X₂ additions must be known both as curly‑arrow schemes and as stepwise descriptions.

烯烃的亲电加成包括亲电试剂首先进攻 π 键、生成碳正离子或卤鎓离子、随后亲核进攻。马氏规则可根据碳正离子稳定性（叔 > 仲 > 伯）预测主要产物。HX、H₂O (H⁺)、H₂SO₄ 和 X₂ 加成的机理必须以弯箭头图和逐步描述两种方式掌握。

Addition Reaction	Electrophile	Intermediate	Regioselectivity	Stereochemistry
HX	H⁺	Carbocation	Markovnikov	Racemic if chiral C formed
X₂ (Br₂, Cl₂)	X⁺ (partial positive on X)	Halonium ion	N/A (both C attacked)	Anti‑addition
H₂O (H⁺)	H⁺	Carbocation	Markovnikov	Racemic if chiral C formed
H₂SO₄	H⁺	Carbocation	Markovnikov	Racemic if chiral C formed

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Electrophilic Addition Exam Focus | A-Level WJEC 化学：亲电加成 考点精讲