Electrophilic Addition in GCSE WJEC Chemistry | GCSE WJEC 化学：亲电加成考点精讲

📚 Electrophilic Addition in GCSE WJEC Chemistry | GCSE WJEC 化学：亲电加成考点精讲

Electrophilic addition is the characteristic reaction of alkenes, the unsaturated hydrocarbons containing a carbon-carbon double bond. In this topic for GCSE WJEC Chemistry, you need to understand why alkenes are more reactive than alkanes, be able to describe the addition of bromine, hydrogen halides and steam, and in some cases predict the products of these reactions using simple mechanistic reasoning.

亲电加成是烯烃（含碳碳双键的不饱和烃）的特征反应。在 GCSE WJEC 化学的这一考点中，你需要理解烯烃为何比烷烃更活泼，能够描述与溴、卤化氢及水蒸气的加成反应，并在某些情况下运用简单的机理推理来预测反应产物。

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

An electrophilic addition reaction is one in which an electrophile attacks an electron-rich carbon-carbon double bond, leading to the formation of a saturated product with two new single bonds. The word ‘electrophile’ literally means ‘electron-loving’ – a species that is attracted to regions of high electron density.

亲电加成反应是指亲电试剂进攻富含电子的碳碳双键，从而形成具有两个新单键的饱和产物。’亲电试剂’一词的字面意思是’爱电子’——即一种被高电子密度区域吸引的物种。

Alkenes undergo addition rather than substitution because the pi bond in the C=C double bond is relatively weak and can be broken to form two new stronger sigma bonds, releasing energy overall.

烯烃发生加成而非取代反应，因为 C=C 双键中的 π 键相对较弱，断裂后能形成两个更强的 σ 键，总体上释放能量。

2. The Electrophile | 亲电试剂

Common electrophiles in GCSE WJEC Chemistry are bromine (Br₂) and hydrogen bromide (HBr). In the first step of the reaction, the electrophile becomes polarised upon approaching the double bond or is already polar. For example, in non-polar bromine, the high electron density of the alkene induces a temporary dipole in the Br₂ molecule, making one bromine atom slightly positive and therefore electrophilic.

GCSE WJEC 化学中常见的亲电试剂有溴 (Br₂) 和溴化氢 (HBr)。反应的第一步中，亲电试剂在接近双键时被极化，或本身就具有极性。例如，在非极性溴中，烯烃的高电子密度使 Br₂ 分子产生瞬时偶极，其中一个溴原子略带正电，因此成为亲电试剂。

In polar molecules like HBr, the hydrogen atom carries a partial positive charge (Hᵟ⁺–Brᵟ⁻) and acts as the electrophile, seeking out the pi electrons of the alkene.

在 HBr 等极性分子中，氢原子带有部分正电荷 (Hᵟ⁺–Brᵟ⁻)，并作为亲电试剂，寻找烯烃的 π 电子。

3. Mechanism of Bromination of Ethene | 乙烯溴化机理

The simplest mechanism for GCSE understanding is the addition of bromine to ethene, drawn using curly arrows. This occurs in two distinct steps.

对 GCSE 而言，最简单的机理是溴与乙烯的加成，使用弯箭头表示。该反应分两步进行。

Step 1: The pi electrons of the C=C bond move towards the slightly positive bromine atom, forming a new C–Br bond. At the same time, the Br–Br bond breaks heterolytically, with both electrons going to the other bromine to form a bromide ion (Br⁻). A carbocation intermediate is formed on the carbon that does not yet have a bromine attached.

第一步：C=C 键的 π 电子移向略带正电的溴原子，形成新的 C–Br 键。同时 Br–Br 键发生异裂，一对电子移向另一个溴原子形成溴负离子 (Br⁻)。此时尚未连接溴的碳上生成一个碳正离子中间体。

Step 2: The bromide ion, acting as a nucleophile, donates its lone pair to the positively charged carbon of the carbocation, forming the second C–Br bond and giving the final product 1,2-dibromoethane.

第二步：溴负离子作为亲核试剂，将孤对电子给予带正电荷的碳正离子碳原子，形成第二个 C–Br 键，得到最终产物 1,2-二溴乙烷。

The use of curly arrows showing movement of electron pairs is essential when illustrating this mechanism in WJEC exams.

在 WJEC 考试中，使用弯箭头表示电子对移动对于图解该机理至关重要。

4. Addition of Hydrogen Halides | 卤化氢加成

Hydrogen halides such as HBr, HCl and HI add across the double bond of alkenes to form halogenoalkanes. The mechanism is similar to bromination: the pi electrons attack the partially positive hydrogen, breaking the H–X bond heterolytically to give a carbocation and a halide ion, then the halide ion attacks the carbocation.

卤化氢如 HBr、HCl 和 HI 可以加成到烯烃的双键上，生成卤代烷。其机理与溴化类似：π 电子进攻带部分正电荷的氢，导致 H–X 键异裂，生成碳正离子和卤负离子，然后卤负离子进攻碳正离子。

For ethene, the addition of HBr produces bromoethane exclusively because the molecule is symmetrical. With longer-chain alkenes, more than one product may be possible, which leads us to Markovnikov’s rule.

对于乙烯，HBr 加成只生成溴乙烷，因为分子是对称的。对于更长碳链的烯烃，可能得到不止一种产物，这就引出了马氏规则。

5. Markovnikov’s Rule | 马氏规则

When an unsymmetrical alkene (e.g. propene) reacts with an unsymmetrical reagent (e.g. HBr), two isomeric products could theoretically form. Markovnikov’s rule states that the hydrogen atom of the reagent attaches to the carbon of the double bond that already has the greater number of hydrogen atoms attached. In other words, ‘the rich get richer’ – the carbon richer in hydrogen gets another hydrogen.

当不对称烯烃（如丙烯）与不对称试剂（如 HBr）反应时，理论上可能生成两种异构产物。马氏规则指出，试剂中的氢原子会加成到双键中含氢较多的那个碳原子上。简单来说，’富者愈富’——含氢多的碳再得一个氢。

Applying this to propene: CH₃–CH=CH₂ reacts with HBr. The major product is 2-bromopropane (CH₃–CHBr–CH₃), not 1-bromopropane, because the secondary carbocation intermediate formed after addition of H⁺ to the terminal carbon is more stable than the primary one.

将此规则应用于丙烯：CH₃–CH=CH₂ 与 HBr 反应。主产物是 2-溴丙烷 (CH₃–CHBr–CH₃)，而不是 1-溴丙烷，因为 H⁺ 加成到端位碳后生成的仲碳正离子中间体比伯碳正离子更稳定。

In WJEC, you may be expected to predict the major product of addition to unsymmetrical alkenes using this rule or by identifying the more stable carbocation intermediate.

在 WJEC 考试中，你可能需要运用该规则或通过判断更稳定的碳正离子中间体，来预测不对称烯烃加成的主要产物。

6. Addition to Unsymmetrical Alkenes | 不对称烯烃的加成

Let us consider propene with bromine. Because Br₂ is symmetrical, only one product forms: 1,2-dibromopropane. The issue of orientation only arises with unsymmetrical electrophiles like HBr or H₂O (in the presence of acid catalyst).

我们来看丙烯与溴的反应。因为 Br₂ 是对称的，只生成一种产物：1,2-二溴丙烷。只有遇到像 HBr 或 H₂O（在酸催化下）这样不对称的亲电试剂时，才会出现取向问题。

For the hydration of propene, the major product is propan-2-ol (CH₃–CHOH–CH₃), following Markovnikov’s rule, because the intermediate carbocation at the central carbon is more stable.

对于丙烯的水化反应，遵循马氏规则，主产物是丙-2-醇 (CH₃–CHOH–CH₃)，因为中间碳上的碳正离子中间体更稳定。

The stability of carbocations increases in the order: methyl < primary < secondary < tertiary. This explains why the more substituted product is favoured thermodynamically and kinetically.

碳正离子的稳定性按以下顺序递增：甲基 < 伯 < 仲 < 叔。这解释了为什么取代更多的产物在热力学和动力学上都更有利。

7. Addition of Water – Hydration | 水的加成——水化

Alkenes can react with steam in the presence of a strongly acidic catalyst (usually concentrated phosphoric acid, H₃PO₄) to form alcohols. This is a key industrial process for producing ethanol from ethene.

烯烃可在强酸催化剂（通常为浓磷酸 H₃PO₄）存在下与水蒸气反应生成醇。这是工业上从乙烯制乙醇的关键过程。

The conditions required are: temperature around 300 °C and pressure around 60-70 atm. The catalyst is phosphoric acid adsorbed onto a solid support. The reaction is reversible, so the yield is optimised by adjusting conditions according to Le Chatelier’s principle.

所需条件为：温度约 300 °C，压力约 60-70 个大气压。催化剂是吸附在固体载体上的磷酸。该反应是可逆的，因此可通过根据勒夏特列原理调整条件来优化产率。

Mechanistically, this is also an electrophilic addition where H⁺ from the acid adds first, followed by water acting as a nucleophile attacking the carbocation. A final deprotonation regenerates the acid catalyst and gives the alcohol.

从机理上讲，这也是一种亲电加成：酸中的 H⁺ 首先加成，然后水作为亲核试剂进攻碳正离子，最后去质子化步骤再生酸催化剂并得到醇。

8. Testing for Unsaturation | 不饱和性检验

Bromine water (orange-brown) is used to test for carbon-carbon double bonds. When an alkene is shaken with bromine water, the solution turns from orange-brown to colourless. This is a standard addition reaction and provides a visual indication of unsaturation.

溴水（橙棕色）用于检验碳碳双键。当烯烃与溴水一起振荡时，溶液由橙棕色变为无色。这是一个标准的加成反应，可直观地指示不饱和性。

Alkanes do not react with bromine water under normal conditions unless exposed to ultraviolet light, which would cause substitution rather than addition. Thus the decolorisation of bromine water without UV light confirms an alkene.

烷烃在通常条件下不与溴水反应，除非在紫外光下发生取代反应而非加成。因此，无紫外光照射下溴水褪色即可确认烯烃的存在。

9. Summary of Key Conditions and Reagents | 关键条件与试剂总结

The table below summarises the main electrophilic addition reactions required for WJEC GCSE Chemistry. Learning these typical conditions is vital for exam success.

下表总结了 WJEC GCSE 化学要求的主要亲电加成反应。掌握这些典型条件对于考试成功至关重要。

Reaction	Reagent	Conditions	Product Type
Bromination	Br₂ (in water or organic solvent)	Room temperature, no catalyst	Dihalogenoalkane
Hydrogen halide addition	HBr, HCl, etc.	Room temperature, no catalyst	Halogenoalkane
Hydration	H₂O (steam)	300 °C, 60-70 atm, H₃PO₄ catalyst	Alcohol
Hydrogenation	H₂	Nickel catalyst, 150 °C	Alkane

Note that hydrogenation (addition of H₂) is technically also an addition reaction, but it is not electrophilic; it involves a metal catalyst and is often classified as catalytic hydrogenation. However, it appears alongside electrophilic addition in the WJEC specification, so it is included here for completeness.

请注意，加氢（添加 H₂）在技术上也是一种加成反应，但并非亲电型；它涉及金属催化剂，通常归类为催化加氢。然而，它在 WJEC 考纲中与亲电加成一起出现，因此为确保完整性，在此一并列出。

10. Common Misconceptions and Exam Tips | 常见误区与应试技巧

Many students confuse addition with substitution. Remember: alkenes add, alkanes substitute (under UV light). In addition reactions, the double bond becomes a single bond and two atoms or groups are added across it.

许多学生混淆加成与取代。记住：烯烃加成，烷烃取代（在紫外光下）。在加成反应中，双键变成单键，两个原子或基团被加在两端。

When drawing mechanisms, always show the curly arrow starting from the pi bond (between the two carbon atoms) and not from the carbon atom itself. The second curly arrow should show the heterolytic fission of the electrophile clearly.

绘制机理时，弯箭头必须从 π 键（两个碳原子之间）出发，而不是从碳原子本身出发。第二个弯箭头应清晰显示亲电试剂的异裂。

In questions on Markovnikov addition, stating ‘the hydrogen adds to the carbon that already has the most hydrogens’ is a straightforward way to earn marks. You may also note that the more stable carbocation intermediate leads to the major product.

在有关马氏加成的题目中，陈述’氢加到已有最多氢的碳上’是得分的直接方法。你还可以指出，更稳定的碳正离子中间体导至主要产物。

Finally, always link theory to practical tests: the decolorisation of bromine water is a quick and reliable test for unsaturation, and being able to explain the colour change using the concept of electrophilic addition shows deeper understanding.

最后，务必将理论与实际检验联系起来：溴水褪色是检验不饱和性的快速可靠方法，能够运用亲电加成概念解释这一颜色变化，则表明理解更为深入。

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Electrophilic Addition in GCSE WJEC Chemistry | GCSE WJEC 化学：亲电加成 考点精讲