📚 Alkenes: IB and WJEC Chemistry Revision | 烯烃：IB与WJEC化学考点精讲
Alkenes are unsaturated hydrocarbons containing at least one carbon–carbon double bond (C=C). This functional group is the centre of reactivity, making alkenes a cornerstone of organic chemistry in both IB and WJEC specifications. Understanding their structure, bonding, isomerism and characteristic addition reactions is essential for high‑stakes examinations.
烯烃是含有至少一个碳碳双键（C=C）的不饱和烃。该官能团是反应活性中心，使烯烃成为IB和WJEC有机化学的核心内容。掌握它们的结构、键合、同分异构现象以及特征性的加成反应，在重大考试中至关重要。
1. Structure and Bonding in Alkenes | 烯烃的结构与键合
The C=C double bond consists of one sigma (σ) bond and one pi (π) bond. The σ bond is formed by end‑on overlap of sp² hybrid orbitals, while the π bond arises from sideways overlap of unhybridised p orbitals. This π bond is weaker and more exposed, accounting for the high reactivity of alkenes.
C=C双键由一个σ键和一个π键组成。σ键由sp²杂化轨道的端对端重叠形成，π键则源于未杂化p轨道的侧向重叠。π键较弱且更暴露，这解释了烯烃的高反应活性。
The carbon atoms in the double bond are sp² hybridised. The three sp² hybrid orbitals lie in a plane 120° apart, giving a trigonal planar geometry around each carbon. The unhybridised p orbital is perpendicular to this plane, enabling the formation of the π bond. The bond angle around the double‑bonded carbon is approximately 120°, and the bond length of C=C is shorter than that of a C–C single bond (134 pm vs 154 pm).
双键碳原子采用sp²杂化。三个sp²杂化轨道处于同一平面，夹角约120°，使每个碳周围呈平面三角形几何形状。未杂化的p轨道垂直于该平面，从而形成π键。双键碳周围的键角约为120°，C=C键长比C–C单键短（134 pm对比154 pm）。
Rotation about the C=C bond is restricted because the π bond would have to be broken. This gives rise to geometric (cis‑trans) isomerism, an important structural feature of alkenes.
C=C键的旋转受到限制，因为π键需要被破坏。这导致了顺反异构（几何异构）现象，这是烯烃的重要结构特征。
2. Nomenclature of Alkenes | 烯烃的命名
Alkenes are named according to IUPAC rules. The parent chain must contain the C=C bond, and the suffix ‘‑ene’ is used. The chain is numbered from the end nearest the double bond, and the position of the double bond is indicated by the lowest number of the first doubly‑bonded carbon.
烯烃按IUPAC规则命名。主链必须包含C=C键，并使用后缀“‑烯”。从最靠近双键的一端开始给碳链编号，双键的位置由第一个双键碳原子的最小编号表示。
Substituents are named as prefixes in alphabetical order, and their positions are indicated by numbers. For cyclic alkenes, the carbons of the double bond are numbered 1 and 2. In WJEC, students are expected to name simple alkenes up to C₆ and some branched examples; IB also covers more complex structures and the use of (E)/(Z) notation for geometric isomers.
取代基按字母顺序作为前缀，其位置用数字标出。对于环烯烃，双键碳原子被编号为1和2。在WJEC中，学生需命名最多六个碳的简单烯烃及一些支链例子；IB还涵盖更复杂的结构以及用(E)/(Z)标记法表示几何异构体。
3. Geometric Isomerism: Cis‑Trans and E/Z | 几何异构：顺反与E/Z标记
Restricted rotation about the double bond leads to stereoisomerism. If each doubly‑bonded carbon carries two different groups, the alkene exhibits geometric isomerism. Cis‑trans terminology applies when there are two identical groups (often H atoms) on the double‑bonded carbons; cis means the two identical groups are on the same side, trans means they are on opposite sides.
双键旋转受限导致立体异构。若每个双键碳上连接两个不同基团，烯烃就表现几何异构。当双键碳上各有两个相同基团（通常是氢原子）时，适用顺反标记；顺式表示两个相同基团在双键同侧，反式表示在异侧。
For more complex alkenes, the E/Z system is used. Priority is assigned to the substituents on each carbon of the double bond using the Cahn–Ingold–Prelog (CIP) rules based on atomic number. (Z) indicates the higher‑priority groups are on the same side (zusammen), while (E) indicates they are on opposite sides (entgegen). IB Diploma Chemistry requires students to assign E/Z configurations, whereas WJEC focuses on cis‑trans for alkenes with hydrogen substituents.
对于更复杂的烯烃，使用E/Z标记法。依据Cahn–Ingold–Prelog（CIP）规则按原子序数给双键每个碳上的取代基分配优先顺序。(Z)表示高优先基团在同侧，(E)表示在异侧。IB文凭课程要求学生能标定E/Z构型，而WJEC主要关注含氢取代基的烯烃的顺反异构。
4. Physical Properties of Alkenes | 烯烃的物理性质
Alkenes are non‑polar molecules; the only intermolecular forces are van der Waals’ (London dispersion) forces. As a result, they have relatively low melting and boiling points that increase with increasing chain length. They are insoluble in water but dissolve in non‑polar organic solvents.
烯烃是非极性分子，分子间作用力仅为范德华力（伦敦色散力）。因此，它们的熔点和沸点相对较低，并随碳链增长而升高。它们不溶于水，但可溶于非极性有机溶剂。
In general, trans isomers have higher melting points but slightly lower boiling points than their cis counterparts. The greater symmetry of trans isomers allows closer packing in the solid state, increasing the lattice energy, while the slight polarity of cis isomers raises their boiling point due to increased dipole‑dipole interactions.
通常，反式异构体的熔点较高，但沸点略低于顺式异构体。反式异构体更高的对称性使其在固态下能更紧密堆积，增加了晶格能；而顺式异构体的微小极性因偶极‑偶极作用增强而使其沸点稍高。
5. Electrophilic Addition Mechanism | 亲电加成机理
The characteristic reaction of alkenes is electrophilic addition. The electron‑rich π bond attracts electrophiles. In the first step, the electrophile accepts a pair of electrons from the π bond, forming a covalent bond to one carbon and leaving a carbocation intermediate on the other. In the second step, a nucleophile (often the halide ion or water) attacks the carbocation to complete the addition.
烯烃的特征反应是亲电加成。富电子的π键吸引亲电试剂。第一步，亲电试剂接受π键的一对电子，与一个碳原子形成共价键，另一个碳上形成碳正离子中间体。第二步，亲核试剂（通常是卤离子或水）攻击碳正离子，完成加成。
This two‑step process is typical for the addition of HX, X₂ (in the presence of a polar solvent), H₂SO₄ and H₂O (acid‑catalysed hydration). Both IB and WJEC require students to draw the mechanism with curly arrows showing electron movement, and to explain the formation of carbocations. IB also expects an understanding of carbocation stability: tertiary (3°) > secondary (2°) > primary (1°) > methyl, which can lead to major and minor products via Markovnikov’s rule.
这一两步过程是HX、X₂（在极性溶剂中）、H₂SO₄和H₂O（酸催化水合）加成的典型机理。IB和WJEC都要求学生用弯箭头画出电子转移的机理，并解释碳正离子的生成。IB还要求理解碳正离子的稳定性顺序：叔（3°）> 仲（2°）> 伯（1°）> 甲基，结合马尔科夫尼科夫规则可预判主产物与副产物。
6. Addition of Hydrogen Halides and Markovnikov’s Rule | 卤化氢的加成与马尔科夫尼科夫规则
Hydrogen halides (HBr, HCl, HI) add across the double bond to form haloalkanes. The reaction with HBr is rapid at room temperature; HCl requires a catalyst or heat. The electrophile is the partially positive hydrogen of the HX molecule. The addition follows Markovnikov’s rule: the hydrogen atom attaches to the carbon with the greater number of hydrogen atoms already present (the less substituted carbon). In mechanistic terms, this occurs because the more stable carbocation intermediate is formed preferentially.
卤化氢（HBr、HCl、HI）对双键加成生成卤代烷。HBr的反应在室温下迅速进行；HCl需要催化剂或加热。亲电试剂是HX分子中部分正电性的氢原子。加成遵循马尔科夫尼科夫规则：氢原子加到已有更多氢原子的碳（即取代较少的碳）上。从机理角度，这是因为优先生成更稳定的碳正离子中间体。
When propene reacts with HBr, the major product is 2‑bromopropane (via the more stable secondary carbocation), not 1‑bromopropane. This regioselectivity is a key exam point for both syllabuses.
当丙烯与HBr反应时，主产物是2‑溴丙烷（经更稳定的仲碳正离子），而不是1‑溴丙烷。这种区域选择性是两套大纲的共同关键考点。
7. Addition of Halogens | 卤素的加成
Alkenes react with bromine or chlorine at room temperature, often in an inert solvent (e.g. CH₂Cl₂) or in water. The reaction occurs readily, and the reddish‑brown colour of bromine is decolourised – this is the standard test for unsaturation.
烯烃与溴或氯在室温下反应，常在惰性溶剂（如CH₂Cl₂）或水中进行。反应迅速发生，溴的红棕色褪去——这是检验不饱和键的标准方法。
The mechanism involves polarisation of the halogen molecule as it approaches the π electron cloud. The π electrons attack one bromine atom, forming a cyclic bromonium ion (in the case of bromine) and a bromide ion. The bromide ion then attacks from the opposite face of the three‑membered ring, giving anti addition (trans product). IB students must describe the bromonium ion intermediate; WJEC may accept a simple carbocation‑based mechanism but the halonium ion is more accurate.
其机理涉及卤素分子靠近π电子云时发生极化。π电子进攻一个溴原子，形成环状溴鎓离子（对溴而言）和一个溴离子。然后溴离子从三元环的反面进攻，得到反式加成产物。IB学生必须描述溴鎓离子中间体；WJEC可能接受基于碳正离子的简单机理，但卤鎓离子更为准确。
8. Hydration of Alkenes | 烯烃的水合反应
Alkenes can be converted to alcohols by acid‑catalysed hydration. This requires concentrated sulfuric acid or phosphoric acid as a catalyst, and water. The first step is the addition of H⁺ to the double bond to form a carbocation, following Markovnikov’s rule. Water then attacks the carbocation, and loss of a proton yields the alcohol. Industrially, this is used to manufacture ethanol from ethene.
烯烃可通过酸催化水合反应转化为醇。这需要使用浓硫酸或磷酸作为催化剂，以及水。第一步是H⁺加成到双键上形成碳正离子，遵循马尔科夫尼科夫规则。水攻击碳正离子，失去质子后得到醇。工业上用此法从乙烯制乙醇。
An alternative industrial route is direct hydration, passing ethene and steam over a phosphoric acid catalyst supported on silica at high temperature (300 °C) and pressure (60–70 atm). Both IB and WJEC syllabi cover this equilibrium reaction, together with its conditions.
另一条工业路线是直接水合法，将乙烯和水蒸气在高温（300 °C）和高压（60–70 atm）下通过负载在二氧化硅上的磷酸催化剂。IB和WJEC大纲都包含这一平衡反应及其条件。
9. Hydrogenation | 加氢反应
Alkenes react with hydrogen gas in the presence of a metal catalyst (Ni, Pt or Pd) at room temperature or moderate heat to form alkanes. This is an addition reaction that occurs on the catalyst surface where hydrogen is adsorbed and dissociated into atoms. The reaction is used to harden unsaturated vegetable oils to make margarine.
烯烃在金属催化剂（Ni、Pt或Pd）存在下于室温或微热条件下与氢气反应生成烷烃。这是一种加成反应，发生在催化剂表面，氢气被吸附并解离成氢原子。此反应用于将不饱和植物油氢化硬化以制造人造黄油。
Hydrogenation is exothermic; the enthalpy change of hydrogenation can be used to compare the stability of alkenes. A less stable alkene releases more heat upon hydrogenation, which is relevant to IB thermochemical questions.
加氢为放热反应；氢化焓变可用来比较烯烃的稳定性。稳定性较低的烯烃在加氢时释放更多热量，这与IB热化学题目相关。
10. Oxidation Reactions | 氧化反应
Alkenes are easily oxidised. With cold, dilute, alkaline KMnO₄ (Baeyer’s test), the purple permanganate colour disappears and a brown precipitate of MnO₂ forms; the alkene is converted into a diol (vicinal diol). This syn addition is a useful test for the presence of a C=C double bond.
烯烃容易被氧化。在冷的稀碱性KMnO₄（拜耳试验）中，紫色高锰酸根褪色，生成棕色MnO₂沉淀；烯烃被转化为邻二醇。这种顺式加成是检验C=C双键存在的一种有效方法。
Under harsher conditions, hot, concentrated, acidified KMnO₄ cleaves the double bond completely. Terminal alkenes produce carbon dioxide and water alongside carboxylic acids; disubstituted alkenes give ketones. WJEC candidates should recall the colour change and the formation of a diol under mild conditions. IB may require predicting the oxidation products from a given alkene structure.
在更剧烈的条件下，热的酸性浓KMnO₄会完全断裂双键。末端烯烃除生成羧酸外，还生成二氧化碳和水；双取代烯烃则得到酮。WJEC考生应记住颜色变化及温和条件下邻二醇的生成。IB可能要求根据给定烯烃结构预测氧化产物。
11. Addition Polymerisation | 加成聚合
Alkenes undergo addition polymerisation to form long‑chain polymers. The C=C bond opens and monomers join together to form a carbon‑carbon backbone. Poly(ethene), poly(propene) and poly(chloroethene) (PVC) are classic examples. The reaction requires heat, pressure and an initiator (often a radical initiator).
烯烃发生加成聚合生成长链聚合物。C=C键打开，单体连接在一起形成碳‑碳主链。聚乙烯、聚丙烯和聚氯乙烯（PVC）是经典例子。此反应需要加热、加压和引发剂（常为自由基引发剂）。
In both IB and WJEC, students are expected to draw the repeating unit from a given monomer, and vice versa. IB may also address issues of plastic waste, recycling and biodegradable polymers. WJEC often includes the environmental impact of PVC and poly(ethene).
在IB和WJEC中，学生都应能从给定单体画出重复单元，反之亦然。IB还可能涉及塑料废弃物、回收和生物可降解聚合物问题。WJEC常涉及PVC和聚乙烯的环境影响。
12. Summary of Key Reactions and Exam Tips | 核心反应总结与应试技巧
| Reaction / 反应 | Reagents/Conditions / 试剂/条件 | Product / 产物 |
|---|---|---|
| Hydrogenation | H₂, Ni/Pd/Pt, room temp. or heat | Alkane |
| Addition of HX | HBr(g) or HCl(g) | Haloalkane (Markovnikov) |
| Halogenation | Br₂ (in CH₂Cl₂ or H₂O) / Cl₂ | Vicinal dihalide (anti) |
| Acid‑catalysed hydration | H₂O, conc. H₂SO₄ / H₃PO₄ | Alcohol (Markovnikov) |
| Mild oxidation (Baeyer’s test) | Cold, dilute, alkaline KMnO₄ | Diol (syn) |
| Strong oxidation | Hot, acidified KMnO₄ | Carboxylic acids / ketones / CO₂ |
| Polymerisation | Heat, pressure, initiator | Addition polymer |
When answering exam questions, always show the mechanism carefully: draw partial charges or dipoles where needed, use double‑headed curly arrows to show electron pair movement, and clearly display the carbocation or bromonium ion intermediate. In IB, pay attention to stereochemistry (anti addition for halogens, syn addition for dihydroxylation) and the use of 3D wedge/dash diagrams. For WJEC, be precise with colour changes and conditions. Practice drawing repeating units and labeling E/Z isomers systematically.
考试作答时，务必仔细展示机理：必要时画出部分电荷或偶极，用双头弯箭头表示电子对移动，并明确呈现碳正离子或溴鎓离子中间体。在IB中，注意立体化学（卤素反式加成，二羟基化顺式加成）并使用三维楔形/虚线图。对于WJEC，要精确描述颜色变化和反应条件。系统练习画出重复单元和标记E/Z异构体。
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