📚 A-Level WJEC Chemistry: Alkenes Essential Revision Guide | A-Level WJEC 化学：烯烃 考点精讲
Alkenes are a fundamental class of unsaturated hydrocarbons that feature prominently in A-Level WJEC Chemistry. Understanding their structure, bonding, reactivity, and mechanisms is vital for success in both AS and A2 units. This guide breaks down every key concept, from E/Z isomerism to electrophilic addition and polymerisation, with targeted revision notes tailored to the WJEC specification.
烯烃是一类重要的不饱和烃，在 A-Level WJEC 化学中占据核心地位。理解它们的结构、成键、反应性与机理，对于 AS 和 A2 阶段的考试都至关重要。本篇精讲将按照 WJEC 考纲拆解每一个核心概念，从 E/Z 异构到亲电加成及聚合反应，为你提供一份精准的复习宝典。
1. General Formula and Bonding | 通式与成键
Alkenes are hydrocarbons with the general formula CnH2n, containing at least one carbon–carbon double bond (C=C). This double bond consists of a sigma (σ) bond and a pi (π) bond. The σ bond is formed by the head-on overlap of sp² hybrid orbitals, while the π bond results from the sideways overlap of adjacent p orbitals. The presence of the π bond restricts rotation around the C=C, giving alkenes their characteristic planar geometry with bond angles of approximately 120°.
烯烃是通式为 CnH2n 的碳氢化合物，分子中含有至少一个碳碳双键 (C=C)。双键由一个 σ 键和一个 π 键组成。σ 键由 sp² 杂化轨道“头对头”重叠形成，而 π 键则来自两个平行 p 轨道的侧向重叠。π 键的存在限制了双键的旋转，使烯烃分子具有平面结构，键角约为 120°。
2. Nomenclature of Alkenes | 烯烃的命名
According to IUPAC rules, the parent chain is the longest carbon chain that contains the double bond. The suffix ‘-ene’ is used, and the position of the double bond is indicated by the lowest possible number before the suffix (e.g., but-1-ene, pent-2-ene). Substituents are named as prefixes with their own position numbers. When multiple double bonds are present, ‘-diene’, ‘-triene’ are used.
按照 IUPAC 命名规则，应选择包含双键的最长碳链为主链，后缀为“-烯”，并用尽可能小的数字表明双键的位置，写在“-烯”之前（如丁-1-烯、戊-2-烯）。取代基以词头表示，并标注位次。若存在多个双键，则使用“-二烯”、“-三烯”等后缀。
3. Isomerism: E/Z and Cis-Trans | 异构现象：E/Z 与顺反异构
Alkenes exhibit stereoisomerism due to the restricted rotation of the C=C bond. Geometric isomerism arises when each carbon of the double bond is attached to two different groups. When the two highest priority groups (determined by Cahn-Ingold-Prelog rules) are on the same side of the double bond, it is designated as Z (zusammen, together); when they are on opposite sides, it is E (entgegen, opposite). For simple cases with identical substituents on each carbon, the terms cis and trans may still be used (cis = Z, trans = E).
由于碳碳双键不能自由旋转，烯烃可存在立体异构。当双键的每个碳原子上连接两个不同的基团时，即可出现几何异构。根据 Cahn-Ingold-Prelog 序列规则，若两个优先基团处于双键同侧，则标记为 Z（zusammen，意为“同”）；若处于对侧，则标记为 E（entgegen，意为“反”）。简单情况下，若每个双键碳上都有一个相同取代基，也可沿用顺反标记（顺 = Z，反 = E）。
4. Physical Properties of Alkenes | 烯烃的物理性质
Alkenes are non-polar or only very slightly polar, so their intermolecular forces are limited to van der Waals (London) forces. Boiling points increase with molecular mass due to greater surface area and stronger induced-dipole interactions. They are insoluble in water but soluble in non-polar organic solvents. Shorter-chain alkenes (C2–C4) are gases at room temperature; mid-chain alkenes are volatile liquids.
烯烃分子非极性或仅具极弱极性，因此分子间作用力仅限于范德华力（伦敦力）。沸点随分子量增大而升高，因为分子表面积增加，诱导偶极作用增强。它们不溶于水，但可溶于非极性有机溶剂。短链烯烃（C2–C4）在室温下为气态；中长链烯烃为易挥发的液体。
5. Preparation of Alkenes (Key Reactions) | 烯烃的制备（关键反应）
In the WJEC specification, alkenes are commonly prepared by elimination reactions. The dehydration of alcohols using concentrated phosphoric(V) acid or hot Al₂O₃ catalyst, and the base-induced elimination of hydrogen halides from halogenoalkanes (using KOH in ethanol, heat). These methods produce the C=C functional group essential for further transformations.
在 WJEC 考纲中，烯烃常通过消除反应制得。醇的脱水反应可使用浓磷酸(V)或热的氧化铝催化剂；卤代烷在强碱（KOH 乙醇溶液，加热）作用下发生消除卤化氢的反应。这些方法都能够产生 C=C 官能团，为后续转化奠定基础。
6. Electrophilic Addition Mechanism | 亲电加成机理
The electron-rich π bond makes alkenes susceptible to attack by electrophiles (electron-pair acceptors). The typical mechanism proceeds in two steps: first, the electrophile is attracted to the double bond, forming a carbocation intermediate and a negatively charged species; second, a nucleophile (often the counter-ion) rapidly combines with the carbocation to give the addition product. Curly-arrow notation is essential: arrow from the C=C bond to the electrophile, then from the nucleophile to the carbocation.
富电子的 π 键使烯烃容易受到亲电试剂（电子对接受体）的进攻。典型的机理分两步进行：首先，亲电试剂被双键吸引，形成碳正离子中间体和带负电的物种；然后，亲核试剂（通常为抗衡离子）迅速与碳正离子结合，得到加成产物。弯箭头的表示极为重要：箭头从 C=C 指向亲电试剂，再从亲核试剂指向碳正离子。
7. Addition of Hydrogen Halides and Markovnikov’s Rule | 卤化氢的加成与马尔可夫尼科夫规则
When HX (X = Cl, Br, I) is added to an unsymmetrical alkene, two possible products can form. Markovnikov’s rule states that the hydrogen atom becomes attached to the carbon with the greater number of hydrogen atoms already present (the carbon less substituted). In modern terms, the more stable carbocation intermediate (tertiary > secondary > primary) is preferentially formed, directing the regioselectivity. For example, propene with HBr yields 2-bromopropane as the major product.
当不对称烯烃与 HX（X = Cl, Br, I）加成时，理论上可生成两种产物。马尔可夫尼科夫规则指出：氢原子会加在原本含氢较多的碳原子上（取代基较少的碳）。现代解释为，反应经过更稳定的碳正离子中间体（叔 > 仲 > 伯），从而决定了区域选择性。例如，丙烯与 HBr 加成，主要产物为 2-溴丙烷。
8. Addition of Halogens and the Bromine Water Test | 卤素的加成与溴水试验
Alkenes react readily with halogens (Br₂, Cl₂) at room temperature to give dihalogenoalkanes. The reaction with bromine dissolved in an inert solvent (or bromine water) is a key test for unsaturation: the orange/brown colour of bromine is decolourised immediately. The mechanism involves the π electrons inducing a dipole in Br₂, forming a cyclic bromonium ion intermediate, followed by nucleophilic attack by the bromide ion.
烯烃在室温下极易与卤素（Br₂、Cl₂）发生加成反应，生成二卤代烷。与溴的有机溶液（或溴水）反应是检验不饱和键的关键测试：溴的橙/红棕色会迅速褪去。其机理为 π 电子使 Br₂ 分子极化，形成环状溴鎓离子中间体，随后溴离子作为亲核试剂进攻开环。
9. Hydration of Alkenes: Alcohol Formation | 烯烃的水合：醇的生成
Alkenes can be hydrated to produce alcohols. Industrially, ethene reacts with steam at high temperature (300–400°C) and pressure (60–70 atm) over a phosphoric(V) acid catalyst to produce ethanol. In the laboratory, the addition of water is achieved by using concentrated sulfuric acid to form an alkyl hydrogensulfate intermediate, followed by hydrolysis. The reaction follows Markovnikov’s rule when unsymmetrical alkenes are used.
烯烃可以通过水合反应转化为醇。工业上，乙烯与水蒸气在高温（300–400°C）高压（60–70 atm）下，通过磷酸(V)催化剂作用生成乙醇。实验室中，常用浓硫酸先与烯烃加成形成硫酸氢酯中间体，再经水解得到醇。当使用不对称烯烃时，水合反应同样遵守马尔可夫尼科夫规则。
10. Oxidation Reactions of Alkenes | 烯烃的氧化反应
Alkenes are oxidised by cold, dilute potassium manganate(VII) (KMnO₄) to form diols (dihydroxylation). The purple colour of MnO₄⁻ disappears and a brown precipitate of MnO₂ forms, providing another test for unsaturation. Under vigorous conditions with hot, concentrated KMnO₄ and acid, the double bond is cleaved to give carbonyl compounds or carboxylic acids, depending on the substitution pattern. This oxidative cleavage is useful for structural determination.
烯烃可被冷稀高锰酸钾 (KMnO₄) 氧化，生成邻二醇（双羟基化）。此时 MnO₄⁻ 的紫色褪去，产生棕色 MnO₂ 沉淀，这也可用于检验不饱和键。在剧烈条件下（热浓高锰酸钾加酸），双键发生断裂，根据取代程度生成羰基化合物或羧酸。这种氧化断键反应在结构测定中十分有用。
11. Polymerisation of Alkenes | 烯烃的聚合反应
Alkenes undergo addition polymerisation, where the π bond breaks open and monomers join together to form long saturated carbon chains. The process requires initiators (often radicals), high pressure, and sometimes catalysts. Poly(ethene), poly(propene), and poly(chloroethene) (PVC) are major industrial polymers with repeating units –(CH₂–CH₂)–, –(CH₂–CH(CH₃))–, and –(CH₂–CHCl)– respectively. The WJEC specification also expects the ability to draw repeat units from given monomers and vice versa.
烯烃可发生加成聚合反应，π 键打开，单体分子相互连接形成长链饱和碳链。聚合过程需要引发剂（常为自由基）、高压，有时还需要催化剂。聚乙烯、聚丙烯和聚氯乙烯是重要的工业聚合物，其重复单元分别为 –(CH₂–CH₂)–、–(CH₂–CH(CH₃))– 和 –(CH₂–CHCl)–。WJEC 考纲要求能够根据单体画出聚合物的重复单元，反之亦然。
12. Summary of Key Reactions and Interconversions | 核心反应与相互转化总结
The chemistry of alkenes centres on the reactive C=C bond. Electrophilic addition with HX, X₂, H₂O (via H₂SO₄), and H₂ (catalytic hydrogenation) forms saturated products. Oxidation with KMnO₄ can either form diols or cleave the double bond. Polymerisation creates commercially essential materials. Mastery of the electrophilic addition mechanism, carbocation stability, and regioselectivity (Markovnikov’s rule) is essential. Be prepared to apply these principles to unfamiliar alkenes in examination questions.
烯烃的化学核心在于活泼的 C=C 双键。与 HX、X₂、H₂O（经 H₂SO₄）及 H₂（催化加氢）发生亲电加成反应，生成饱和产物。高锰酸钾氧化可生成二醇或使双键断裂。聚合反应则创造商业必需的高分子材料。掌握亲电加成机理、碳正离子稳定性及区域选择性（马尔可夫尼科夫规则）至关重要。考试中要能将这些原理灵活应用于陌生的烯烃。
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