📚 A-Level AQA Chemistry: Alkenes Revision Guide | A-Level AQA 化学：烯烃考点精讲
Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond, making them far more reactive than alkanes. For AQA A-Level Chemistry, mastering the structure, isomerism, and characteristic reactions of alkenes is essential – especially the electrophilic addition mechanism, geometric isomerism, and industrial significance. This revision guide distils all key concepts and common pitfalls to help you secure top marks.
烯烃是含有至少一个碳碳双键的不饱和烃，反应活性远高于烷烃。在AQA A-Level化学中，掌握烯烃的结构、异构现象和特征反应至关重要，特别是亲电加成机理、几何异构以及工业重要性。本考点精讲浓缩了所有关键概念和常见易错点，助你稳拿高分。
1. Introduction to Alkenes | 烯烃介绍
Alkenes have the general formula CₙH₂ₙ and are characterised by a carbon-carbon double bond. Because the double bond is an area of high electron density, alkenes readily undergo addition reactions with electrophiles. They are far more reactive than alkanes, which only contain single C–C and C–H bonds, and are the feedstock for a huge range of polymers and industrial chemicals.
烯烃的通式为CₙH₂ₙ，其特征是含有一个碳碳双键。由于双键区域电子密度高，烯烃极易与亲电试剂发生加成反应。它们的反应活性远高于只含单键的烷烃，并且是大量聚合物及工业化学品的原料。
2. Structure of the Double Bond: Sigma and Pi Bonds | 双键结构：σ键与π键
A carbon atom in a C=C double bond is sp² hybridised, forming three σ bonds in a trigonal planar arrangement (bond angles ≈120°). The double bond consists of one strong σ bond (head-on overlap of sp² orbitals) and one weaker π bond (sideways overlap of p orbitals). The π electrons sit above and below the plane of the molecule, creating a region of high electron density that attracts electrophiles. Because the π bond is weaker, it breaks easily during addition reactions.
双键中的碳原子为sp²杂化，呈平面三角形（键角≈120°），形成三个σ键。双键由一个强σ键（sp²轨道头对头重叠）和一个较弱的π键（p轨道肩并肩重叠）组成。π电子云分布在分子平面的上下方，形成一个高电子密度区域，容易吸引亲电试剂。由于π键较弱，在加成反应中易于断裂。
3. Geometric Isomerism: E/Z Nomenclature | 几何异构：E/Z命名
Restricted rotation about the C=C double bond gives rise to geometric isomerism when each carbon of the double bond is attached to two different groups. The E/Z system uses the Cahn–Ingold–Prelog (CIP) priority rules: look at the two groups on each carbon; the atom with the higher atomic number has higher priority. If the two higher-priority groups are on opposite sides of the double bond, the isomer is E (from German entgegen, opposite); if they are on the same side, it is Z (zusammen, together). Note that the old cis/trans system only works when one of the groups on each carbon is the same, typically hydrogen.
由于碳碳双键旋转受限，当双键碳原子上各自连接两个不同的基团时，便产生几何异构。E/Z命名体系采用Cahn-Ingold-Prelog (CIP) 次序规则：比较每个碳上两个基团中直接相连原子的原子序数，序数大的优先。若两个较优基团在双键异侧，则为E构型；若在同侧，则为Z构型。顺反异构体系仅适用于每个碳上有一个相同基团（通常是氢）的情况。
Example / 示例:
- (E)-but-2-ene: CH₃ groups are on opposite sides; (Z)-but-2-ene: CH₃ groups on the same side.
- (E)-1,2-dichloroethene: Cl atoms are opposite; (Z)-isomer: Cl atoms adjacent.
In exam questions, you must be able to assign E or Z to alkenes with alkyl, halogen, or other substituent groups.
考试中，你需要能够为含有烷基、卤素或其他取代基的烯烃指定E或Z构型。
4. Electrophilic Addition Mechanism | 亲电加成机理
Alkenes undergo electrophilic addition: the π electrons attack an electron-deficient species (electrophile). The general mechanism has two steps. First, the electrophile accepts the π electrons to form a covalent bond to one carbon, leaving a carbocation on the other carbon. Second, a nucleophile (often a negatively charged ion) rapidly attacks the carbocation to complete the addition. The reaction is heterolytic fission: both electrons of the π bond go to form the new bond, and the electrophile is polarised or already positively charged.
烯烃发生亲电加成反应：π电子进攻缺电子物种（亲电试剂）。机理通常分两步：第一步，亲电试剂接受π电子，与一个碳原子形成共价键，另一个碳上产生碳正离子；第二步，亲核试剂（通常是带负电荷的离子）迅速进攻碳正离子，完成加成。反应属于异裂：π键的电子对全部用来形成新键，亲电试剂则需被极化或本身带正电。
Curly arrow conventions: a curly arrow starts from the π bond or from a lone pair and points towards the atom accepting the electrons. In AQA mark schemes, showing the movement of the electron pair correctly is vital.
弯箭头画法：弯箭头从π键或孤对电子出发，指向接受电子的原子。在AQA评分方案中，正确表示电子对转移极为关键。
5. Reaction with Bromine and the Test for Unsaturation | 与溴反应及不饱和度测试
When an alkene is shaken with orange-brown bromine water (or bromine in an organic solvent), the colour quickly disappears as the bromine adds across the double bond. This decolourisation is the standard test for unsaturation. The mechanism proceeds via a bromonium ion, not a simple carbocation. The Br–Br bond becomes polarised as it approaches the π cloud, with the δ+ Br acting as the electrophile. The π electrons attack this Br, forming a cyclic bromonium ion and a bromide ion (Br⁻). The bromide then attacks the bromonium ion from the opposite side (anti addition), yielding the vicinal dibromoalkane as a trans product if stereochemistry is relevant.
将烯烃与橙黄色的溴水（或溴的有机溶液）振荡，颜色迅速消失，表明溴加至双键上。这种褪色现象是检验不饱和度的标准方法。该反应的机理经过溴鎓离子而非简单的碳正离子。Br–Br键在接近π电子云时被极化，带部分正电荷的δ+ Br作为亲电试剂。π电子进攻该溴原子，形成环状溴鎓离子和溴离子（Br⁻）。随后溴离子从溴鎓离子的背面进攻（反式加成），生成邻二溴代烷；如果涉及立体化学，产物为反式构型。
CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br
This reaction is selective for alkenes; alkanes do not react with bromine water under these conditions.
此反应对烯烃具有选择性；烷烃在此条件下不与溴水反应。
6. Reaction with Hydrogen Halides and Markovnikov’s Rule | 与卤化氢的反应与马氏规则
Alkenes react with hydrogen halides (HCl, HBr, HI) by electrophilic addition. With unsymmetrical alkenes, two products are possible. The major product follows Markovnikov’s rule: the hydrogen atom attaches to the carbon that already has more hydrogen atoms (the less substituted carbon), leading to the more stable carbocation intermediate. For example, propene with HBr gives mainly 2-bromopropane, not 1-bromopropane, because the secondary carbocation formed is more stable than the primary one. This stability order arises from alkyl groups’ electron-donating inductive effect, which disperses the positive charge.
烯烃与卤化氢（HCl, HBr, HI）发生亲电加成。当烯烃不对称时，可能得到两种产物。主要产物遵循马氏规则：氢原子加到原本氢较多的碳上（取代度较低的碳），从而经由更稳定的碳正离子中间体。例如，丙烯与HBr主要生成2-溴丙烷而非1-溴丙烷，因为形成的二级碳正离子比一级碳正离子更稳定。这种稳定性来自烷基的给电子诱导效应，能分散正电荷。
CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (major product)
When writing a mechanism for HX addition, the first step produces a carbocation; there is no cyclic bromonium ion because HBr is a stronger acid and the bromide ion is formed directly after protonation.
在书写HX加成的机理时，第一步生成碳正离子；不会形成溴鎓离子，因为HBr是强酸，质子化后直接得到溴离子。
7. Hydration of Alkenes: Industrial and Laboratory Methods | 烯烃的水合：工业与实验室方法
Alkenes can be hydrated to produce alcohols. Industrially, ethene is reacted with steam at high temperature and pressure using a phosphoric acid (H₃PO₄) catalyst on a solid support. This is a direct hydration: CH₂=CH₂ + H₂O(g) → CH₃CH₂OH. A laboratory-scale route involves two steps: addition of cold concentrated sulfuric acid to form an alkyl hydrogensulfate, followed by hydrolysis with water. The net result is the addition of water across the double bond with Markovnikov regioselectivity. For example, propene yields propan-2-ol, not propan-1-ol.
烯烃可通过水合反应制备醇。工业上，乙烯与水蒸气在高温高压及磷酸（H₃PO₄）固体催化下直接水合：CH₂=CH₂ + H₂O(g) → CH₃CH₂OH。实验室路线则需要两步：先用冷浓硫酸与烯烃反应生成硫酸氢酯，再用水水解。净结果是水以马氏规则选择性加成到双键上。例如丙烯水合得到丙-2-醇，而非丙-1-醇。
This two-step method using H₂SO₄ is often examined as it links to ester chemistry and demonstrates the principle of electrophilic addition.
用浓硫酸的两步法常被考查，因为它关联到酯化学，同时展示了亲电加成原理。
8. Addition Polymerisation | 加聚反应
Alkenes and substituted alkenes can undergo addition polymerisation, where the π bonds break open to link monomers into long chains without any small molecules being eliminated. The polymer is named by putting ‘poly’ in front of the monomer name. Examples include poly(ethene) from ethene, poly(propene) from propene, poly(chloroethene) (PVC) from chloroethene, and poly(phenylethene) (polystyrene) from phenylethene. You must be able to draw the repeating unit from a monomer and identify the monomer from a given polymer section.
烯烃及其衍生物可以发生加聚反应，π键打开，单体相互连接成长链，且没有小分子脱去。聚合物以“聚”前缀命名，如乙烯生成聚（乙烯），丙烯生成聚（丙烯），氯乙烯生成聚（氯乙烯）（PVC），苯乙烯生成聚（苯乙烯）（PS）。必须能够从单体画出重复单元，并由聚合物片段反推单体。
n CH₂=CHX → –(CH₂–CHX)–ₙ where X = H, CH₃, Cl, C₆H₅, etc.
Polymers are typically inert and non-biodegradable, which raises environmental issues regarding disposal and recycling. These societal implications are often part of AQA examination questions.
聚合物通常化学惰性且不可生物降解，由此带来的废弃与回收问题常在AQA考题中出现。
9. Oxidation of Alkenes | 烯烃的氧化反应
Although not the central focus of AQA mechanisms, alkenes are easily oxidised. Cold, dilute, acidified potassium manganate(VII) (KMnO₄) turns from purple to colourless when shaken with an alkene, forming a diol. This is another test for unsaturation, similar to bromine water, but it is less selective. Under more vigorous conditions, oxidative cleavage can occur, breaking the C=C bond completely. In exams, you may be expected to state that alkenes decolourise acidified KMnO₄, indicating the presence of a double bond.
尽管氧化反应不是AQA机理考查的重心，烯烃仍易被氧化。冷的稀酸性高锰酸钾（KMnO₄）与烯烃一同振荡，紫色褪去，生成邻二醇，这是另一种不饱和度检验方法，与溴水类似但选择性较低。更剧烈的条件下可发生氧化断裂，完全破坏双键。考试中可能要求指出烯烃能使酸性高锰酸钾褪色，从而证明双键的存在。
10. Industrial Importance and Cracking | 工业重要性与裂解
Short-chain alkenes like ethene and propene are in high demand for polymer production and chemical synthesis. They are obtained by cracking long-chain alkanes from petroleum fractions. Thermal cracking uses high temperature and pressure to produce a high proportion of alkenes. Catalytic cracking uses a zeolite catalyst at moderate temperatures, producing branched alkanes and aromatic compounds as well. Understanding the economic demand for alkenes explains why cracking is a vital industrial process.
短链烯烃如乙烯和丙烯在聚合物生产与化学合成中需求极大。它们由石油馏分中的长链烷烃经裂解获得。热裂解在高温高压下进行，主要得到烯烃；催化裂解使用沸石催化剂，在中等温度下进行，同时生成支链烷烃和芳香族化合物。理解烯烃的经济需求有助于解释为何裂解是关键的工业过程。
11. Summary and Common Pitfalls | 总结与常见易错点
- Misidentifying the electrophile: In Br₂ addition, the electrophile is the polarised Br atom, not Br⁻. In HBr addition, it is H⁺. Always show the polarisation of Br–Br in your mechanism.
- 认为亲电试剂是Br⁻或H⁻： 在Br₂加成中，亲电试剂是极化的δ+ Br原子，而非Br⁻。HBr加成中则是H⁺。务必在机理中表示出Br–Br的极化。
- Confusing E/Z with cis/trans: E/Z is the modern system that works for any alkene, whereas cis/trans is limited. Always use Cahn–Ingold–Prelog rules for E/Z assignment.
- 混淆E/Z与顺/反： E/Z是现代命名法，适用于任何烯烃；顺/反则有限制。务必用CIP规则确定E/Z。
- Forgetting Markovnikov’s rule: With unsymmetrical alkenes, always draw the major product arising from the more stable carbocation, unless specifically asked for the minor product.
- 遗漏马氏规则： 遇到不对称烯烃，除非要求写次要产物，否则都应画出经由更稳定碳正离子的主要产物。
- Incomplete mechanism drawings: In AQA mechanisms, every curly arrow must start from a bond or a lone pair. Missing the arrow from the bromide ion to the carbocation, or forgetting to show the Br⁻ ion, loses marks.
- 机理图不完整： AQA机理中，每个弯箭头必须从键或孤对电子出发。漏掉溴离子进攻碳正离子的箭头，或忘记画出Br⁻离子，都会失分。
- Polymer repeating units: The repeating unit must have the correct connectivity and the subscript ‘n’ outside the bracket. Show the extended bonds through the brackets.
- 聚合物的重复单元： 重复单元连接方式必须正确，括号外加下标’n’，并画出贯穿括号的延伸键。
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