📚 Alkenes Revision for A-Level CIE Chemistry | A-Level CIE 化学：烯烃 考点精讲
Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C). They are a cornerstone of organic chemistry in the CIE A-Level syllabus, appearing in questions on structure, bonding, isomerism, and a rich variety of addition reactions. Understanding the reactivity of the electron-rich double bond is essential for mastering mechanisms and predicting products. This article provides a focused, bilingual breakdown of all key concepts you need to excel in the exam.
烯烃是含有至少一个碳碳双键 (C=C) 的不饱和烃。它们是 CIE A-Level 有机化学的基石，常出现在结构、键合、异构现象和多种加成反应的考题中。理解富电子双键的反应活性，对于掌握反应机理和预测产物至关重要。本文将对所有你需要掌握的考点进行清晰的双语讲解，助你考场夺分。
1. Structure and Bonding in Alkenes | 烯烃的结构与键合
A carbon-carbon double bond consists of one sigma (σ) bond and one pi (π) bond. The σ bond is formed by the head-on overlap of sp² hybrid orbitals, while the π bond arises from the sideways overlap of adjacent p orbitals. This π bond restricts rotation, giving rise to geometrical isomerism.
碳碳双键由一个 σ 键和一个 π 键组成。σ 键由 sp² 杂化轨道的头对头重叠形成，而 π 键则由相邻 p 轨道的侧面重叠产生。这个 π 键限制了旋转，从而导致了顺反异构现象。
Each carbon atom in the C=C bond uses three sp² hybrid orbitals to form three σ bonds in a trigonal planar arrangement with bond angles of approximately 120°. The remaining unhybridised p orbital is perpendicular to this plane and participates in π bonding.
双键中的每个碳原子使用三个 sp² 杂化轨道形成三个 σ 键，呈平面三角形排列，键角约为 120°。剩余的一个未杂化 p 轨道垂直于该平面，参与 π 键的形成。
The electron density of the π bond lies above and below the plane of the molecule. This region of high electron density makes alkenes susceptible to attack by electrophiles (electron-pair acceptors), which is the basis of their characteristic addition reactions.
π 键的电子云分布在分子平面的上方和下方。这个高电子密度的区域使得烯烃容易受到亲电试剂（电子对接受体）的攻击，这是它们特征加成反应的基础。
2. General Formula and Nomenclature | 通式与命名
Alkenes with one double bond follow the general formula CₙH₂ₙ, which shows they have two fewer hydrogen atoms than the corresponding alkane (CₙH₂ₙ₊₂). Each double bond or ring structure corresponds to one degree of unsaturation.
含有单个双键的烯烃遵循通式 CₙH₂ₙ，表明它们比相应的烷烃 (CₙH₂ₙ₊₂) 少两个氢原子。每含一个双键或一个环状结构，对应于一个不饱和度。
The systematic IUPAC name of an alkene is derived from the parent alkane by changing the ending from ‘-ane’ to ‘-ene’. The position of the double bond is indicated by the lowest possible number placed before the ‘-ene’ suffix, e.g., but-1-ene and but-2-ene.
烯烃的系统命名由母体烷烃而来，将词尾 ‘-ane’ 改为 ‘-ene’。双键的位置用尽可能小的编号表示，放在 ‘-ene’ 后缀之前，例如 but-1-ene 和 but-2-ene。
When substituents are present, the carbon chain is numbered to give the double bond the lowest number, and substituents are named as prefixes with their positions, e.g., 3-methylbut-1-ene.
当有取代基时，碳链编号应使双键位次最小，取代基以其位置和名称作为前缀，例如 3-methylbut-1-ene。
3. Isomerism in Alkenes | 烯烃的异构现象
Alkenes exhibit both structural isomerism and stereoisomerism. Structural isomers occur when the carbon skeleton or the position of the double bond differs, e.g., but-1-ene and but-2-ene are position isomers.
烯烃表现出构造异构和立体异构。构造异构出现在碳骨架或双键位置不同时，例如 but-1-ene 和 but-2-ene 属于位置异构体。
Stereoisomerism in alkenes arises from the restricted rotation about the C=C bond, leading to geometrical (cis-trans) isomerism. For cis-trans isomerism to occur, each carbon of the double bond must be attached to two different groups.
烯烃的立体异构源于碳碳双键的旋转受限，导致几何（顺反）异构。要产生顺反异构，双键上的每个碳原子必须连接两个不同的基团。
The cis isomer has the two highest-priority groups on the same side of the double bond, while the trans isomer has them on opposite sides. This is formally assigned using the Cahn-Ingold-Prelog (CIP) rules as E/Z configuration, where E (entgegen) means opposite and Z (zusammen) means together.
顺式异构体中，两个优先基团在双键的同侧；反式异构体中，它们在对侧。正式命名使用 Cahn-Ingold-Prelog (CIP) 规则按 E/Z 构型标记，E (entgegen) 表示相反，Z (zusammen) 表示同侧。
4. Physical Properties of Alkenes | 烯烃的物理性质
Alkenes are non-polar molecules, so the only intermolecular forces are weak van der Waals’ (London dispersion) forces. This results in relatively low melting and boiling points that increase with molecular size due to greater surface contact and more electrons.
烯烃是非极性分子，分子间作用力仅为微弱的范德华力（色散力）。因此它们的熔点和沸点相对较低，并随分子体积增大、表面接触面积和电子数增多而升高。
Alkenes are insoluble in water because they cannot form hydrogen bonds with water molecules, but they dissolve readily in non-polar organic solvents such as hexane.
烯烃不溶于水，因为它们无法与水分子形成氢键，但易溶于非极性有机溶剂，如己烷。
Boiling points of cis and trans isomers can differ slightly: the cis isomer often has a slightly higher boiling point due to a small net dipole moment, whereas the trans isomer is more symmetrical and packs more efficiently in the solid state, sometimes giving a higher melting point.
顺反异构体的沸点可能略有不同：顺式异构体通常因存在小的净偶极矩而沸点稍高，而反式异构体更为对称，固态堆积更紧密，有时熔点更高。
5. Electrophilic Addition Mechanism | 亲电加成机理
The characteristic reaction of alkenes is electrophilic addition, in which the π bond is broken and two new σ bonds are formed. The mechanism proceeds via a carbocation intermediate (or a cyclic intermediate in the case of halogens) and is a key topic for CIE exam questions.
烯烃的特征反应是亲电加成，反应中 π 键断裂，形成两个新的 σ 键。机理通过碳正离子中间体（或卤素与烯烃的环状中间体）进行，是 CIE 考试的核心考查内容。
Step 1: The electrophile, attracted by the high electron density of the π bond, accepts a pair of electrons, forming a bond to one of the carbon atoms. This creates a carbocation on the other carbon and releases the leaving group (if part of the electrophile).
第一步：亲电试剂被 π 键的高电子密度吸引，接受一对电子，与其中一个碳原子成键。这使得另一个碳原子形成碳正离子，并释放离去基团（若亲电试剂含有）。
Step 2: The negatively charged species (or nucleophile, e.g., Br⁻, Cl⁻, HSO₄⁻) rapidly attacks the carbocation, donating a pair of electrons to form a second new σ bond, completing the addition.
第二步：带负电的物种（或亲核试剂，例如 Br⁻、Cl⁻、HSO₄⁻）迅速进攻碳正离子，提供一对电子形成第二个新的 σ 键，完成加成。
For symmetrical alkenes like ethene, only one product is possible. For unsymmetrical alkenes, the stability of the possible carbocation intermediates determines the major product, as explained by Markovnikov’s rule.
对于对称烯烃如乙烯，只可能生成一种产物。对于不对称烯烃，可能生成的碳正离子中间体的稳定性决定了主要产物，这由马氏规则解释。
6. Addition of Hydrogen Halides and Markovnikov’s Rule | 卤化氢加成与马氏规则
Hydrogen halides (HX, where X = Cl, Br, I) add across the double bond to form halogenoalkanes. With unsymmetrical alkenes, two regioisomers can form, but Markovnikov’s rule predicts the major product.
卤化氢（HX，其中 X = Cl、Br、I）与双键发生加成反应生成卤代烷。对于不对称烯烃，可能生成两种区域异构体，但马氏规则可预测主要产物。
Markovnikov’s rule: In the addition of HX to an unsymmetrical alkene, the hydrogen atom attaches to the carbon with the greater number of hydrogen atoms already attached (the less substituted carbon), and the halogen attaches to the more substituted carbon. This is because the more stable carbocation intermediate is formed preferentially.
马氏规则：在 HX 与不对称烯烃的加成中，氢原子加到已连有较多氢原子的碳（取代度较低的碳）上，卤原子加到取代度较高的碳上。这是因为更稳定的碳正离子中间体优先形成。
Carbocation stability order: tertiary (3°) > secondary (2°) > primary (1°) > methyl. The greater the number of alkyl groups attached to the positively charged carbon, the more the charge is dispersed through positive inductive effect and hyperconjugation, increasing stability.
碳正离子稳定性顺序：叔 (3°) > 仲 (2°) > 伯 (1°) > 甲基。连接到带正电碳上的烷基越多，通过正诱导效应和超共轭作用，电荷越分散，稳定性越高。
For example, addition of HBr to propene gives mainly 2-bromopropane (via a secondary carbocation) rather than 1-bromopropane (via a primary carbocation).
例如，HBr 与丙烯加成主要生成 2-溴丙烷（通过仲碳正离子），而不是 1-溴丙烷（通过伯碳正离子）。
7. Addition of Halogens (Bromination) | 卤素加成（溴化反应）
Alkenes react rapidly with halogens (Br₂, Cl₂) at room temperature in the dark. The reaction with bromine is particularly important as a test for unsaturation: the orange-brown colour of bromine water is decolourised as the addition product, a dibromoalkane, is colourless.
烯烃在室温暗处与卤素（Br₂、Cl₂）迅速反应。与溴的反应特别重要，可用于检验不饱和键：溴水的橙棕色褪去，因为加成产物二溴代烷是无色的。
The mechanism for bromine addition involves the formation of a cyclic bromonium ion intermediate, not a classical carbocation. The π electrons polarise the Br–Br bond, and the slightly positive bromine atom acts as the electrophile, while the bromide ion is released.
溴加成机理涉及环状溴鎓离子中间体的形成，而非经典碳正离子。π 电子使 Br–Br 键极化，略带正电的溴原子充当亲电试剂，同时释放出溴负离子。
The bromonium ion is then attacked from the opposite side by Br⁻ in an Sₙ2-like step, leading to overall anti addition. This means the two bromine atoms end up on opposite faces of the original double bond plane.
然后溴负离子从环对面进攻溴鎓离子，类似于 Sₙ2 过程，导致整体反式加成。这意味着两个溴原子最终位于原双键平面的两侧。
Adding bromine water to an alkene also produces a bromohydrin alongside the dibromoalkane, as water can compete as a nucleophile. This is relevant for CIE questions on competing reactions.
向烯烃中加入溴水，除了生成二溴代烷外，还会产生溴代醇，因为水可以作为亲核试剂参与竞争。这与 CIE 考试中关于竞争反应的考点相关。
8. Acid-Catalysed Hydration of Alkenes | 烯烃的酸催化水合
Alkenes react with steam in the presence of a strong acid catalyst, typically concentrated phosphoric acid (H₃PO₄) or sulfuric acid (H₂SO₄), to form alcohols. This is the industrial route for manufacturing ethanol from ethene.
烯烃在水蒸气和强酸催化剂（通常是浓磷酸 H₃PO₄ 或硫酸 H₂SO₄）存在下反应生成醇。这是工业上由乙烯制乙醇的路线。
The mechanism proceeds via electrophilic addition, with H⁺ (from the acid) acting as the electrophile. The carbocation formed then reacts with a water molecule, followed by loss of a proton to regenerate the acid catalyst.
机理遵循亲电加成，H⁺（来自酸）充当亲电试剂。生成的碳正离子再与水分子反应，随后失去一个质子重新生成酸催化剂。
The addition follows Markovnikov’s rule: for unsymmetrical alkenes, the major alcohol product has the –OH group on the more substituted carbon. For example, propene yields propan-2-ol as the major product, not propan-1-ol.
加成遵循马氏规则：对于不对称烯烃，主要醇产物中 –OH 基团连在取代度更高的碳上。例如，丙烯主要生成丙-2-醇，而非丙-1-醇。
Conditions: 300°C, 60–70 atm pressure, concentrated phosphoric acid catalyst. These conditions shift the equilibrium to favour the alcohol while maintaining a reasonable rate without excessive energy costs.
反应条件：300°C，60–70 atm 压力，浓磷酸催化剂。此条件使平衡向生成醇的方向移动，同时保持合适的反应速率且不过度耗能。
9. Oxidation Reactions of Alkenes | 烯烃的氧化反应
Alkenes can be oxidised under different conditions to yield a variety of products. Combustion (complete oxidation) produces CO₂ and H₂O, but alkenes often burn with a smokier flame than alkanes due to the higher carbon-to-hydrogen ratio.
烯烃可在不同条件下被氧化，生成多种产物。完全燃烧（彻底氧化）生成 CO₂ 和 H₂O，但由于碳氢比更高，烯烃燃烧时的火焰通常比烷烃更多黑烟。
Mild oxidation with cold, dilute, alkaline potassium manganate(VII) (KMnO₄) solution results in the formation of a diol (dihydroxylation). The purple colour of the manganate(VII) ion changes to dark brown as manganese(IV) oxide (MnO₂) precipitates. This reaction is also used as a test for unsaturation.
在冷的、稀的、碱性的高锰酸钾 (KMnO₄) 溶液中温和氧化，烯烃生成二醇（二羟基化）。高锰酸根离子的紫色变为深棕色，同时生成二氧化锰 (MnO₂) 沉淀。该反应也可用于检验不饱和键。
Stronger oxidation with hot, concentrated, acidified KMnO₄ leads to oxidative cleavage of the double bond. The products depend on the substitution pattern of the alkene: terminal =CH₂ groups are oxidised to CO₂ and H₂O, =CHR units become carboxylic acids, and =CRR’ units become ketones.
在热的、浓的、酸性 KMnO₄ 条件下的强氧化会导致双键断裂。产物取决于烯烃的取代情况：端基 =CH₂ 基团被氧化成 CO₂ 和 H₂O，=CHR 单元变成羧酸，=CRR’ 单元变成酮。
Ozonolysis (reaction with O₃ followed by reductive work-up) also cleaves the double bond and is used in structural determination. Each carbon of the double bond becomes part of a carbonyl group (aldehyde or ketone). This reaction is important in CIE analytical chemistry contexts.
臭氧分解（与 O₃ 反应后还原处理）也可断裂双键，常用于结构测定。双键上的每个碳原子都变成羰基的一部分（醛或酮）。该反应在 CIE 分析化学背景中很重要。
10. Polymerisation of Alkenes | 烯烃的聚合反应
Alkenes undergo addition polymerisation, in which the π bonds of monomer molecules open up and link together to form long saturated carbon chains. No other product is formed, so it is classified as addition polymerisation.
烯烃发生加成聚合反应，单体分子的 π 键打开并相互连接，形成长的饱和碳链。反应没有其他副产物，因此归类为加成聚合。
The monomer is typically ethene or a substituted ethene (e.g., propene, chloroethene, styrene). The polymer is named by putting ‘poly’ in front of the monomer name, e.g., poly(ethene), poly(propene).
单体通常是乙烯或取代乙烯（如丙烯、氯乙烯、苯乙烯）。聚合物的命名是在单体名称前加上 ‘poly’，如 poly(ethene)、poly(propene)。
Representing polymerisation equations requires drawing the repeating unit with the double bond opened and bonds extending through brackets. CIE often asks for the structure of the polymer from a given monomer, or the monomer from a polymer repeat unit.
书写聚合反应方程式时，需要画出打开双键后扩展延伸的重复单元，并用括号括起。CIE 常要求学生根据给定单体画出聚合物结构，或根据聚合物重复单元推导出单体。
The properties of addition polymers depend on the nature of the side groups and the chain length. Poly(ethene) is flexible and used for plastic bags, while poly(chloroethene) (PVC) is harder and used for pipes. Disposal and recycling issues are related contexts in CIE chemistry.
加成聚合物的性质取决于侧基性质和链长。聚乙烯柔韧，用于塑料袋；聚氯乙烯 (PVC) 较硬，用于管道。废弃处理与回收利用是 CIE 化学的相关情境考点。
11. Testing for Alkenes: Unsaturation Tests | 烯烃检验：不饱和性测试
Two main chemical tests distinguish alkenes from alkanes and other saturated compounds: the bromine water test and the acidified potassium manganate(VII) test. Both rely on the reactivity of the double bond.
区分烯烃与烷烃及其他饱和化合物的主要化学检验方法有两种：溴水测试和酸性高锰酸钾测试。两者都基于双键的反应活性。
Bromine water test: Add a few drops of orange-brown bromine water to the sample and shake. If an alkene is present, the solution rapidly turns colourless. This is due to electrophilic addition forming a dibromoalkane (and possibly a bromohydrin). Alkanes do not react in the dark without UV light.
溴水测试：向试样中加入几滴橙棕色溴水并振荡。若存在烯烃，溶液迅速变为无色。这是因为发生了亲电加成，生成了二溴代烷（也可能生成溴代醇）。烷烃在无紫外光照的黑暗条件下不反应。
Acidified KMnO₄ test: Add a few drops of purple acidified potassium manganate(VII) solution. With alkenes, the purple colour disappears and a colourless solution (Mn²⁺ ions) or brown precipitate (MnO₂) is observed, depending on conditions. This test is also positive for other easily oxidisable functional groups, so it is less specific.
酸性高锰酸钾测试：加入几滴紫色的酸性高锰酸钾溶液。对于烯烃，紫色消失，根据条件可能出现无色溶液（Mn²⁺ 离子）或棕色沉淀 (MnO₂)。该测试对其他易氧化的官能团也呈阳性，因此专一性较低。
12. Summary of CIE Exam Tips for Alkenes | CIE 烯烃考点总结
Ensure you can draw and label the σ and π bond formation clearly. Always show the partial charges and curly arrows correctly in mechanisms, starting from the middle of the π bond or from a lone pair, and terminating at the electron-deficient atom.
务必能清晰绘制并标注 σ 键和 π 键的形成。在机理中准确表示部分电荷和弯箭头：从 π 键中间或孤对电子出发，指向缺电子的原子。
Remember Markovnikov’s rule and be able to explain it by comparing the stability of carbocation intermediates. Use terms like ‘positive inductive effect’ and ‘hyperconjugation’ for top marks.
牢记马氏规则，并能通过比较碳正离子中间体的稳定性加以解释。使用“正诱导效应”和“超共轭”等术语来获取高分。
Be familiar with the conditions and products for hydration and oxidative cleavage. For ozonolysis, practice deducing the original alkene structure from the carbonyl products given.
熟悉水合和氧化裂解的条件与产物。对于臭氧分解，练习从给定的羰基产物反推出原始烯烃结构。
In polymerisation questions, pay close attention to drawing the repeating unit correctly, with bonds extending through the brackets, and ensure the monomer-to-polymer transformation is consistent with addition polymerisation.
在聚合题中，注意准确绘制重复单元，让化学键穿出括号，并确保单体到聚合物的转化符合加成聚合的特点。
When describing test results, always include the colour change and the species responsible for the colour. For bromine water, it’s the disappearance of Br₂ colour; for KMnO₄, it’s the decolourisation of MnO₄⁻ ions.
描述测试结果时，务必包括颜色变化以及对应的有色物种。溴水测试是 Br₂ 颜色消失；高锰酸钾测试是 MnO₄⁻ 离子褪色。
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