📚 Alkenes: A Comprehensive Exam Guide | 烯烃：全面考点精讲
Alkenes form one of the most conceptually rich and synthetically important topics in IB and CIE A‑Level Chemistry. Their chemistry is defined by the presence of a carbon–carbon double bond, which makes them far more reactive than alkanes. In examinations, you will be tested on IUPAC naming, stereoisomerism, electrophilic addition mechanisms, Markovnikov’s rule, and a wide range of addition and oxidation reactions. This article breaks down every key learning objective into clear, bilingual sections, equipping you for top marks.
烯烃是 IB 和 CIE A‑Level 化学中概念最丰富、合成应用最广泛的主题之一。碳碳双键决定了烯烃的化学性质，使其远比烷烃活泼。考试将考查 IUPAC 命名、立体异构、亲电加成机理、马氏规则以及多种加成与氧化反应。本文以清晰的中英双语形式拆解每一个核心考点，助你冲击高分。
1. Structure and Bonding | 结构与成键
The functional group of an alkene is the C=C double bond, consisting of one sigma (σ) bond and one pi (π) bond. The σ bond arises from head‑on overlap of sp² hybrid orbitals, while the π bond results from sideways overlap of adjacent unhybridized p orbitals. The two carbon atoms and the four atoms directly attached to them lie in the same plane, with bond angles of approximately 120°. The π electron cloud sits above and below the plane, making alkenes electron‑rich and susceptible to attack by electrophiles.
烯烃的官能团是碳碳双键 C=C，由一个 σ 键和一个 π 键组成。σ 键来自 sp² 杂化轨道的头对头重叠，π 键则来自相邻未杂化 p 轨道的肩并肩重叠。双键上的两个碳原子以及与它们直接相连的四个原子共平面，键角约为 120°。π 电子云分布在平面的上下方，使烯烃成为富电子体系，容易受到亲电试剂的进攻。
The restricted rotation around the C=C double bond is a direct consequence of the π bond geometry; rotating one end would break the sideways overlap, requiring a large activation energy. This restriction gives rise to geometrical isomerism (cis‑trans / E‑Z).
C=C 双键不能自由旋转，这是由 π 键的几何特点直接导致的；旋转会破坏 p 轨道的肩并肩重叠，需要较高的活化能。这种限制正是几何异构（顺反 / E‑Z 异构）产生的根本原因。
2. General Formula and Homologous Series | 通式与同系物
Alkenes with one double bond follow the general formula CₙH₂ₙ. They form a homologous series where each successive member differs by a –CH₂– group. Physical properties, such as boiling point and viscosity, increase gradually with chain length owing to stronger London dispersion forces.
含一个双键的烯烃通式为 CₙH₂ₙ。它们构成同系物，相邻成员相差一个 –CH₂– 基团。随着碳链增长，色散力增强，沸点、黏度等物理性质呈规律性变化。
Cycloalkenes with one double bond have the formula CₙH₂ₙ₋₂, while polyenes and alkadienes require formulas reflecting the number of double bonds. Be precise in writing molecular formulas in exam answers.
含一个双键的环烯烃通式为 CₙH₂ₙ₋₂，而多烯和二烯需根据双键个数给出分子式。考试中务必准确书写分子式，避免扣分。
3. IUPAC Nomenclature | IUPAC 命名规则
Select the longest continuous chain that contains the C=C bond. Replace the –ane suffix of the corresponding alkane with –ene. Number the chain from the end that gives the double bond the lowest possible locant. The locant is placed immediately before –ene (e.g., pent‑1‑ene, not 1‑pentene). For cycloalkenes, the double bond is assumed to be between C1 and C2, so numbering starts there.
选取包含 C=C 双键的最长碳链为主链，将对应烷烃词尾 ‑ane 改为 ‑ene。从靠近双键的一端开始编号，使双键位次最小。位次写在 ‑ene 之前（如 pent‑1‑ene，而非 1‑pentene）。环烯烃默认双键位于 C1 和 C2 之间，并由此处开始编号。
When substituents are present, they are listed alphabetically with their respective locants. For dienes, use –diene and indicate both positions. If the double bond and substituents conflict, the double bond takes priority for lowest numbering.
支链按字母顺序列出，并标注位次。对于二烯烃，使用 –diene 并标出两个双键位置。当双键与取代基编号冲突时，优先保证双键位次最小。
4. Geometrical Isomerism (E/Z and Cis‑Trans) | 几何异构（E/Z 与顺反异构）
Geometric isomerism arises because restricted rotation about the C=C bond prevents interconversion. Cis‑trans terminology applies when each carbon of the double bond carries two different groups, and at least one pair of identical groups exists across the double bond. Cis isomers have the higher‑priority groups on the same side; trans isomers have them opposite.
由于双键旋转受阻，两组原子/基团无法互相转换，由此产生几何异构。当双键两端碳原子各连有两个不同基团，且两端存在至少一对相同基团时，可使用顺反命名。顺式 (cis) 中高优先级基团在同侧，反式 (trans) 中在异侧。
The Cahn‑Ingold‑Prelog (CIP) system assigns E/Z unambiguously. Rank the substituents at each carbon by atomic number; if the highest‑priority groups are on opposite sides, the configuration is E (entgegen); if on the same side, it is Z (zusammen). This system is compulsory for compounds with four different substituents.
Cahn‑Ingold‑Prelog (CIP) 规则通过原子序数排定基团优先级，可严格区分构型。两端优先级较高的基团位于异侧为 E (entgegen)，同侧为 Z (zusammen)。当四个取代基均不同时，必须用 E/Z 命名。
5. Electrophilic Addition Mechanism | 亲电加成机理
The characteristic reaction of alkenes is electrophilic addition (AE), in which the π bond attracts an electrophile and forms two new σ bonds. The mechanism generally proceeds in two steps:
烯烃的特征反应是亲电加成 (AE)，π 键吸引亲电试剂并生成两个新的 σ 键。机理通常分两步进行：
Step 1: The electrophile E⁺ is attracted to the electron‑rich π bond. A carbocation intermediate is formed when the π bond breaks and forms a bond to the electrophile, while the other carbon becomes positively charged.
第一步：亲电试剂 E⁺ 被富电子的 π 键吸引。π 键断裂，一端与亲电试剂成键，另一端形成带正电的碳正离子中间体。
Step 2: The negatively charged nucleophile (or a neutral nucleophile like H₂O) rapidly attacks the carbocation, donating a pair of electrons to form the second σ bond.
第二步：带负电荷的亲核试剂（或中性的 H₂O 等）迅速进攻碳正离子，提供一对电子生成第二个 σ 键。
Heterolytic bond cleavage of the reagent (e.g., Br–Br → Br⁺ + Br⁻) often generates the electrophile; this can be assisted by the polarisation induced by the approaching π electrons.
试剂常通过异裂产生亲电体（如 Br–Br → Br⁺ + Br⁻），这一过程可被烯烃 π 电子云产生的诱导极化所促进。
6. Stability of Carbocations and Markovnikov’s Rule | 碳正离子稳定性与马氏规则
Carbocation stability follows the order: tertiary (3°) > secondary (2°) > primary (1°) > methyl. Alkyl groups donate electron density through the inductive effect and hyperconjugation, dispersing the positive charge and stabilising the intermediate. Whenever an unsymmetrical alkene reacts with an unsymmetrical reagent, the more stable carbocation pathway dominates.
碳正离子稳定性顺序：叔 (3°) > 仲 (2°) > 伯 (1°) > 甲基。烷基通过诱导效应和超共轭效应提供电子云，分散正电荷，使中间体更稳定。当不对称烯烃与不对称试剂反应时，倾向于经由更稳定的碳正离子路径进行。
Markovnikov’s rule summarises this behaviour: in the addition of HX to an unsymmetrical alkene, the hydrogen atom attaches to the carbon that already has the greater number of hydrogen atoms (the less substituted carbon), while X attaches to the more substituted carbon. This results from preferential formation of the more stable carbocation.
马氏规则正是该规律的总结：在 HX 与不对称烯烃的加成中，氢原子加在含氢较多的碳上（取代度较低的碳），X 则加在取代度较高的碳上。其本质是优先生成更稳定的碳正离子。
7. Addition Reactions: Hydrogenation, Halogenation, and Hydrohalogenation | 加成反应：加氢、卤化与加卤化氢
Hydrogenation (H₂, Pt/Pd/Ni, heat): Alkene + H₂ → alkane. The reaction is exothermic and used to compare alkene stability via heats of hydrogenation. A more substituted alkene is thermodynamically more stable and releases less energy upon hydrogenation.
加氢 (H₂, Pt/Pd/Ni, 加热): 烯烃 + H₂ → 烷烃。反应放热，通过氢化热可比较烯烃的热力学稳定性。取代度越高的烯烃越稳定，氢化放热越少。
Halogenation (Br₂ or Cl₂, inert solvent): Alkene + X₂ → vicinal dihalide. Bromine water (orange → colourless) serves as a simple test for unsaturation. The mechanism proceeds through a cyclic bromonium (or chloronium) ion, leading to anti‑addition.
卤化 (Br₂ 或 Cl₂, 惰性溶剂): 烯烃 + X₂ → 邻二卤代物。溴水褪色（橙 → 无色）是检验不饱和键的经典方法。反应经环状溴鎓离子（或氯鎓离子）中间体，产物为反式加成。
Hydrohalogenation (HX, where X = Cl, Br, I): Alkene + HX → haloalkane, governed by Markovnikov’s rule unless peroxides are present. With HBr and peroxides, the anti‑Markovnikov product forms via a free‑radical mechanism.
加卤化氢 (HX，X = Cl, Br, I): 烯烃 + HX → 卤代烷，除有过氧化物外，均遵循马氏规则。HBr 在过氧化物存在下通过自由基机理生成反马氏产物。
8. Hydration of Alkenes | 烯烃的水合反应
Industrial hydration of ethene to ethanol uses phosphoric acid catalyst on a silica support at ~300°C and 60–70 atm: C₂H₄ + H₂O ⇌ C₂H₅OH. This is a reversible, acid‑catalysed electrophilic addition. Laboratory hydration of alkenes uses concentrated sulfuric acid followed by dilution and heating (indirect hydration). The intermediate is an alkyl hydrogensulfate, which is then hydrolysed.
工业上乙烯水合制乙醇采用磷酸/硅胶催化剂，~300°C、60–70 atm：C₂H₄ + H₂O ⇌ C₂H₅OH。这是酸催化、可逆的亲电加成。实验室则先用浓硫酸吸收烯烃生成硫酸氢酯，再加水加热水解（间接水合），中间体为烷基硫酸氢酯。
Both routes produce alcohols according to Markovnikov’s orientation where applicable; e.g., propene gives propan‑2‑ol, not propan‑1‑ol. The reaction is vital for the synthesis of alcohols from petroleum fractions.
两种方法在适用时均得到马氏规则产物，如丙烯水合生成 2‑丙醇而非 1‑丙醇。该反应是利用石油馏分合成醇类的重要途径。
9. Oxidation Reactions of Alkenes | 烯烃的氧化反应
(a) Mild oxidation with cold, dilute, alkaline KMnO₄: Alkene + [O] → diol (syn‑addition). The purple permanganate is reduced to brown MnO₂ precipitate; the reaction is used as the Baeyer test for unsaturation.
(a) 冷、稀、碱性 KMnO₄ 温和氧化： 烯烃 + [O] → 邻二醇（顺式加成）。紫色的高锰酸钾被还原成棕色 MnO₂ 沉淀，即贝耶尔试验，用于不饱和键检验。
(b) Vigorous oxidation with hot, concentrated, acidified KMnO₄ or acidified K₂Cr₂O₇: The double bond is cleaved oxidatively. Unsubstituted =CH₂ is oxidised to CO₂ and H₂O; =CHR gives a carboxylic acid; =CRR′ gives a ketone. This pattern enables structural elucidation of unknown alkenes from the fragments obtained.
(b) 热、浓、酸性 KMnO₄ 或酸化 K₂Cr₂O₇ 强氧化： 双键断裂。端基 =CH₂ 被氧化为 CO₂ 和 H₂O；=CHR 生成羧酸；=CRR′ 生成酮。通过断裂产物可反推未知烯烃结构，是结构分析的重要手段。
(c) Ozonolysis: O₃, then Zn/H₂O (reductive workup) cleaves the double bond to carbonyls. This reaction, like vigorous oxidation, yields aldehydes (or ketones) depending on substitution.
(c) 臭氧化： O₃ 然后 Zn/H₂O（还原后处理）将双键断裂为羰基化合物。与强氧化类似，产物为醛或酮取决于取代情况。
10. Polymerisation of Alkenes | 烯烃的聚合反应
Addition polymerisation converts alkene monomers into long‑chain polymers, retaining all atoms of the monomer. The π bond breaks, and monomers link via single bonds. Poly(ethene), poly(propene), poly(chloroethene) (PVC), and poly(tetrafluoroethene) (PTFE) are classic examples.
加成聚合将烯烃单体转化为长链聚合物，单体原子全部保留。π 键打开，单体通过单键连接。聚乙烯、聚丙烯、聚氯乙烯 (PVC) 和聚四氟乙烯 (PTFE) 是典型例子。
The process can be initiated by free radicals, cations, or coordination catalysts (Ziegler–Natta). The tacticity (isotactic, syndiotactic, atactic) of the polymer affects its crystallinity and physical properties. Disposal and recycling of polymers are highly relevant to environmental chemistry, a frequent context‑based question.
聚合可由自由基、阳离子或配位催化剂（齐格勒‑纳塔）引发。聚合物的立构规整度（全同、间同、无规）影响其结晶性和物理性质。高分子材料的处理与回收与环境化学紧密相关，常作为情景题出现。
11. Laboratory Preparation and Test for Alkenes | 烯烃的实验室制备与检验
Alkenes are commonly prepared in the lab by (i) dehydration of alcohols using heated Al₂O₃ or concentrated H₂SO₄, and (ii) dehydrohalogenation of haloalkanes with hot ethanolic KOH. Both are elimination reactions and produce an alkene via a Zaitsev‑oriented pathway when possible.
实验室常通过以下两种消除反应制备烯烃：(i) 醇在加热的 Al₂O₃ 或浓 H₂SO₄ 催化下脱水；(ii) 卤代烷与热的氢氧化钾乙醇溶液发生脱卤化氢。若有两种以上烯烃可能，主要产物遵循扎伊采夫规则（生成较稳定烯烃）。
The definitive test for an alkene is the decolorisation of bromine water (orange to colourless, addition reaction). The Baeyer test (purple KMnO₄ to brown MnO₂) is also used but is less specific. Oxygen‑containing compounds that decolourise bromine water by substitution (e.g., phenols) can sometimes be distinguished by the absence of HBr fumes in the bromine reaction (but careful in exam context).
烯烃确证性检验是溴水褪色（橙→无色，亲电加成）。贝耶尔试验（紫色 KMnO₄ → 棕色 MnO₂）也常用，但专一性稍差。含氧化合物如酚类可使溴水通过取代反应褪色，但会同时生成 HBr 白雾（考试需注意区分）。
12. Summary Comparison: Alkanes vs Alkenes | 烷烃与烯烃对比总结
| Property / Reaction | Alkanes | Alkenes |
|---|---|---|
| Functional group | C–C and C–H single bonds | At least one C=C double bond |
| General formula | CₙH₂ₙ₊₂ | CₙH₂ₙ |
| Typical reactions | Free‑radical substitution, combustion | Electrophilic addition, oxidation, polymerisation |
| Reactivity | Low; requires UV light or heat for substitution | High; readily attacked by electrophiles |
| Bromine water test | No immediate colour change (requires UV) | Rapid decolorisation (orange → colourless) |
| Baeyer test (cold KMnO₄) | No reaction | Purple → brown precipitate |
Understanding these differences forms the backbone of most functional‑group‑based questions. The double bond not only dictates the reactivity pattern but also introduces stereochemical complexity that examiners love to probe. Master the electrophilic addition mechanism, the stability arguments behind Markovnikov’s rule, and the oxidation‑cleavage patterns, and you will be equipped to handle any alkene problem in IB and CIE examinations.
掌握烷烃与烯烃的核心差异是攻克官能团考题的基础。双键不仅决定了反应类型，还引入了立体化学的复杂性，这些都是考试的热点。吃透亲电加成机理、马氏规则背后的稳定性逻辑以及氧化断键规律，你就能从容应对 IB 与 CIE 化学中任何烯烃考题。
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