📚 Alkenes: Key Exam Points for IB and AQA Chemistry | IB AQA 化学：烯烃 考点精讲
Alkenes are a cornerstone of organic chemistry, whether you're tackling IB Higher Level or AQA A-Level specifications. Understanding their structure, reactivity, and mechanisms is essential for predicting products, justifying regioselectivity, and connecting reaction types. This article systematically covers every high-yield exam point on alkenes, blending IB and AQA requirements, with clear explanations, mechanisms, and examples to strengthen your exam responses.
无论你是在攻克 IB 高级课程还是 AQA A-Level 考试，烯烃都是有机化学的核心基石。掌握它们的结构、反应活性与机理，对于预测产物、解释区域选择性以及串联各反应类型至关重要。本文系统梳理了烯烃每个高频考点，融合 IB 与 AQA 的要求，通过清晰的解释、机理与示例，助你提升应试能力。
1. Structure and Bonding in Alkenes | 烯烃的结构与键合
Alkenes contain at least one carbon–carbon double bond consisting of one σ (sigma) bond and one π (pi) bond. The σ bond is formed by head-on overlap of sp² hybrid orbitals, while the π bond arises from sideways overlap of adjacent p orbitals. This arrangement restricts rotation, giving rise to geometric (cis/trans) isomerism.
烯烃至少含有一个碳碳双键，它由一个 σ 键和一个 π 键组成。σ 键由 sp² 杂化轨道正面重叠形成，而 π 键源于相邻 p 轨道的侧向重叠。这种成键方式限制了旋转，从而产生了几何（顺/反）异构现象。
The carbon atoms in the double bond adopt sp² hybridization with trigonal planar geometry and bond angles of approximately 120°. Two of the sp² orbitals form σ bonds with two other atoms, while the third sp² orbital holds a non-bonding electron pair in carbocations or forms another σ bond. The remaining unhybridized p orbital on each carbon overlaps to create the π bond, which has electron density above and below the plane of the molecule.
双键上的碳原子采取 sp² 杂化，呈平面三角形，键角约 120°。其中两个 sp² 轨道与其他原子形成 σ 键，第三个 sp² 轨道在碳正离子中容纳孤对电子或形成另一个 σ 键。每个碳原子上未参与杂化的 p 轨道相互重叠形成 π 键，其电子云分布在分子平面的上方和下方。
The π bond is weaker than the σ bond; average bond energies: C– 267 kJ mol⁻¹. The electron-rich π cloud makes alkenes vulnerable to electrophilic attack.
π 键弱于 σ 键；平均键能：C–C σ ≈ 347 kJ mol⁻¹，C=C π ≈ 267 kJ mol⁻¹。富电子的 π 云使烯烃极易受到亲电试剂的进攻。
2. Nomenclature and Isomerism | 烯烃的命名与异构
IUPAC naming for alkenes follows standard rules with priority for the longest chain containing the double bond. Numbering gives the double bond the lowest possible locant, indicated by the prefix 'en' and a number before the parent chain (e.g., but-1-ene, but-2-ene). Substituents are named as usual with their positions. For cyclic alkenes, the double bond is assumed to be between C1 and C2.
烯烃的 IUPAC 命名遵循标准规则，优先选取含双键的最长链。编号时使双键位次尽可能小，以“-烯”及数字前缀表示（如丁-1-烯、丁-2-烯）。取代基按常规方式标明位置。对于环烯烃，双键默认为位于 C1 与 C2 之间。
. Cis/trans notation is used when each double-bond carbon bears two different groups. If one carbon carries identical groups, cis/trans is not applicable. The more general E/Z system uses Cahn–Ingold– e higher priority groups are on the same side, E (entgegen) if they are on opposite sides.
由于 C=C 双键旋转受限而产生几何异构。当双键的每个碳原子上连有两个不同基团时，可使用顺/反标记法。若某个碳上连有两个相同基团，则不适用顺/反标记。更通用的 E/Z 系统采用 Cahn-Ingold-Prelog 次序规则：若高优先基团在同侧则为 Z（同侧），在异侧则为 E（对侧）。
Exam tip: For molecules like 2-methylbut-2-ene, one double-bond carbon has two methyl groups – no geometric isomerism. Distinguish constitutional isomers (e.g., position of double bond, branching) from stereoisomers clearly.
考试提示：对于像 2-甲基丁-2-烯这样的分子，其中一个双键碳连有两个甲基——因此不存在几何异构。务必清楚区分构造异构体（如双键位置、支链）与立体异构体。
3. Electrophilic Addition Mechanism | 亲电加成机理
The dominant reaction of alkenes is electrophilic addition, in which the π bond acts as a nucleophile and attacks an electrophile. The mechanism proceeds in two steps: (1) formation of a carbocation intermediate by addition of the electrophile to one carbon of the double bond, and (2) rapid attack by a nucleophile on the carbocation to yield the saturated product.
烯烃的主要反应是亲电加成，其中 π 键作为亲核试剂进攻亲电试剂。该机理分两步进行：(1) 亲电试剂加到双键的一个碳上，形成碳正离子中间体；(2) 亲核试剂迅速进攻碳正离子，得到饱和产物。
Step 1: The π electrons attack the electrophile E⁺, breaking the π bond and forming a new σ bond between one carbon and the electrophile. The other carbon becomes a carbocation. This is the rate-determining step.
第一步：π 电子进攻亲电试剂 E⁺，π 键断裂，在一个碳与亲电试剂间形成新的 σ 键。另一个碳形成碳正离子。这是决定反应速率的步骤。
Step 2: The nucleophile (often the counterion or solvent) donates a pair of electrons to the carbocation, forming a second σ bond and completing the addition.
第二步：亲核试剂（通常为抗衡离子或溶剂）向碳正离子提供一对电子，形成第二个 σ 键，完成加成。
CH₂Br. The electrophile is H⁺ (from HBr polarization), carbocation is CH₃CH₂⁺, nucleophile is Br⁻.
示例：乙烯与 HBr 加成得到溴乙烷：H₂C=CH₂ + HBr → CH₃CH₂Br。其中亲电试剂为 H⁺（来自于 HBr 极化），碳正离子为 CH₃CH₂⁺，亲核试剂为 Br⁻。
Students must show curly arrows correctly: from the π bond to the electrophile (or the H atom of H–X), then from the nucleophile's lone pair to the carbocation. IB and AQA both require accurate mechanistic curly arrows in exams.
考生需正确使用弯箭头：从 π 键指向亲电试剂（或 H–X 中的 H），再从亲核试剂的孤对电子指向碳正离子。IB 与 AQA 考试均要求准确画出机理弯箭头。
4. Carbocation Stability and Markovnikov's Rule | 碳正离子稳定性与马氏规则
Carbocations are classified as primary, secondary, or tertiary based on the number of alkyl groups attached to the positively charged carbon. Stability increases with alkyl substitution due to the +I (inductive) effect and hyperconjugation: tertiary > secondary > primary > methyl. This stability order directly governs the regioselectivity of electrophilic addition to unsymmetrical alkenes.
碳正离子根据带正电碳所连烷基数分为伯、仲、叔。由于 +I 效应和超共轭作用，稳定性随烷基取代增多而增强：叔碳正离子 > 仲 > 伯 > 甲基。这一稳定顺序直接决定了不对称烯烃亲电加成的区域选择性。
Markovnikov's rule: In the addition of H–X to an unsymmetrical alkene, the hydrogen atom attaches to the carbon with the greater number of hydrogen atoms originally, i.e., the carbon that forms the more stable carbocation intermediate. The halogen (or other group) ends up on the more substituted carbon.
马氏规则：在不对称烯烃与 H–X 加成时，氢原子加在原来含氢较多的碳上，即生成更稳定碳正离子中间体的那个碳。卤素（或其他基团）最终加在取代较多的碳上。
ane (major), not 1-bromopropane. The secondary carbocation intermediate is more stable than the primary one.
示例：丙烯 + HBr → 2-溴丙烷（主要产物），而非 1-溴丙烷。仲碳正离子中间体比伯碳正离子更稳定。
Electrophilic addition of water (hydration) also follows Markovnikov orientation because the mechanism involves a carbocation intermediate. However, hydration requires an acid catalyst (e.g., concentrated H₂SO₄ or H₃PO₄) and proceeds via protonation of the alkene, followed by water attack and deprotonation.
水的亲电加成（水合）同样遵循马氏规则取向，因为其机理也经过碳正离子中间体。但水合需要酸催化剂（如浓 H₂SO₄ 或 H₃PO₄），经历烯烃质子化、水进攻以及去质子化步骤。
5. Addition of Hydrogen (Catalytic Hydrogenation) | 加氢反应（催化氢化）
Alkenes react with hydrogen gas in the presence of a metal catalyst (Pt, Pd, or Ni) to form alkanes. The reaction is an example of heterogeneous catalysis, where the alkene and hydrogen adsorb onto the metal surface, weakening the H–H and π bonds.
烯烃在金属催化剂（Pt、Pd 或 Ni）存在下与氢气反应生成烷烃。此反应为多相催化的一个实例，烯烃和氢气吸附于金属表面，削弱 H–H 键和 π 键。
The overall enthalpy change is exothermic because a π bond and an H–H σ bond are broken, while two C–H σ bonds and a C–C σ bond are formed. Typical values: ΔHᵒ = –120 to – determine the number of double bonds in an unknown compound by measuring hydrogen uptake.
总反应焓变为放热，因为断裂了一个 π 键和一个 H–H σ 键，同时形成了两个 C–H σ 键和一个 C–C σ 键。简单烯烃的 ΔHᵒ 为 –120 至 –130 kJ mol⁻¹。催化氢化常通过测量氢气消耗量来确定未知化合物中的双键数目。
This reaction has no regioselectivity issues because both carbons ultimately gain a hydrogen. It is a syn addition on the catalyst surface, although stereochemistry is often not required at AQA/IB level unless discussing fats and oils.
该反应不存在区域选择性问题，因为两个碳最终各加一个氢。在催化剂表面是顺式加成，但 AQA/IB 阶段通常不要求分析立体化学，除非涉及油脂硬化。
6. Addition of Halogens (Bromination) | 卤素加成（溴化）
Alkenes decolorize bromine water or bromine in CCl₄ at room temperature, a classic test for unsaturation. The reaction is an electrophilic addition where the Br₂ molecule is polarized as it approaches the π cloud.
烯烃在室温下可使溴水或溴的四氯化碳溶液褪色，这是检验不饱和度的经典方法。反应为亲电加成，当 Br₂ 靠近 π 云时发生极化。
Mechanism: The π electrons induce a dipole in Br₂, making the nearer Br electron-poor (Brᵟ⁺). The alkene attacks this Br, expelling Br⁻, but simultaneously a cyclic bromonium ion intermediate is formed (bridged three-membered ring). The bromide ion then attacks from the opposite face of the bromonium ion, giving anti addition (trans product).
机理：π 电子在 Br₂ 中诱导出偶极，使较近的 Br 缺电子 (Brᵟ⁺)。烯烃进攻该 Br，同时形成环状溴鎓离子中间体（桥式三元环）。溴离子随后从溴鎓离子的反面进攻，得到反式加成产物。
ns-1,2-dibromocyclopentane. This anti stereospecificity is a key difference from addition of H– ures). IB often asks for the mechanism with the bromonium ion; AQA may accept either carbocation or bromonium ion depending on the context, but the bromonium ion explains the stereochemistry.
例如，环戊烯 + Br₂ → 反式-1,2-二溴环戊烷。这种反式立体专一性是卤素加成区别于 H–X 加成的重要特征（后者可能产生对映体或混合物）。IB 常要求画出含溴鎓离子的机理；AQA 可根据上下文接受碳正离子或溴鎓离子，但溴鎓离子能更好地解释立体化学。
7. Addition of Hydrogen Halides and Sulfuric Acid | 卤化氢与硫酸的加成
Dry hydrogen halides (HCl, HBr, HI) react rapidly with alkenes at room temperature to give haloalkanes. Reactivity order: HI > HBr > HCl, correlating with H–X bond strength. Gaseous HCl sometimes requires an AlCl₃ catalyst. The mechanism is straightforward electrophilic addition via carbocation, subject to Markovnikov orientation.
干燥的卤化氢（HCl、HBr、HI）在室温下与烯烃迅速反应生成卤代烷。反应活性顺序为 HI > HBr > HCl，这与 H–X 键强度相关。HCl 气体有时需要 AlCl₃ 催化剂。机理为经碳正离子中间体的标准亲电加成，遵循马氏规则取向。
Cold concentrated sulfuric acid also adds to alkenes, forming alkyl hydrogensulfates. The electrophile is effectively H⁺, generating a carbocation, which then attacks HSO₄⁻. Subsequent hydrolysis with water yields an alcohol; this is an indirect hydration method. The addition follows Markovnikov's rule, and overall anti-Markovnikov products are not obtained without special reagents (e.g., hydroboration-oxidation).
冷的浓硫酸也可与烯烃加成，生成硫酸氢烷基酯。有效亲电试剂为 H⁺，形成碳正离子后进攻 HSO₄⁻。随后用水水解即得醇，这是一种间接水合法。加成遵循马氏规则，若无特殊试剂（如硼氢化-氧化），无法获得反马氏产物。
Exam Tip: Be prepared to explain why 2-bromopropane is the major product from propene with HBr but 1-bromopropane is not. Use carbocation stability arguments, and cite Markovnikov's rule or the electrophilic addition mechanism.
应试提示：务必能解释为何丙烯与 HBr 反应主要生成 2-溴丙烷而非 1-溴丙烷。运用碳正离子稳定性论证，并引用马氏规则或亲电加成机理。
8. Oxidation Reactions: Baeyer's Test and Ozonolysis | 氧化反应：拜耳试验与臭氧化
Cold, dilute alkaline potassium permanganate (KMnO₄) oxidizes alkenes to vicinal diols (glycols), with the purple color disappearing and a brown MnO₂ precipitate forming. This is Baeyer's test for unsaturation. The stereochemistry is syn addition, as the MnO₄⁻ ion forms a cyclic intermediate that hydrolyzes to give the cis-diol.
冷的稀碱性高锰酸钾溶液可将烯烃氧化为邻二醇（乙二醇类），同时紫色褪去并产生棕色 MnO₂ 沉淀。这就是检验不饱和度的拜耳试验。立体化学为顺式加成，因为 MnO₄⁻ 离子形成环状中间体水解后得到顺式二醇。
ds: ketones, aldehydes, or CO₂ depending on substitution. Unsubstituted =CH₂ ends become CO₂, R–CH= ends become RCOOH, R₂C= ends become R₂C=O.
在剧烈条件下（热酸性 KMnO₄ 或热浓 KMnO₄），烯烃在双键处发生氧化断裂，生成羰基化合物：酮、醛或 CO₂，取决于取代情况。未取代的 =CH₂ 端生成 CO₂，R–CH= 端生成 RCOOH，R₂C= 端生成 R₂C=O。
Ozonolysis (O₃ followed by reducing workup, e.g., Zn/H₂O or (CH₃)₂S) also cleaves the double bond, yielding aldehydes or ketones. This reaction is often used to determine the position of the double bond in an unknown alkene by identifying the carbonyl products. Under oxidative workup (H₂O₂), aldehydes are further oxidized to carboxylic acids.
臭氧化反应（O₃ 随后还原处理，如 Zn/H₂O 或 (CH₃)₂S）同样可切断双键，生成醛或酮。该反应常用于通过鉴定羰基产物来确定未知烯烃中双键的位置。若采用氧化性后处理（H₂O₂），醛会进一步氧化为羧酸。
9. Polymerization of Alkenes | 烯烃的聚合反应
Alkenes can undergo addition polymerization, where the π bond of a monomer opens up to form long saturated carbon chains. This is a chain reaction requiring an initiator (free radical, cationic, or Ziegler–Natta catalyst). Both IB and AQA focus on free-radical addition polymerization, but also mention high-density vs low-density polyethene (HDPE vs LDPE).
烯烃可发生加成聚合，单体的 π 键打开形成长链饱和碳链。这是一种链反应，需要引发剂（自由基、阳离子引发或齐格勒-纳塔催化剂）。IB 与 AQA 均侧重自由基加成聚合，但也提及高密度聚乙烯与低密度聚乙烯（HDPE 与 LDPE）。
Free-radical polymerization mechanism: (1) Initiation – an initiator (e.g., benzoyl peroxide) generates radicals, which add to the alkene. (2) Propagation – the monomer radical adds to another alkene repeatedly, growing the chain. (3) Termination – two radicals combine or disproportionate. AQA exams may ask for a section of the polymer chain with repeating units, using 'n' to indicate many repeats.
自由基聚合机理：(1) 引发——引发剂（如过氧化苯甲酰）产生自由基，加到烯烃上。(2) 链增长——单体自由基不断与另一烯烃加成，增长链。(3) 终止——两个自由基结合或歧化。AQA 考试可能要求写出聚合物链段的重复单元，以 'n' 表示重复次数。
Poly(ethene) properties depend on reaction conditions: high pressure free-radical polymerization yields LDPE with branched chains (lower density, softer), while Ziegler–Natta catalysts at lower pressure produce HDPE with straight chains (higher density, stronger). Poly(propene) also has isotactic and atactic forms depending on catalyst stereocontrol.
聚乙烯的性质取决于反应条件：高压自由基聚合得到含支链的 LDPE（密度低、较软），而低压下齐格勒-纳塔催化剂则产生直链的 HDPE（密度高、更强韧）。聚丙烯也因催化剂立体控制能力不同而存在等规和无规形态。
Recognize common addition polymers: poly(ethene), poly(propene), poly(chloroethene) (PVC), poly(phenylethene) (polystyrene), and PTFE (from tetrafluoroethene). Be able to draw the monomer given the polymer, and vice versa.
要熟悉常见的加成聚合物：聚乙烯、聚丙烯、聚氯乙烯（PVC）、聚苯乙烯以及聚四氟乙烯（PTFE，由四氟乙烯聚合而成）。要求能由聚合物推出单体，反之亦然。
10. Summary of Key Reactions and Interconversions | 关键反应与相互转化总结
Below is a concise reference table of alkene reactions and their outcomes, highly useful for synthesis and flowchart questions:
下表为烯烃反应及其产物的简明参考表，对合成推断和流程图题型极有帮助：
| Reaction | Reagents/Conditions | Product(s) | Key Point |
|---|---|---|---|
| Hydrogenation | H₂, Pt/Pd/Ni, heat | Alkane | Syn addition |
| Halogenation | Br₂ or Cl₂, CCl₄ or water, RT | Vicinal dihalide | Anti addition; decolorizes Br₂ |
| Hydrohalogenation | HX (dry), RT; or HX + AlCl₃ for HCl | Haloalkane | Markovnikov |
| Hydration (direct) | H₂O(g) + H₃PO₄ catalyst, 300 °C, 60 atm | Alcohol | Markovnikov; industrial ethanol |
| Hydration (indirect) | Conc. H₂SO₄, then H₂O, heat | Alcohol | Via alkyl hydrogensulfate |
| Baeyer oxidation | Cold dilute KMnO₄, alkaline | Cis-diol | Purple → brown/colorless |
| Oxidative cleavage | Hot acidic KMnO₄ or O₃/Zn/H₂O | Carbonyl compounds / acids | Locates double bond position |
| Polymerization | Initiator, high pressure / Ziegler–Natta | Addition polymer | Monomer must have C=C |
For IB exam preparation, ensure you can deduce structures of unknown alkenes from ozonolysis or oxidation product data, predict major/minor products using Markovnikov's rule and carbocation stability, and write complete mechanisms with curly arrows for electrophilic addition and bromination via bromonium ion. For AQA, be ready to interpret polymer repeat units, compare industrial hydration processes (direct vs indirect), and apply reaction conditions.
在 IB 备考中，要确保能从臭氧化或氧化产物数据推导未知烯烃结构，运用马氏规则和碳正离子稳定性预测主/次产物，并会书写完整的亲电加成与溴鎓离子机理（含弯箭头）。对于 AQA，需会解读聚合物重复单元，比较工业水合法（直接法与间接法），并能准确应用反应条件。
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