IB Chemistry: Alkenes Key Concepts | IB 化学：烯烃考点精讲

📚 IB Chemistry: Alkenes Key Concepts | IB 化学：烯烃考点精讲

Alkenes are a fundamental class of unsaturated hydrocarbons in organic chemistry. They contain at least one carbon–carbon double bond (C=C) and display a rich variety of chemical reactions, particularly electrophilic addition. For IB Chemistry students, understanding the structure, isomerism, reactivity patterns and mechanisms of alkenes is essential for both Standard Level and Higher Level assessments.

烯烃是有机化学中一类基本的不饱和烃。它们至少含有一个碳碳双键（C=C），并表现出丰富多样的化学反应，尤其是亲电加成反应。对于 IB 化学的学生而言，理解烯烃的结构、异构现象、反应规律及其机理，对标准水平和高水平考试都至关重要。

1. What Are Alkenes? | 什么是烯烃？

Alkenes are hydrocarbons with the general formula CₙH₂ₙ for non-cyclic structures. They are characterised by the presence of at least one C=C double bond, which makes them unsaturated and far more reactive than alkanes. The double bond is the functional group responsible for the typical reactions of alkenes.

烯烃是通式为 CₙH₂ₙ（非环状）的烃类，其特征是至少含有一个 C=C 双键，因此它们是不饱和的，远比烷烃活泼。双键是导致烯烃典型反应的官能团。

In IB, the simplest alkene is ethene (C₂H₄), followed by propene (C₃H₆), butene (C₄H₈), and so on. The double bond introduces rigidity and the possibility of geometrical isomerism, which has significant consequences for physical and chemical properties.

在 IB 中，最简单的烯烃是乙烯（C₂H₄），其次是丙烯（C₃H₆）、丁烯（C₄H₈）等。双键带来了刚性，并可能导致几何异构现象，这对物理和化学性质有重要影响。

2. Naming Alkenes | 烯烃的命名

IUPAC nomenclature for alkenes follows these steps: identify the longest carbon chain containing the double bond, name it by replacing the -ane suffix of the corresponding alkane with -ene, number the chain to give the double bond the lowest possible number, and indicate its position before the -ene suffix. Substituent positions and names are added as prefixes, sorted alphabetically.

烯烃的 IUPAC 命名遵循以下步骤：找出含有双键的最长碳链，将相应烷烃的 -ane 词尾改为 -ene；给碳链编号，使得双键的位置编号尽可能小，并在 -ene 前标出双键位置；取代基的位置和名称作为前缀，按字母顺序排列。

Example: CH₃–CH=CH–CH₃ is but-2-ene, not but-3-ene. If there are multiple double bonds, use -diene, -triene, etc. Cycloalkenes are named by placing cyclo- before the alkene name, with the double bond assumed to be between carbons 1 and 2 unless otherwise indicated.

例如：CH₃–CH=CH–CH₃ 是丁-2-烯，而不是丁-3-烯。如果有多个双键，则使用 -二烯、-三烯等词尾。环烯烃在烯烃名称前加上“环”字，并默认双键位于碳 1 和碳 2 之间，除非另有说明。

3. Structure and Bonding | 结构与键合

The carbon atoms of a C=C double bond are sp² hybridised. Each carbon uses three sp² hybrid orbitals to form three σ bonds (two to other atoms and one to the other carbon), leaving one unhybridised p orbital. These p orbitals overlap sideways to form a π bond. The combination of one σ bond and one π bond gives the double bond its strength and restricted rotation.

C=C 双键的碳原子是 sp² 杂化的。每个碳用三个 sp² 杂化轨道形成三个 σ 键（两个与其他原子成键，一个与另一个碳成键），并留下一个未杂化的 p 轨道。这些 p 轨道侧向重叠形成 π 键。一个 σ 键和一个 π 键共同构成双键，使其键能较高且旋转受限。

The bond angle around the sp² carbon is approximately 120°, giving a trigonal planar geometry. The restricted rotation about the C=C bond is the reason alkenes can exhibit cis–trans (E–Z) isomerism. The π electrons are more exposed and are the reactive site for electrophilic attack.

sp² 碳周围的键角约为 120°，呈平面三角形构型。C=C 双键的旋转受限是烯烃可以出现顺反（E–Z）异构的原因。π 电子较为暴露，是发生亲电进攻的反应位点。

4. Geometric Isomerism (Cis–Trans and E–Z) | 几何异构（顺反与 E–Z 异构）

Geometric isomerism occurs in alkenes when both carbon atoms of the double bond carry two different groups. In cis–trans nomenclature, cis means the two higher-priority groups are on the same side of the double bond, while trans indicates they are on opposite sides. E–Z nomenclature uses the Cahn–Ingold–Prelog rules: Z (zusammen) for same side, E (entgegen) for opposite, based on atomic number priorities.

当双键的两个碳原子各连有两个不同的基团时，烯烃会出现几何异构。在顺反命名法中，顺式表示两个优先基团在双键的同侧，反式则表示在异侧。E–Z 命名法依据 Cahn–Ingold–Prelog 规则：按原子序数确定优先次序，同侧为 Z（德语 zusammen），异侧为 E（entgegen）。

For IB, it is crucial to recognise that compounds with two identical groups on one carbon of the double bond do NOT show cis–trans isomerism (e.g., propene). The different physical properties of geometric isomers, such as boiling points and dipole moments, are often examined.

在 IB 中，必须认识到如果一个双键碳上连有两个相同的基团（如丙烯），则不显示顺反异构。几何异构体在沸点、偶极矩等物理性质上的差异经常是考点。

5. Physical Properties of Alkenes | 烯烃的物理性质

Alkenes are non-polar or weakly polar molecules, so the main intermolecular forces are van der Waals’ (London dispersion) forces. Boiling points increase with increasing molecular mass and surface area. Branched alkenes have lower boiling points than their straight-chain isomers due to reduced contact area. Alkenes are insoluble in water but dissolve in non-polar organic solvents.

烯烃是非极性或弱极性分子，因此分子间作用力主要是范德华力（伦敦色散力）。沸点随分子质量和表面积的增大而升高。支链烯烃由于分子间接触面积减小，沸点低于同碳数的直链异构体。烯烃不溶于水，但可溶于非极性有机溶剂。

Cis isomers tend to have slightly higher boiling points than trans isomers because the polar bonds in the cis form create a net dipole, leading to stronger dipole–dipole interactions in addition to London forces. Trans isomers are often more symmetrical and pack better in a crystal lattice, hence they usually have higher melting points.

顺式异构体的沸点通常略高于反式异构体，因为顺式结构中的极性键会产生净偶极矩，从而在伦敦力的基础上增加更强的偶极-偶极相互作用。反式异构体往往对称性更高，在晶格中堆积得更好，因此熔点通常更高。

6. General Addition Reactions | 加成反应概述

The most characteristic reaction of alkenes is electrophilic addition. The π bond, being electron-rich, is attacked by electrophiles (electron-pair acceptors). The general mechanism involves the electrophile accepting electrons from the π bond, forming a carbocation intermediate (in most cases), followed by rapid attack by a nucleophile. This breaks the double bond and adds atoms or groups across it.

烯烃最典型的反应是亲电加成。富电子的 π 键受到亲电试剂（电子对受体）的进攻。一般机理为：亲电试剂从 π 键接受电子，通常形成碳正离子中间体，然后亲核试剂迅速进攻。这一过程使得双键断裂，并在其上加上原子或基团。

The addition reactions covered in IB include hydrogenation, halogenation, hydrohalogenation, hydration, and polymerisation. Understanding the stepwise mechanism is higher level material, while SL students focus on the overall reaction equations and conditions.

IB 中涉及的加成反应包括加氢、卤化、加卤化氢、水化和聚合反应。分步机理的理解属于高水平内容，而 SL 学生主要关注总反应方程式和反应条件。

7. Catalytic Hydrogenation | 催化加氢

Alkenes react with hydrogen gas (H₂) in the presence of a metal catalyst (Pt, Pd, or Ni) at room temperature or moderate heat to form alkanes. This is an addition reaction where two H atoms add across the double bond. Heat is released; it is an exothermic reaction. Hydrogenation is used industrially to convert unsaturated vegetable oils into saturated solid margarines.

烯烃在金属催化剂（Pt、Pd 或 Ni）存在下，于室温或适度加热条件下与氢气（H₂）反应生成烷烃。这是两个氢原子加成到双键上的加成反应。反应放热，是一个放热反应。工业上利用加氢反应将不饱和的植物油转化为饱和的固体人造黄油。

The reaction is heterogeneous catalysis: the metal surface adsorbs both H₂ and the alkene, weakening bonds and facilitating addition. Equation: CₙH₂ₙ + H₂ → CₙH₂ₙ₊₂. The catalyst does not alter the equilibrium position but speeds up the rate by providing an alternative pathway with lower activation energy.

该反应属于多相催化：金属表面吸附 H₂ 和烯烃，削弱化学键，促进加成反应的发生。方程式：CₙH₂ₙ + H₂ → CₙH₂ₙ₊₂。催化剂不改变平衡位置，但通过提供活化能更低的替代路径而加快反应速率。

8. Halogenation (Addition of Halogens) | 卤化（卤素加成）

Alkenes react readily with halogens (Cl₂, Br₂) at room temperature in the dark to give vicinal dihaloalkanes. For example, ethene reacts with bromine to give 1,2-dibromoethane. This reaction is often used as a test for unsaturation: the orange-brown colour of bromine water is rapidly decolourised by alkenes, whereas alkanes do not react under these conditions.

烯烃在室温、避光条件下即可与卤素（Cl₂、Br₂）迅速反应，生成邻二卤代烷。例如，乙烯与溴反应生成 1,2-二溴乙烷。该反应常被用作不饱和键的检验方法：溴水的橙棕色会被烯烃迅速褪去，而烷烃在相同条件下不反应。

The mechanism involves the polarisation of the Br–Br bond as it approaches the π electron cloud. The Br atom acting as electrophile forms a cyclic bromonium ion intermediate (HL detail), which then opens by nucleophilic attack of Br⁻ to give the trans addition product. This stereospecificity can be discussed at HL.

其机理包括：当 Br–Br 键靠近 π 电子云时发生极化；亲电的 Br 原子形成环状溴鎓离子中间体（HL 细节），随后受到 Br⁻ 的亲核进攻而开环，得到反式加成产物。这种立体专一性可在 HL 中讨论。

9. Hydrohalogenation and Markovnikov’s Rule | 加卤化氢与马氏规则

Alkenes add hydrogen halides (HX; X = Cl, Br, I) to form haloalkanes. With unsymmetrical alkenes, two products are theoretically possible, but one predominates. Markovnikov’s rule states that the hydrogen atom of HX attaches to the carbon of the double bond that already has more hydrogen atoms, and the halogen attaches to the more substituted carbon.

烯烃可与卤化氢（HX；X = Cl、Br、I）加成生成卤代烷。对于不对称烯烃，理论上有两种可能产物，但其中一种占优势。马尔科夫尼科夫规则指出：HX 中的氢原子加到含氢较多的双键碳上，而卤素加到取代较多的碳上。

The rule can be explained by the stability of the carbocation intermediate: tertiary carbocations are more stable than secondary, which are more stable than primary. The more stable carbocation forms faster, leading to the major product. In IB HL, students should be able to predict the major product and outline the mechanism with curly arrows.

该规则可用碳正离子中间体的稳定性来解释：叔碳正离子比仲碳正离子稳定，仲碳正离子又比伯碳正离子稳定。更稳定的碳正离子形成更快，从而得到主要产物。在 IB HL 中，学生应能预测主产物并用弯箭头表示反应机理。

Alkene 烯烃	HX 卤化氢	Major Product 主要产物
CH₃–CH=CH₂ 丙烯	HBr	CH₃–CHBr–CH₃ (2-bromopropane) 2-溴丙烷
(CH₃)₂C=CH₂ 2-甲基丙烯	HCl	(CH₃)₃CCl (2-chloro-2-methylpropane) 2-氯-2-甲基丙烷

10. Hydration of Alkenes | 烯烃的水化

In the presence of an acid catalyst (usually concentrated H₂SO₄ or H₃PO₄) and steam at high temperature and pressure, alkenes add water to form alcohols. This is the industrial method for manufacturing ethanol from ethene. The addition follows Markovnikov’s rule: the H atom attaches to the less substituted carbon, and the OH group to the more substituted carbon.

在酸催化剂（通常为浓 H₂SO₄ 或 H₃PO₄）和高温高压水蒸汽存在下，烯烃与水加成生成醇。这是工业上从乙烯制备乙醇的方法。加成反应遵循马氏规则：H 原子加到含氢较多的碳上，OH 基则加到取代较多的碳上。

Example: CH₂=CH₂ + H₂O → CH₃CH₂OH (ethanol). For propene, the product is predominantly propan-2-ol (isopropyl alcohol), not propan-1-ol. IB higher level students may study the direct vs. indirect hydration methods and the equilibria involved, including temperature and pressure effects.

例如：CH₂=CH₂ + H₂O → CH₃CH₂OH（乙醇）。对于丙烯，主要产物是丙-2-醇（异丙醇），而不是丙-1-醇。IB 高水平的学生可能需要学习直接水化法与间接水化法，以及涉及到的平衡问题，包括温度和压强的影响。

11. Oxidation of Alkenes | 烯烃的氧化反应

Alkenes can be oxidised by cold, dilute, alkaline potassium manganate(VII) (KMnO₄) to form diols (dihydroxylation). The purple colour of the MnO₄⁻ ion is discharged, and a brown precipitate of MnO₂ appears. This reaction is sometimes used as a test for alkenes, though bromine water is more common. The full oxidation with hot, concentrated acidified KMnO₄ cleaves the double bond, producing carbonyl compounds and/or CO₂ depending on substitution.

烯烃可被冷、稀的碱性高锰酸钾（KMnO₄）氧化生成二醇（双羟基化）。MnO₄⁻ 离子的紫色褪去，出现棕色的 MnO₂ 沉淀。该反应有时用于检验烯烃，不过溴水检验更为常用。用热的、酸化的浓 KMnO₄ 彻底氧化则会使双键断裂，根据取代情况生成羰基化合物和/或 CO₂。

At SL, recognition of the colour change and the formula of the diol is sufficient. At HL, oxidative cleavage details may be required: disubstituted alkene carbons yield ketones, monosubstituted yield carboxylic acids (further oxidised possibly), and unsubstituted =CH₂ yield CO₂ and H₂O. Careful condition control is emphasised.

SL 水平要求识别颜色变化和二醇的结构式。HL 水平可能需要了解氧化裂解的细节：双取代烯烃碳生成酮，单取代的生成羧酸（可能进一步氧化），未取代的 =CH₂ 则生成 CO₂ 和 H₂O。需要强调反应条件的控制。

12. Polymerisation | 聚合反应

Alkenes undergo addition polymerisation, where the double bond opens and monomers link together to form long saturated carbon chains. This requires high pressure, a catalyst (often a Ziegler–Natta catalyst or organic peroxides) and heat. Poly(ethene), poly(propene), poly(chloroethene) (PVC) and poly(tetrafluoroethene) (PTFE) are all made this way.

烯烃可发生加成聚合反应：双键打开，单体相互连接形成长的饱和碳链。这需要高压、催化剂（常为齐格勒-纳塔催化剂或有机过氧化物）和加热。聚乙烯、聚丙烯、聚氯乙烯（PVC）和聚四氟乙烯（PTFE）都是通过这一方法制造的。

In IB, students must be able to draw the repeating unit of an addition polymer from a given monomer, and deduce the monomer from a polymer section. The difference between the empirical formula of the monomer and polymer is often tested. Environmental issues (non-biodegradability, recycling) are relevant to the Option or TOK discussions.

在 IB 中，学生必须能够由给定单体画出加成聚合物的重复单元，并根据一段聚合物推断出单体。单体和聚合物经验式之间的差异经常被考查。环境问题（不可生物降解性、回收利用）与选修部分或 TOK 讨论相关。

General equation: n CH₂=CHX → [–CH₂–CHX–]ₙ. For example, n CH₂=CHCl → [–CH₂–CHCl–]ₙ (PVC). The polymer backbone is saturated and inert, making these plastics durable but persistent in the environment.

总方程式：n CH₂=CHX → [–CH₂–CHX–]ₙ。例如，n CH₂=CHCl → [–CH₂–CHCl–]ₙ (PVC)。聚合物主链是饱和的且化学惰性，使得这类塑料经久耐用，但在环境中持久存在。

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IB Chemistry: Alkenes Key Concepts | IB 化学：烯烃 考点精讲