AS Chemistry: Alkenes Key Points | AS 化学：烯烃 考点精讲

📚 AS Chemistry: Alkenes Key Points | AS 化学：烯烃 考点精讲

Alkenes are unsaturated hydrocarbons containing at least one carbon–carbon double bond. Their general formula is CₙH₂ₙ, and they serve as the foundation for a vast array of organic reactions, particularly electrophilic addition. This article covers the essential concepts required for AS-level Chemistry, including structure, bonding, isomerism, characteristic reactions, and polymerisation.

烯烃是含有至少一个碳碳双键的不饱和烃，通式为 CₙH₂ₙ。它们是众多有机反应（尤其是亲电加成）的基础。本文涵盖了 AS 化学中必需的核心概念，包括结构、键合、异构现象、特征反应和聚合反应。

1. Structure and Bonding in Alkenes | 烯烃的结构与键合

The carbon atoms of the double bond are sp² hybridised. Each carbon forms three sigma bonds (two to other atoms and one to a carbon) using sp² hybrid orbitals, leaving one unhybridised p orbital perpendicular to the plane. The sideways overlap of two parallel p orbitals creates a pi (π) bond, which is weaker than the sigma bond. This combination of one sigma and one pi bond constitutes the double bond.

双键碳原子为 sp² 杂化。每个碳利用 sp² 杂化轨道形成三个 σ 键（两个与其他原子，一个与碳原子），留下一个垂直于平面的未杂化 p 轨道。两个平行 p 轨道侧向重叠形成 π 键，π 键比 σ 键弱。一个 σ 键加一个 π 键共同构成双键。

The restricted rotation about the C=C bond gives rise to geometric isomerism. The high electron density in the π bond makes alkenes susceptible to attack by electrophiles.

由于 C=C 双键不能自由旋转，导致了几何异构现象。π 键的高电子密度使烯烃容易被亲电试剂进攻。

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2. Naming Alkenes | 烯烃的命名

Select the longest carbon chain containing the double bond and replace the -ane ending with -ene. Number the chain from the end nearest the double bond, and indicate the position of the double bond by the lower-numbered carbon involved. For branched alkenes, follow IUPAC priority rules. Example: but-1-ene, but-2-ene.

选择含双键的最长碳链，将词尾 -ane 改为 -ene。从靠近双键的一端开始编号，用双键中编号较小的碳原子标示其位置。对于支链烯烃，遵循 IUPAC 优先规则。例如：丁-1-烯、丁-2-烯。

The general formula for non-cyclic alkenes with one double bond is CₙH₂ₙ. For cyclic alkenes, the formula is CₙH₂ₙ₋₂.

含一个双键的非环状烯烃通式为 CₙH₂ₙ，环状烯烃则为 CₙH₂ₙ₋₂。

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3. Isomerism: Structural and Stereoisomerism | 异构现象：构造异构与立体异构

Alkenes exhibit chain isomerism, position isomerism, and functional group isomerism (cycloalkanes). More importantly, they show E/Z isomerism (geometric isomerism) due to the restricted rotation of the double bond. For E/Z assignment, the Cahn–Ingold–Prelog priority rules are used: the higher the atomic number of the atom directly attached to the double-bonded carbon, the higher the priority. If the two higher-priority groups are on opposite sides, the isomer is E (entgegen); if they are on the same side, it is Z (zusammen).

烯烃存在碳链异构、位置异构和官能团异构（环烷烃）。更重要的是，由于双键旋转受限，它们表现出 E/Z 异构（几何异构）。E/Z 标记采用 Cahn–Ingold–Prelog 优先规则：直接连接在双键碳上的原子序数越大，优先级越高。若两个优先基团位于双键异侧，则为 E 型（entgegen）；位于同侧则为 Z 型（zusammen）。

Cis–trans terminology is a restricted form used only when each carbon of the double bond carries at least one identical group.

顺反标记法仅适用于双键的每个碳上至少连有一个相同基团的情况。

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

Alkenes are non-polar or only weakly polar molecules, so the only intermolecular forces are van der Waals (London) forces. Boiling points increase with increasing chain length and decrease with branching. They are insoluble in water but dissolve in non-polar organic solvents.

烯烃为非极性或弱极性分子，分子间仅存在范德华力。沸点随碳链增长而升高，随支链增多而降低。它们不溶于水，但可溶于非极性有机溶剂。

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5. Reactivity of the Double Bond | 双键的反应活性

The π bond is an area of high electron density, which makes alkenes susceptible to attack by electrophiles (electron-deficient species). This is the basis of electrophilic addition, the characteristic reaction of alkenes. The π bond breaks heterolytically, forming a carbocation intermediate.

π 键区域电子密度高，使得烯烃易受亲电试剂（缺电子物种）进攻。这是亲电加成反应的基础，也是烯烃的特征反应。π 键发生异裂，生成碳正离子中间体。

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6. Electrophilic Addition Mechanism | 亲电加成机理

The general mechanism involves two steps. Step 1: The electrophile attacks the double bond, accepting a pair of electrons from the π bond to form a new σ bond. This results in a carbocation and a negative ion. Step 2: The negatively charged species (or nucleophile) rapidly bonds with the carbocation to complete the addition.

一般机理分两步。第一步：亲电试剂进攻双键，从 π 键接受一对电子，形成新的 σ 键，同时生成碳正离子和一个负离子。第二步：带负电的物种（或亲核试剂）迅速与碳正离子结合，完成加成。

Using ethene and hydrogen bromide as an example:

以乙烯与溴化氢的反应为例：

CH₂=CH₂ + HBr → CH₃CH₂Br

The mechanism:

反应机理如下：

Step 1: H⁺ attacks the π bond → CH₃–C⁺H₂ + Br⁻

Step 2: CH₃–C⁺H₂ + Br⁻ → CH₃CH₂Br

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7. Addition of Hydrogen Halides and Sulfuric Acid | 卤化氢与硫酸的加成

Alkenes react with hydrogen halides (HCl, HBr, HI) at room temperature to form halogenoalkanes. With unsymmetrical alkenes, two products are possible, and the major product is predicted by Markovnikov’s rule.

烯烃与卤化氢（HCl、HBr、HI）在室温下反应生成卤代烷。对于不对称烯烃，可能有两种产物，主要产物可根据马氏规则预测。

Markovnikov’s rule states that 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 carbon that was originally less substituted), and the halide attaches to the more substituted carbon. This is explained by the stability of the carbocation intermediate: tertiary > secondary > primary > methyl.

马氏规则指出：HX 与不对称烯烃加成时，氢原子加到原来连氢较多的碳上（取代基较少的碳），卤素则加到取代基较多的碳上。这可以根据碳正离子中间体的稳定性来解释：叔碳正离子 > 仲碳正离子 > 伯碳正离子 > 甲基碳正离子。

Cold concentrated sulfuric acid also adds to alkenes to form alkyl hydrogensulfates. Subsequent hydrolysis with water yields alcohols, providing an indirect hydration route that follows Markovnikov’s rule.

冷的浓硫酸也可与烯烃加成生成硫酸氢烷基酯，随后水解得到醇类。这是一种间接水合法，同样遵循马氏规则。

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8. Addition of Halogens | 卤素的加成

Alkenes react rapidly with bromine or chlorine at room temperature. The reaction with bromine is used as a test for unsaturation: the orange-brown colour of bromine water is decolourised. The mechanism proceeds via a cyclic bromonium ion (or halonium ion), leading to anti-addition.

烯烃在室温下与溴或氯快速反应。与溴的反应常用于检验不饱和键：溴水的橙棕色会褪去。反应机理经过环状溴鎓离子（或卤鎓离子），导致反式加成。

Ethene with Br₂ gives 1,2-dibromoethane: CH₂=CH₂ + Br₂ → BrCH₂CH₂Br.

乙烯与 Br₂ 反应生成 1,2-二溴乙烷：CH₂=CH₂ + Br₂ → BrCH₂CH₂Br。

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9. Hydrogenation | 催化加氢

Alkenes react with hydrogen in the presence of a metal catalyst (Ni, Pt, or Pd) at moderate temperature and pressure to form alkanes. This is an addition reaction used in the hardening of vegetable oils. The enthalpy change of hydrogenation provides information about the stability of alkenes.

烯烃在金属催化剂（Ni、Pt 或 Pd）存在下，于中等温度和压力下与氢气反应生成烷烃。该加成反应用于植物油的硬化。氢化反应的焓变可提供烯烃稳定性的信息。

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10. Oxidation Reactions | 氧化反应

Alkenes can be oxidised by cold, dilute, acidified potassium manganate(VII). The purple solution turns colourless, and a diol (dihydroxylation) is formed. This can also be used as a test for unsaturation. Under more vigorous conditions, oxidative cleavage of the double bond occurs, producing carbonyl compounds or carboxylic acids.

烯烃可被冷的、稀的酸化高锰酸钾溶液氧化。紫色溶液褪为无色，生成二醇（二羟基化产物）。这也可用于检验不饱和键。在更剧烈的条件下，双键发生氧化断裂，生成羰基化合物或羧酸。

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11. Addition Polymerisation | 加成聚合

Alkenes and substituted alkenes can undergo addition polymerisation to form long-chain polymers. The π bond breaks, and monomers join together without the loss of any small molecules. Common examples include poly(ethene), poly(propene), and poly(chloroethene) (PVC).

烯烃及其取代衍生物可发生加成聚合，形成长链高分子。π 键断裂，单体连接在一起而不脱去小分子。常见例子有聚乙烯、聚丙烯和聚氯乙烯（PVC）。

The repeating unit is derived from the monomer by opening the double bond. The polymer’s properties depend on the length of the chains, branching, and the nature of the side groups.

重复单元由单体打开双键衍生而来。聚合物的性质取决于链长、支化度和侧基的性质。

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12. Key Comparisons and Summary Table | 关键对比与总结表

The following table summarises the key addition reactions of alkenes.

下表总结了烯烃的主要加成反应。

Reagent / Reaction | 试剂 / 反应	Conditions | 条件	Product | 产物	Notes | 备注
Hydrogen (H₂) | 氢气	Ni/Pt/Pd catalyst, heat | Ni/Pt/Pd 催化剂，加热	Alkane | 烷烃	Addition reaction | 加成反应
Bromine (Br₂) | 溴	Room temperature, in the dark | 室温，避光	1,2-dibromoalkane | 1,2-二溴代烷	Test for unsaturation; anti-addition | 不饱和性检验；反式加成
Hydrogen bromide (HBr) | 溴化氢	Room temperature | 室温	Bromoalkane | 溴代烷	Markovnikov’s rule applies | 遵循马氏规则
Concentrated H₂SO₄ | 浓硫酸	Cold | 冷	Alkyl hydrogensulfate | 硫酸氢烷基酯	Then water, heat → alcohol | 然后加水加热 → 醇
Acidified KMnO₄ (cold, dilute) | 酸化高锰酸钾（冷稀）	Cold, dilute | 冷、稀	Diol (glycol) | 二醇	Purple → colourless | 紫色 → 无色
Polymerisation | 聚合	High pressure, catalyst | 高压，催化剂	Poly(alkene) | 聚烯烃	Addition polymer | 加聚物

Understanding the mechanisms, regioselectivity (Markovnikov’s rule), and the stereochemistry of addition (where applicable) is crucial for success in AS Chemistry. Alkenes not only provide a model for electrophilic addition but also link to polymers, isomers, and organic synthesis pathways.

掌握反应机理、区域选择性（马氏规则）以及加成反应的立体化学（如适用）对 AS 化学取得成功至关重要。烯烃不仅是亲电加成的模型，还与聚合物、异构体和有机合成路线密切相关。

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