Alkenes: Structure and Properties | 烯烃的结构与性质

📚 Alkenes: Structure and Properties | 烯烃的结构与性质

Alkenes are a fundamental family of hydrocarbons in A-Level chemistry, characterised by the presence of at least one carbon-carbon double bond. Their structure gives rise to a distinct set of physical and chemical properties, making them highly important in both industrial processes and biological systems.

烯烃是 A-Level 化学中一类重要的碳氢化合物，其结构特征是含有至少一个碳碳双键。这一结构赋予了烯烃一系列独特的物理和化学性质，使其在工业过程和生物体系中都具有重要意义。

1. General Formula and Structure | 通式与结构

The general formula for acyclic alkenes containing one double bond is CₙH₂ₙ, where n is the number of carbon atoms. For example, ethene is C₂H₄ and propene is C₃H₆. Each alkene has two fewer hydrogen atoms than the corresponding alkane because the double bond replaces two single bonds.

含有一个双键的开链烯烃的通式为 CₙH₂ₙ，其中 n 为碳原子数。例如，乙烯是 C₂H₄，丙烯是 C₃H₆。每个烯烃比对应的烷烃少两个氢原子，因为双键取代了两条单键。

The carbon atoms involved in the double bond are sp² hybridised. Each carbon forms three sigma (σ) bonds using three sp² orbitals, and the remaining unhybridised p orbital overlaps sideways to form a pi (π) bond. This gives a planar arrangement around the double bond with bond angles of approximately 120°.

参与双键的碳原子采用 sp² 杂化。每个碳原子用三个 sp² 轨道形成三条 sigma（σ）键，剩余未杂化的 p 轨道侧面重叠形成一条 pi（π）键。这使得双键周围呈平面排列，键角约为 120°。

2. The Pi Bond and Restricted Rotation | π 键与受限旋转

The pi bond is formed by the sideways overlap of two p orbitals, one from each carbon. This overlap creates an electron cloud above and below the plane of the molecule. Because the p orbitals must remain parallel for effective overlap, rotation about the C=C bond is severely restricted.

π 键由两个碳原子各一个 p 轨道侧面重叠形成。这种重叠在分子平面的上下方产生电子云。由于 p 轨道必须保持平行才能有效重叠，围绕 C=C 键的旋转受到严重限制。

This restricted rotation leads to stereoisomerism in alkenes. When each carbon of the double bond is attached to two different groups, two distinct arrangements are possible: the E isomer, where the higher-priority groups are on opposite sides, and the Z isomer, where they are on the same side.

这种受限旋转导致烯烃出现立体异构现象。当双键的每个碳原子连接两个不同基团时，可能存在两种不同的排列：E 异构体，即优先级较高的基团在异侧；Z 异构体，即它们在同侧。

3. Nomenclature of Alkenes | 烯烃的命名

Alkenes are named by identifying the longest carbon chain containing the double bond and changing the suffix from -ane to -ene. The position of the double bond is indicated by the lowest possible number assigned to the first carbon of the double bond.

烯烃的命名是找出含双键的最长碳链，并将后缀从 -ane 改为 -ene。双键的位置用尽可能小的数字标明，该数字对应双键的第一个碳原子。

Substituents are listed alphabetically, and stereochemical descriptors E or Z may be added as prefixes when necessary. For example, CH₃CH=CHCH₂CH₃ is named pent-2-ene, and if the higher-priority groups are on the same side, it is (Z)-pent-2-ene.

取代基按字母顺序排列，必要时可在前面添加立体化学描述符 E 或 Z。例如，CH₃CH=CHCH₂CH₃ 称为戊-2-烯，若优先级较高的基团在同侧，则称为 (Z)-戊-2-烯。

4. Physical Properties | 物理性质

Alkenes are non-polar molecules because the C-H and C-C bonds have very small electronegativity differences. As a result, the only intermolecular forces present are weak London dispersion forces. This explains their relatively low boiling points compared to polar molecules of similar molar mass.

烯烃是非极性分子，因为 C-H 和 C-C 键的电负性差异非常小。因此，分子间仅存在较弱的伦敦色散力。这解释了与摩尔质量相近的极性分子相比，烯烃沸点较低的原因。

Alkene	Formula	Boiling point / °C
Ethene	C₂H₄	-104
Propene	C₃H₆	-47
But-1-ene	C₄H₈	-6

As chain length increases, the surface area increases, leading to stronger London forces and higher boiling points. Alkenes are also immiscible with water but soluble in organic solvents such as hexane and dichloromethane.

随着链长增加，分子表面积增大，伦敦力增强，沸点升高。烯烃不溶于水，但可溶于己烷、二氯甲烷等有机溶剂。

5. Reactivity: The Electron-Rich Double Bond | 反应性：富电子的双键

The pi bond in an alkene is relatively weak and electron-rich. It acts as a source of electrons and readily reacts with electrophiles. This is why the most characteristic reactions of alkenes are electrophilic addition reactions, in which the pi bond breaks and two new sigma bonds form.

烯烃中的 π 键相对较弱且电子密度高，可作为电子源，容易与亲电试剂反应。因此，烯烃最具特征的反应是亲电加成反应，其中 π 键断裂，形成两条新的 sigma 键。

The double bond is also a region of high electron density, which makes alkenes susceptible to attack by oxidising agents and reducing agents. In contrast, alkanes, which lack pi bonds, are generally unreactive under similar conditions.

双键区域电子密度高，因此烯烃容易被氧化剂和还原剂进攻。相比之下，没有 π 键的烷烃在相同条件下通常不反应。

6. Addition of Hydrogen Halides | 卤化氢的加成

When an alkene reacts with a hydrogen halide such as HBr or HCl, the product is a halogenoalkane. The hydrogen atom adds to one carbon of the double bond, and the halogen atom adds to the other. For symmetrical alkenes such as ethene, only one product is possible.

当烯烃与 HBr 或 HCl 等卤化氢反应时，产物是卤代烷烃。氢原子加到双键的一个碳上，卤素原子加到另一个碳上。对于乙烯这样的对称烯烃，只可能生成一种产物。

For unsymmetrical alkenes, the major product is determined by Markovnikov’s rule: the hydrogen atom adds to the carbon atom that already has the greater number of hydrogen atoms. For example, propene reacts with HBr to give 2-bromopropane as the major product.

对于不对称烯烃，主要产物由马尔可夫尼科夫规则决定：氢原子加到已有较多氢原子的碳上。例如，丙烯与 HBr 反应，主要产物是 2-溴丙烷。

CH₃CH=CH₂ + HBr → CH₃CHBrCH₃

The rule arises because the intermediate carbocation formed during the reaction is more stable when the positive charge is on the more substituted carbon. Tertiary carbocations are more stable than secondary, which are more stable than primary.

该规则源于反应中间体碳正离子的稳定性：正电荷位于取代程度更高的碳上时更稳定。叔碳正离子比仲碳正离子稳定，仲碳正离子比伯碳正离子稳定。

7. Hydration of Alkenes | 烯烃的水合反应

Alkenes react with steam in the presence of an acid catalyst, typically concentrated phosphoric acid, to form alcohols. This is an industrial method for producing ethanol from ethene. The reaction is reversible and exothermic.

烯烃在酸催化剂（通常是浓磷酸）存在下与水蒸气反应生成醇。这是工业上由乙烯制备乙醇的方法。该反应可逆且放热。

C₂H₄ + H₂O ⇌ C₂H₅OH

In the laboratory, alkenes can also be hydrated by heating with dilute sulfuric acid. However, the direct addition of water across a double bond is slow in the absence of a catalyst. The mechanism proceeds via protonation of the double bond to form a carbocation, followed by nucleophilic attack by water and deprotonation.

在实验室中，烯烃也可用稀硫酸加热水合。但无催化剂时，水在双键上的直接加成很慢。其机理是双键先被质子化形成碳正离子，然后水作为亲核试剂进攻，最后去质子化。

8. Addition of Halogens | 卤素的加成

Alkenes react rapidly with bromine or chlorine at room temperature to form vicinal dihalides. For example, ethene reacts with bromine to give 1,2-dibromoethane. The reaction with bromine is often used as a test for unsaturation because the orange-brown bromine water is decolourised.

烯烃在室温下与溴或氯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。与溴的反应常被用作不饱和性检测，因为橙棕色的溴水会褪色。

C₂H₄ + Br₂ → CH₂BrCH₂Br

The reaction is an electrophilic addition. The bromine molecule becomes polarised as it approaches the electron-rich double bond, and the positive bromine is attracted to the pi electrons. A cyclic bromonium ion intermediate is formed, which is then attacked by a bromide ion from the opposite side, resulting in anti addition.

该反应是亲电加成。溴分子接近富电子的双键时发生极化，带正电的溴被 π 电子吸引，形成环状溴鎓离子中间体，然后溴离子从背面进攻，结果发生反式加成。

9. Addition of Steam and Hydrogen | 水与氢的加成

Hydrogenation is the addition of hydrogen across the double bond in the presence of a metal catalyst such as nickel, palladium, or platinum. This reaction converts alkenes into alkanes and is widely used in the food industry to harden vegetable oils into margarine.

氢化是在镍、钯或铂等金属催化剂存在下，氢气在双键上的加成。该反应将烯烃转化为烷烃，广泛应用于食品工业中将植物油硬化为人造黄油。

C₂H₄ + H₂ → C₂H₆

Catalytic hydrogenation is an addition reaction of great industrial importance. The catalyst provides a surface on which both the alkene and hydrogen are adsorbed, bringing them into close proximity and lowering the activation energy. The product is a saturated alkane.

催化氢化是重要的工业加成反应。催化剂提供表面，使烯烃和氢气吸附其上，彼此靠近并降低活化能。产物是饱和烷烃。

Steam addition, as mentioned in section 7, is also an addition reaction. Unlike hydrogenation, which adds H₂, hydration adds H and OH across the double bond. Both reactions follow the general pattern of electrophilic addition.

如第 7 节所述，水蒸气加成也是加成反应。与添加 H₂ 的氢化不同，水合在双键上加成 H 和 OH。两者都属于亲电加成的普遍模式。

10. Oxidation Reactions | 氧化反应

Alkenes are readily oxidised by cold dilute potassium manganate(VII) to form diols. A solution of KMnO₄ changes from purple to colourless, which can be used as a test for the presence of a double bond. The product is a vicinal diol, such as ethane-1,2-diol from ethene.

烯烃可被冷稀高锰酸钾氧化生成二醇。KMnO₄ 溶液从紫色变为无色，可用于检测双键的存在。产物是邻二醇，例如乙烯生成乙烷-1,2-二醇。

CH₂=CH₂ + [O] + H₂O → CH₂OHCH₂OH

With hot concentrated acidified KMnO₄, alkenes undergo oxidative cleavage. The double bond is completely broken, and the products depend on the structure of the alkene. A terminal CH₂ group is oxidised to CO₂, a CHR group to a carboxylic acid, and a CR₂ group to a ketone.

在热浓酸性 KMnO₄ 条件下，烯烃发生氧化断裂。双键完全断裂，产物取决于烯烃的结构。末端 CH₂ 基团被氧化为 CO₂，CHR 基团氧化为羧酸，CR₂ 基团氧化为酮。

This oxidative cleavage can be used to deduce the structure of an unknown alkene, because the identities of the products reveal the groups attached to the double bond in the original reactant.

这种氧化断裂可用于推断未知烯烃的结构，因为产物的种类揭示了原反应物中双键所连接的基团。

11. Polymerisation of Alkenes | 烯烃的聚合

Alkenes undergo addition polymerisation, in which many small monomer units join together to form a long-chain polymer. The double bond opens up and each monomer forms two single bonds to its neighbours. Polyethene is produced from ethene monomers.

烯烃可发生加成聚合反应，即许多小单体单元连接形成长链聚合物。双键打开，每个单体与相邻单体形成两条单键。聚乙烯由乙烯单体聚合而成。

n CH₂=CH₂ → [-CH₂-CH₂-]ₙ

The reaction requires high pressure, a catalyst, and elevated temperature. The product is a saturated, chemically inert polymer. Substituted alkenes such as propene and chloroethene can also polymerise, giving polypropene and PVC respectively.

该反应需要高压、催化剂和高温。产物是饱和、化学惰性的聚合物。取代烯烃如丙烯和氯乙烯也可聚合，分别得到聚丙烯和聚氯乙烯。

Polymerisation of alkenes is a major industrial process, as polymers are widely used in packaging, construction, clothing, and medical devices. The properties of the polymer, such as flexibility and melting point, depend on the structure of the monomer and the degree of branching in the polymer chain.

烯烃聚合是重要的工业过程，因为聚合物广泛用于包装、建筑、服装和医疗器械。聚合物的性能，如柔韧性和熔点，取决于单体的结构和聚合物链的分支程度。

12. Summary and Exam Tips | 总结与考试提示

Alkenes are characterised by the presence of a carbon-carbon double bond consisting of one sigma bond and one pi bond. The pi bond makes alkenes reactive towards electrophiles and enables addition reactions, which are the defining chemical property of this family.

烯烃的特征是含有碳碳双键，包括一条 σ 键和一条 π 键。π 键使烯烃易与亲电试剂反应，能够发生加成反应，这是烯烃家族最具代表性的化学性质。

In CIE A-Level exams, you should be able to draw the mechanism for electrophilic addition using curly arrows, apply Markovnikov’s rule to predict major products, and interpret the results of bromine water and KMnO₄ tests. You should also be able to name alkenes including E/Z stereoisomers.

在 CIE A-Level 考试中，你需要能够用弯箭头画出亲电加成的机理，应用马尔可夫尼科夫规则预测主要产物，并解释溴水和 KMnO₄ 的检测结果。还需要能够对烯烃（包括 E/Z 立体异构体）进行命名。

Practise writing balanced equations for each addition reaction and be careful to show the correct structural formulas for products. Understanding the relationship between structure, bonding, and reactivity is essential for achieving high marks in organic chemistry questions.

多加练习为每个加成反应写出配平方程式，并注意正确书写产物的结构式。理解结构、成键和反应性之间的关系，是在有机化学题目中取得高分的关键。

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Alkenes: Structure and Properties | 烯烃的结构与性质