Alkenes: Structure, Reactions and Mechanisms | 烯烃：结构、反应与机理

📚 Alkenes: Structure, Reactions and Mechanisms | 烯烃：结构、反应与机理

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C). This functional group makes them chemically reactive, particularly in electrophilic addition reactions. Understanding the structure, isomerism, and reaction pathways of alkenes is essential for AQA A-level Chemistry.

烯烃是含有至少一个碳碳双键（C=C）的不饱和烃。该官能团使其具有较高的化学反应活性，尤其是在亲电加成反应中。理解烯烃的结构、异构现象及反应路径，是 AQA A-level 化学学习的关键内容。

1. Structure and Bonding of Alkenes | 烯烃的结构与成键

In a C=C double bond, each carbon atom is sp² hybridised. This gives a planar arrangement with bond angles of approximately 120° around each double-bonded carbon. The double bond consists of one sigma (σ) bond, formed by the overlap of sp² hybrid orbitals, and one pi (π) bond, formed by the sideways overlap of two unhybridised p orbitals.

在 C=C 双键中，每个碳原子采取 sp² 杂化。这使得双键碳周围的原子呈平面排列，键角约为 120°。双键由一根 σ 键和一根 π 键组成：σ 键由 sp² 杂化轨道头碰头重叠形成，π 键由两个未杂化的 p 轨道肩并肩重叠形成。

The key features of the π bond are as follows:

π 键的关键特征如下：

The p orbitals overlap above and below the plane of the molecule, creating an electron-rich region.
π 轨道在分子平面上方和下方重叠，形成一个电子云密集区域。
The π bond is weaker than the σ bond and is readily broken during chemical reactions.
π 键比 σ 键弱，在化学反应中容易被打开。
Rotation about the C=C bond is restricted, giving rise to stereoisomerism.
围绕 C=C 双键的旋转受阻，从而产生立体异构现象。

2. E/Z and Cis-Trans Isomerism | E/Z 异构与顺反异构

Because rotation around the double bond is restricted, alkenes can exist as stereoisomers. E/Z isomerism arises when each carbon of the double bond is bonded to two different groups. The Cahn-Ingold-Prelog priority rules are used to assign E or Z configuration.

由于双键不能自由旋转，烯烃可以存在立体异构体。当双键上的每个碳原子连接两个不同的基团时，就会产生 E/Z 异构。根据 Cahn-Ingold-Prelog 顺序规则，可以确定 E 构型或 Z 构型。

Z isomer: the two higher-priority groups are on the same side of the double bond.
E isomer: the two higher-priority groups are on opposite sides of the double bond.
Z 构型：两个优先级较高的基团位于双键的同一侧。
E 构型：两个优先级较高的基团位于双键的相反两侧。

Cis-trans isomerism is a special case of E/Z isomerism where one group on each carbon is hydrogen. ‘Cis’ corresponds to Z and ‘trans’ corresponds to E.

顺反异构是 E/Z 异构的特例，此时每个碳原子上均有一个基团为氢原子。’Cis’（顺式）对应 Z 构型，’Trans’（反式）对应 E 构型。

3. Electrophilic Addition: The Core Reaction | 亲电加成：核心反应

The electron-rich π bond attracts electrophiles (electron-pair acceptors). The general mechanism of electrophilic addition involves two steps: formation of a carbocation intermediate, followed by nucleophilic attack.

富电子的 π 键会吸引亲电试剂（电子对受体）。亲电加成的一般机理包含两步：形成碳正离子中间体，随后受到亲核试剂进攻。

Alkene + Electrophile → Carbocation → Product

烯烃 + 亲电试剂 → 碳正离子 → 产物

For example, the addition of hydrogen bromide (HBr) to propene:

例如，溴化氢（HBr）与丙烯的加成反应：

CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (major) + CH₃CH₂CH₂Br (minor)

The mechanism is as follows:

反应机理如下：

Step 1: The π bond donates a pair of electrons to the H atom of HBr, forming a C-H bond. The H-Br bond breaks heterolytically, producing a carbocation and a bromide ion.
Step 2: The bromide ion attacks the positively charged carbocation, forming the final product.
步骤 1：π 键向 HBr 中的氢原子提供一对电子，形成 C-H 键。H-Br 键发生异裂，生成碳正离子和溴离子。
步骤 2：溴离子进攻带正电荷的碳正离子，生成最终产物。

4. Markovnikov’s Rule and Carbocation Stability | 马氏规则与碳正离子稳定性

When an unsymmetrical alkene reacts with an unsymmetrical reagent such as HBr, two possible products can form. Markovnikov’s rule states that the hydrogen atom adds to the carbon of the double bond that already has the greater number of hydrogen atoms. The major product is the one formed via the more stable carbocation intermediate.

当不对称烯烃与不对称试剂（如 HBr）反应时，可能生成两种产物。马氏规则指出：氢原子加到双键上含氢较多的碳原子。主要产物是经过更稳定的碳正离子中间体生成的产物。

The relative stability of carbocations follows the order:

碳正离子的相对稳定性顺序为：

tertiary > secondary > primary > methyl

叔碳 > 仲碳 > 伯碳 > 甲基

This stability is due to the electron-donating effect of alkyl groups, which reduces the positive charge density on the carbocation. For example, in the reaction of propene with HBr, the secondary carbocation (CH₃CH⁺CH₃) is more stable than the primary carbocation (CH₃CH₂CH₂⁺), so 2-bromopropane is the major product.

这种稳定性源于烷基的给电子效应，它可以降低碳正离子上的正电荷密度。例如，在丙烯与 HBr 的反应中，仲碳正离子（CH₃CH⁺CH₃）比伯碳正离子（CH₃CH₂CH₂⁺）更稳定，因此 2-溴丙烷是主要产物。

5. Addition of Halogens and Testing for Alkenes | 卤素加成与烯烃的检验

Alkenes react readily with halogens, such as bromine, in a process called halogenation. This is a useful test for unsaturation.

烯烃可以与卤素（如溴）迅速发生反应，这一过程称为卤化反应。这是检验不饱和性的常用方法。

CH₂=CH₂ + Br₂ → CH₂BrCH₂Br

When bromine water (an orange solution) is added to an alkene, the colour changes from orange to colourless. This decolourisation indicates the presence of a double bond. No such colour change occurs with alkanes under normal conditions.

当溴水（橙色溶液）加入烯烃中时，溶液由橙色变为无色。褪色现象表明存在双键。而烷烃在通常条件下不会发生这种颜色变化。

Reaction type: electrophilic addition
Conditions: room temperature, no catalyst required
Observation: orange to colourless
反应类型：亲电加成
条件：室温，无需催化剂
现象：橙色变为无色

6. Addition of Water (Hydration) | 水的加成（水合反应）

Alkenes can be converted to alcohols by the addition of steam. This is an industrial process used to produce ethanol from ethene.

烯烃可以通过与水蒸气加成转化为醇。这是由乙烯制备乙醇的工业方法。

CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH

The reaction conditions are important:

反应条件十分重要：

Temperature: 300 °C
Pressure: 60–70 atm
Catalyst: concentrated phosphoric acid (H₃PO₄), often supported on silica
温度：300 °C
压力：60–70 大气压
催化剂：浓磷酸（H₃PO₄），通常负载在硅胶上

The reverse reaction, dehydration of ethanol to ethene, is carried out using excess concentrated sulfuric acid or phosphoric acid at 170 °C. This reversible nature makes the process industrially important and economically viable.

逆反应（乙醇脱水生成乙烯）在过量浓硫酸或磷酸存在下、170 °C 条件下进行。这种可逆性使该过程在工业上具有重要意义，且经济可行。

7. Oxidation of Alkenes | 烯烃的氧化反应

Alkenes can be oxidised under different conditions, leading to different products. The most common oxidising agents are acidified potassium manganate(VII) (KMnO₄) and ozone.

烯烃可在不同条件下被氧化，生成不同的产物。最常见的氧化剂是酸化的高锰酸钾（KMnO₄）和臭氧。

Cold, dilute KMnO₄: This produces a diol (vicinal diol). A purple to colourless colour change is observed.

冷稀 KMnO₄：生成邻二醇。可观察到紫色变为无色。

CH₂=CH₂ + [O] + H₂O → CH₂OHCH₂OH

Warm, concentrated KMnO₄: This causes oxidative cleavage of the C=C bond. The products depend on the structure of the alkene:

热浓 KMnO₄：导致 C=C 键氧化断裂。产物取决于烯烃的结构：

Terminal alkene (RCH=CH₂) → carboxylic acid (RCOOH) and CO₂
Alkene with RCH=CHR’ → two carboxylic acids (RCOOH and R’COOH)
Alkene with R₂C=CHR’ → ketone + carboxylic acid
末端烯烃（RCH=CH₂）→ 羧酸（RCOOH）和 CO₂
RCH=CHR’ 型烯烃 → 两种羧酸（RCOOH 和 R’COOH）
R₂C=CHR’ 型烯烃 → 酮 + 羧酸

8. Polymerisation of Alkenes | 烯烃的聚合反应

Alkenes undergo addition polymerisation to form long-chain polymers. The C=C double bond opens up, allowing monomers to link together in a repeated chain.

烯烃通过加成聚合形成长链聚合物。C=C 双键打开后，单体可以连接形成重复的链结构。

n CH₂=CH₂ → [–CH₂–CH₂–]ₙ

For substituted alkenes such as propene and chloroethene, the polymer structure is regular, giving materials such as poly(propene) and poly(chloroethene), commonly known as PVC.

对于取代烯烃（如丙烯和氯乙烯），聚合物的结构规整，得到聚丙烯和聚氯乙烯（俗称 PVC）等材料。

n CH₂=CHCl → [–CH₂–CHCl–]ₙ

Conditions: high pressure and temperature, often with a catalyst (e.g. Ziegler-Natta catalyst)
Mechanism: free-radical or coordination polymerisation
Products are chemically inert and non-biodegradable
条件：高温高压，通常需要使用催化剂（如齐格勒-纳塔催化剂）
机理：自由基聚合或配位聚合
产物化学惰性高，不可生物降解

9. Environmental Impact and Sustainability | 环境影响与可持续发展

The disposal of polyalkenes, such as plastics, poses significant environmental challenges. They are non-biodegradable and accumulate in landfills and oceans, causing pollution and harm to wildlife.

聚烯烃类塑料的处置带来了严峻的环境挑战。此类材料不可生物降解，会在地填和海洋中不断累积，造成污染并危害野生动物。

Several strategies are being developed to address this issue:

目前正在发展多种应对策略：

Feedstock recycling: converting polymers back into monomers or fuels.
Bioplastics: producing polymers from renewable resources such as corn starch.
Photodegradable polymers: incorporating light-sensitive groups into the polymer chain.
Substitution: replacing non-biodegradable materials with biodegradable alternatives.
原料回收：将聚合物重新转化为单体或燃料。
生物塑料：利用玉米淀粉等可再生资源生产聚合物。
光降解聚合物：在聚合物链中引入光敏基团。
替代：用可生物降解的替代品取代不可降解材料。

Additionally, chemists are designing new catalysts and reaction routes to produce polymers with better recyclability and lower energy demands, aligning with the principles of green chemistry.

此外，化学家正在设计新的催化剂和反应路线，以生产可回收性更好、能耗更低的聚合物，这符合绿色化学的原则。

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Alkenes: Structure, Reactions and Mechanisms | 烯烃：结构、反应与机理