📚 Addition Reactions of the Alkenes | 烯烃的加成反应
ts of a strong σ bond and a weaker π bond. The presence of the electron-rich π bond makes alkenes highly reactive towards electrophiles, giving rise to a wide range of addition reactions. In an addition reaction, the double bond opens up and two atoms or groups add across the two carbon atoms, converting the alkene into a saturated compound. This article covers the major addition reactions of alkenes encountered in Cambridge A-Level Chemistry, including hydrogenation, halogenation, addition of hydrogen halides, hydration, and polymerisation, along with the underlying electrophilic addition mechanism, Markovnikov's rule, and the peroxide effect.
烯烃是含有碳碳双键 (C=C) 的不饱和烃。该双键由一个强σ键和一个较弱的π键组成。富含电子的π键使烯烃对亲电试剂具有高度反应活性，从而引发多种加成反应。在加成反应中，双键打开，两个原子或基团分别加到两个碳原子上，将烯烃转化为饱和化合物。本文涵盖剑桥A-Level化学中涉及的主要烯烃加成反应，包括加氢、卤化、卤化氢加成、水合及聚合反应，并详细阐述亲电加成机理、马氏规则和过氧化物效应。
1. Introduction to Addition Reactions of Alkenes | 烯烃加成反应简介
n be donated to an electrophile (electron-loving species). Therefore, the typical reactivity of alkenes is electrophilic addition. The general equation can be written as: R-CH=CH-R' + X-Y → R-CHX-CHY-R' ition reactions of alkenes are important in synthesis, analysis, and industrial processes.
烯烃中的碳碳双键因裸露的π电子而成为高电子密度区域。这些π电子结合松散，可以提供给亲电试剂（缺电子物种）。因此，烯烃的典型反应是亲电加成。通用方程式可写作：R-CH=CH-R' + X-Y → R-CHX-CHY-R'。产物是饱和分子，其中X和Y原子已加到原先的双键两侧。烯烃的加成反应在有机合成、分析化学和工业过程中都具有重要意义。
2. Catalytic Hydrogenation | 催化加氢
catalytic hydrogenation. The conditions are typically 150 °C and a nickel catalyst, though platinum or palladium can be used at room temperature. The reaction is exothermic and involves the addition of H₂ across the double bond. The general equation is: CₙH₂ₙ + H₂ → CₙH₂ₙ₊₂. For example, ethene is hydrogenated to ethane: CH₂=CH₂ + H₂ → CH₃-CH₃. This reaction is used to convert unsaturated vegetable oils into solid margarine by partially hydrogenating the double bonds present in the fatty acid chains.
烯烃在镍、铂或钯等金属催化剂存在下与氢气反应生成烷烃。该反应称为催化加氢。条件通常为150 °C并使用镍催化剂，而铂或钯可在室温下使用。反应放热，涉及H₂加至双键上。通用方程式为：CₙH₂ₙ + H₂ → CₙH₂ₙ₊₂。例如，乙烯加氢生成乙烷：CH₂=CH₂ + H₂ → CH₃-CH₃。该反应用于将不饱和植物油转化为固体人造黄油，通过部分氢化脂肪酸链中的双键来实现。
3. Electrophilic Addition Mechanism | 亲电加成机理
Most addition reactions of alkenes proceed by an electrophilic addition mechanism. The mechanism involves two main steps. In the first, rate-determining step, the π electrons attack the electrophile (E⁺), forming a covalent bond to one carbon atom and generating a carbocation intermediate on the adjacent carbon. In the second, fast step, the negatively charged species (nucleophile, Nu⁻) attacks the carbocation to form the final product. For symmetrical alkenes and symmetrical reagents, only one product is possible. When the reagents or alkenes are unsymmetrical, multiple products may form, and the major product is governed by carbocation stability or Markovnikov's rule.
大多数烯烃的加成反应遵循亲电加成机理。该机理包含两个主要步骤。第一步是速率决定步骤，π电子进攻亲电试剂 (E⁺)，与一个碳原子形成共价键，同时在相邻碳原子上生成碳正离子中间体。第二步是快速步骤，带负电荷的物种（亲核试剂，Nu⁻）进攻碳正离子，生成最终产物。对于对称烯烃和对称试剂，只能得到一种产物。当试剂或烯烃不对称时，可能生成多种产物，主要产物取决于碳正离子稳定性或马氏规则。
4. Addition of Halogens: Bromination | 卤素加成：溴化反应
ine water) is a classic test for unsaturation: the orange-brown colour of bromine is decolourised. The reaction with ethene produces 1,2-dibromoethane: CH₂=CH₂ + Br₂ → CH₂Br-CH₂Br. The mechanism involves the polarisation of the Br-Br bond as it approaches the π electron cloud, leading to the formation of a cyclic bromonium ion intermediate and a bromide ion. The bromide ion then attacks from the opposite side, giving overall anti-addition.
烯烃在室温下无需光照即可与卤素（Cl₂、Br₂）快速反应。溴的加成（在有机溶剂中或使用溴水）是检验不饱和度的经典实验：溴的红棕色褪去。与乙烯反应生成1,2-二溴乙烷：CH₂=CH₂ + Br₂ → CH₂Br-CH₂Br。机理涉及当Br-Br键靠近π电子云时发生极化，形成环状溴鎓离子中间体和溴负离子。然后溴负离子从反面进攻，总体为反式加成。
5. Addition of Hydrogen Halides | 卤化氢加成
des or with concentrated aqueous acids. For example, ethene reacts with hydrogen bromide to give bromoethane: CH₂=CH₂ + HBr → CH₃CH₂Br. The mechanism: the π bond accepts H⁺ from HBr, forming a carbocation and Br⁻, which then combines. With unsymmetrical alkenes such as propene, two products are possible, and the major product is determined by the stability of the carbocation intermediate.
卤化氢（HCl、HBr、HI）加至烯烃双键上生成卤代烷。气态卤化氢或浓酸水溶液可快速进行该反应。例如，乙烯与溴化氢反应生成溴乙烷：CH₂=CH₂ + HBr → CH₃CH₂Br。机理为：π键接受来自HBr的H⁺，形成碳正离子和Br⁻，随后两者结合。对于不对称烯烃如丙烯，可能得到两种产物，主要产物由碳正离子中间体的稳定性决定。
6. Markovnikov's Rule | 马氏规则
mmetrical alkene, the hydrogen atom of H-X attaches to the carbon atom of the double bond that already has the greater number of hydrogen atoms. The halogen attaches to the more substituted carbon. This is Markovnikov' inly 2-bromopropane (CH₃CHBrCH₃), not 1-bromopropane. The reason lies in carbocation stability: secondary and tertiary carbocations are more stable than primary due to positive inductive effect and hyperconjugation. The reaction proceeds via the more stable carbocation, leading to the Markovnikov product.
当卤化氢与不对称烯烃加成时，H-X 中的氢原子加到双键中原本氢原子较多的碳上，而卤素加到取代较多的碳上。这就是马氏规则。例如，丙烯与 HBr 反应主要生成 2-溴丙烷 (CH₃CHBrCH₃)，而非 1-溴丙烷。原因在于碳正离子稳定性：由于正诱导效应和超共轭效应，仲碳正离子和叔碳正离子比伯碳正离子更稳定。反应经由更稳定的碳正离子进行，从而得到马氏规则产物。
7. Addition of Water: Hydration of Alkenes | 水的加成：烯烃水合
id). This reaction is called hydration. Because water is a poor electrophile, the acid catalyst provides H⁺ to initiate electrophilic addition. The conditions are: high temperature (300-330 °C) and high pressure (60-70 atm) with a phosphoric(V) acid catalyst on a solid silica support. For example, ethene is hydrated to ethanol: CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH. With unsymmetrical alkenes like propene, the addition follows Markovnikov's rule, giving propan-2-ol as the major product.
烯烃可以通过酸催化（通常是浓磷酸或硫酸）与水直接加成转化为醇，该反应称为水合。由于水是弱的亲电试剂，酸催化剂提供 H⁺ 以启动亲电加成。反应条件为：高温（300-330 °C）、高压（60-70 atm），并使用负载于固体二氧化硅上的磷酸催化剂。例如，乙烯水合生成乙醇：CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH。对于不对称烯烃如丙烯，加成遵循马氏规则，主要生成丙-2-醇。
8. Addition of Sulfuric Acid: Indirect Hydration | 硫酸加成：间接水合
drogensulfates. For example, ethene + H₂SO₄ → ethyl hydrogensulfate (CH₃CH₂OSO₂OH). This is an addition reaction across the double bond, and with unsymmetrical alkenes, Markovnikov' CH₃CH₂OSO₂OH + H₂O → CH₃CH₂OH + H₂SO₄. This indirect hydration was historically important before direct catalytic hydration became widespread.
一种较旧的从烯烃制醇方法涉及两步。首先，烯烃与冷的浓硫酸反应生成硫酸氢烷基酯。例如，乙烯 + H₂SO₄ → 硫酸氢乙酯 (CH₃CH₂OSO₂OH)。该反应为双键上的加成，对于不对称烯烃适用马氏规则。第二步，硫酸氢烷基酯与水加热水解，得到醇并再生硫酸：CH₃CH₂OSO₂OH + H₂O → CH₃CH₂OH + H₂SO₄。在直接催化水合普及之前，这种间接水合法在历史上具有重要意义。
9. Anti-Markovnikov Addition (Peroxide Effect) | 反马氏加成（过氧化物效应）
When hydrogen bromide (but not HCl or HI) adds to an unsymmetrical alkene in the presence of organic peroxides or oxygen, the product is the anti-Markovnikov isomer. This is known as the peroxide effect or Kharasch effect. For example, propene + HBr with peroxides gives 1-bromopropane as the major product, rather than 2-bromopropane. The reaction proceeds via a free-radical mechanism instead of the ionic electrophilic addition. The Br• radical adds first to the less substituted end of the double bond, generating the more stable secondary radical, which then abstracts H• from another HBr molecule. This effect is specific to HBr because the H-Cl and H-I bond strengths are less suited for the radical chain propagation steps.
当溴化氢（而非 HCl 或 HI）在有机过氧化物或氧气存在下与不对称烯烃加成时，产物为反马氏规则异构体。这被称为过氧化物效应或卡拉施效应。例如，丙烯与 HBr 在过氧化物存在下反应，主要生成 1-溴丙烷，而非 2-溴丙烷。该反应通过自由基机理进行，而非离子型亲电加成。Br• 自由基首先加到双键取代较少的一端，生成较稳定的仲自由基，然后该自由基从另一 HBr 分子中夺取 H•。此效应仅对 HBr 有效，因为 H-Cl 和 H-I 的键能不适宜自由基链增长步骤。
10. Addition Polymerisation | 加成聚合
onds to form a long-chain polymer. This is an addition reaction because no small molecules are eliminated. For example, ethene polymerises to poly(ethene), commonly known as polythene: n CH₂=CH₂ → -[CH₂-CH₂]-n. The polymer is a saturated alkane backbone with repeating units. Poly(propene), poly(styrene), and poly(tetrafluoroethene) (PTFE) are other important examples. Polymerisation requires high pressure, a catalyst, or a free-radical initiator. The mechanism involves initiation, propagation, and termination steps in a free-radical chain reaction. Addition polymers are widely used in plastics and synthetic materials.
烯烃及取代烯烃可发生加成聚合反应，许多小单体分子通过打开双键而连接成高分子长链。因为不脱去小分子，这属于加成反应。例如，乙烯聚合生成聚（乙烯），常称为聚乙烯：n CH₂=CH₂ → -[CH₂-CH₂]-n。该聚合物为具有饱和烷烃主链的重复单元结构。聚丙烯、聚苯乙烯和聚四氟乙烯 (PTFE) 是其他重要例子。聚合反应需要高压、催化剂或自由基引发剂。机理涉及自由基链反应中的引发、增长和终止步骤。加成聚合物广泛用于塑料和合成材料。
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