The Alkenes | 烯烃

📚 The Alkenes | 烯烃

Alkenes are one of the most reactive organic compound families in A‑Level Chemistry. Their characteristic carbon–carbon double bond opens the door to a huge range of addition reactions, stereoisomerism and industrial polymerisation processes that make them indispensable in the modern chemical world.

烯烃是 A‑Level 化学中反应活性最高的有机化合物系列之一。它们特有的碳碳双键为丰富多彩的加成反应、立体异构现象以及工业聚合过程打开了大门，使烯烃在现代化学世界中不可或缺。

1. What Are Alkenes? | 什么是烯烃？

Alkenes are unsaturated hydrocarbons that contain at least one carbon– e bond (C=C). They follow the general molecular formula CₙH₂ₙ for monounsaturated straight‑ he π electron cloud is the source of the high reactivity of alkenes towards electrophiles.

烯烃是含有至少一个碳碳双键 (C=C) 的不饱和烃。对于单不饱和直链和支链烯烃，它们遵循通式 CₙH₂ₙ。双键由一个较强的 σ 键和一个较弱的 π 键组成，而 π 电子云正是烯烃对亲电试剂表现出高反应活性的根源。

2. Nomenclature of Alkenes | 烯烃的命名

the end nearest to the double bond, indicate the position of the double bond by the lower‑numbered carbon atom involved, and use the suffix -ene Substituents are named as prefixes with their positions. For example, CH₂=CHCH₂CH₃ is but‑1‑ene, while CH₃CH=CHCH₃ is but‑2‑ene.

烯烃的 IUPAC 命名遵循以下关键规则：选择含有 C=C 双键的最长连续碳链作为主链，从靠近双键一端开始给碳原子编号，用参与双键的编号较小的碳原子标示双键位置，词尾用 -ene（中文用“烯”）。取代基作为前缀并标明位次。例如，CH₂=CHCH₂CH₃ 是 1‑丁烯，而 CH₃CH=CHCH₃ 是 2‑丁烯。

3. Isomerism in Alkenes | 烯烃的同分异构现象

Alkenes exhibit both structural isomerism and stereoisomerism. Structural isomers arise from different positions of the double bond or branching of the carbon skeleton. For example, but‑1‑ene and but‑2‑ene are positional isomers. More importantly, alkenes with two different groups attached to each carbon of the double bond show cis‑trans (geometric) isomerism. When the higher‑priority groups are on the same side of the double bond, it is the Z isomer (from German zusammen); when they are on opposite sides, it is the E isomer (entgegen). The Cahn–Ingold–Prelog (CIP) rules assign priority based on atomic number.

烯烃既有构造异构也有立体异构。构造异构源自双键位置不同或碳骨架支链化。例如，1‑丁烯和2‑丁烯就是位置异构体。更重要的是，当双键的每个碳原子上连接了两个不同的基团时，烯烃表现出顺‑反（几何）异构现象。Cahn–Ingold–Prelog (CIP) 规则根据原子序数确定优先基团。较高优先基团在双键同侧时为 Z 构型（德语 zusammen），在异侧时为 E 构型（entgegen）。

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

The physical properties of alkenes resemble those of the corresponding alkanes. They are non‑polar molecules, so they are insoluble in water but soluble in non‑polar organic solvents. Boiling points increase with increasing molecular mass because of larger van der Waals forces. However, an alkene typically has a slightly lower boiling point than the alkane with the same carbon skeleton due to a less efficient packing shape. Melting points also rise with molar mass, but symmetry plays an additional role; for instance, the trans (E) isomer often packs better and has a higher melting point than the cis (Z) isomer.

烯烃的物理性质与相应烷烃相似。它们是非极性分子，因此不溶于水但可溶于非极性有机溶剂。沸点随分子量增大而升高，这是由于范德华力增强。不过，由于分子形状堆积效率较低，烯烃的沸点通常比碳骨架相同的烷烃略低。熔点也随摩尔质量增加而升高，但对称性也起重要作用；比如 trans (E) 异构体往往堆积得更紧密，熔点往往高于 cis (Z) 异构体。

5. Electrophilic Addition Reactions | 亲电加成反应

Alkenes are electron‑rich because of the exposed π bond. They therefore react with electrophiles (electron‑deficient species) in electrophilic addition reactions. The general mechanism involves two steps: the π electrons attack the electrophile, forming a carbocation intermediate and releasing a leaving group or generating a new bond; then a nucleophile rapidly combines with the carbocation to give the saturated product. This mechanism explains the addition of hydrogen halides (HX), halogens (X₂), concentrated sulfuric acid and water (steam, acid‑catalysed).

由于暴露的 π 键，烯烃是富电子体，因此会与亲电试剂（缺电子物种）发生亲电加成反应。一般机理分两步：π 电子进攻亲电试剂，形成碳正离子中间体并释放离去基团或形成新键；然后亲核试剂快速与碳正离子结合生成饱和产物。这一机理解释了卤化氢 (HX)、卤素 (X₂)、浓硫酸和水（水蒸气，酸催化）的加成反应。

6. Markovnikov's Rule | 马氏规则

When a hydrogen halide (HX) or water adds to an unsymmetrical alkene, two constitutional isomers are possible. Markovnikov's rule states that the hydrogen atom attaches to the carbon atom that already has the greater number of hydrogen atoms (the less substituted carbon), while the halide (or –OH group) attaches to the more substituted carbon. This outcome is governed by carbocation stability: tertiary > secondary > primary > methyl. The more stable carbocation intermediate forms faster, leading to the Markovnikov product as the major product.

当卤化氢 (HX) 或水与不对称烯烃加成时，原则上可能生成两种构造异构体。马氏规则指出：氢原子加到含氢较多的碳原子（取代较少的碳）上，而卤素（或 –OH 基团）加到取代较多的碳上。这一规律由碳正离子稳定性决定：三级 > 二级 > 一级 > 甲基。越稳定的碳正离子中间体生成越快，因此马氏加成产物是主要产物。

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7. Addition of Hydrogen Halides and Halogens | 与卤化氢和卤素的加成

Alkenes react readily with HCl, HBr and HI at room temperature to give the corresponding haloalkanes. With bromine or chlorine, the addition occurs rapidly in the dark at room temperature without a catalyst. The bromine test is a classic chemical test for unsaturation: when an alkene is shaken with orange‑brown bromine water, the colour immediately disappears, producing a colourless dibromoalkane. The mechanism for halogen addition involves a cyclic bromonium (or chloronium) ion intermediate, which accounts for the anti‑stereospecificity of the addition – the two halogen atoms add from opposite faces of the double bond.

烯烃在室温下即可与 HCl、HBr 和 HI 加成，生成相应的卤代烷。对溴或氯而言，不需催化剂，室温暗处即可迅速发生加成反应。溴水试验是检验不饱和键的经典化学方法：当烯烃与橙黄色的溴水摇动混合时，颜色立即褪去，生成无色的二溴代烷。卤素加成的机理涉及环状溴鎓（或氯鎓）离子中间体，这解释了加成的反式立体专一性——两个卤原子从双键的相反面加上去。

8. Addition of Water (Hydration) and Sulfuric Acid | 水的加成（水合）与硫酸

Alkenes do not react directly with water, but in the presence of a strong acid catalyst – commonly concentrated phosphoric(V) acid on a solid support or concentrated sulfuric acid – hydration occurs to give alcohols. The industrial process for ethanol uses ethene and steam with a H₃PO₄ catalyst at about 300 °C and 60–70 atm. In the laboratory, a two‑step route can be used: an alkene is first treated with cold concentrated H₂SO₄ to form an alkyl hydrogen sulfate, which is then hydrolysed by warming with water to produce the alcohol. Both routes follow Markovnikov's rule.

烯烃不能与水直接反应，但在强酸催化剂存在下——通常是负载的浓磷酸或浓硫酸——则可发生水合反应生成醇。工业上由乙烯和水蒸气在磷酸催化剂、约 300 °C 和 60–70 大气压下生产乙醇。实验室可采用两步法：先用冷的浓硫酸处理烯烃生成硫酸氢烷基酯，然后加水加热水解得到醇。两种途径都遵循马氏规则。

9. Oxidation Reactions of Alkenes | 烯烃的氧化反应

formation of a diol; the purple colour of MnO₄⁻ disappears and a brown precipitate of MnO₂ is observed. This reaction is also a test for unsaturation (Baeyer test). Strong oxidation with hot, acidified KMnO₄ leads to cleavage of the double bond and gives carbonyl compounds or carboxylic acids depending on the substitution pattern of the alkene. For example, ethene gives CO₂, a disubstituted alkene like CH₃CH=CHCH₃ yields carboxylic acids, and a terminal alkene gives a carboxylic acid plus CO₂.

烯烃可在不同条件下被氧化。用冷的、稀的碱性高锰酸钾 (KMnO₄) 温和氧化，生成邻二醇；MnO₄⁻ 的紫色消失并观察到棕色 MnO₂ 沉淀。该反应也可用于检验不饱和键（拜耳试验）。用热的、酸化的高锰酸钾进行强氧化，则导致双键断裂，根据烯烃的取代模式生成羰基化合物或羧酸。例如，乙烯被氧化成 CO₂，二取代烯烃如 CH₃CH=CHCH₃ 生成羧酸，而末端烯烃生成羧酸和 CO₂。

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

long saturated carbon chain. Poly(ethene) is produced from ethene under high pressure (low‑density LDPE) or using a Ziegler–Natta catalyst (high‑ y important addition polymers. The repeating unit is enclosed in brackets with a subscript n to indicate the degree of polymerisation.

烯烃进行加成聚合反应，大量单体分子通过打开 C=C 双键相互连接，形成长长的饱和碳链。聚乙烯可由乙烯在高压下（低密度聚乙烯 LDPE）或用齐格勒‑纳塔催化剂（高密度聚乙烯 HDPE）制备。聚丙烯、聚氯乙烯 (PVC) 和聚苯乙烯等其他加成聚合物也具有重要商业价值。重复单元用方括号括起来，下标 n 表示聚合度。

11. Preparation of Alkenes | 烯烃的制备

Two main laboratory methods prepare alkenes: dehydration of alcohols and dehydrohalogenation of haloalkanes. Acid‑catalysed dehydration of alcohols using concentrated H₂SO₄ or Al₂O₃ at high temperature eliminates a water molecule to form the alkene; it is an E1 elimination for secondary and tertiary alcohols. The dehydrohalogenation of haloalkanes with hot ethanolic potassium hydroxide proceeds via an E2 elimination mechanism. Both methods yield the more substituted alkene where isomerism is possible, according to Zaitsev's rule.

实验室制备烯烃主要有两种方法：醇的脱水和卤代烷的脱卤化氢。用浓硫酸或氧化铝在高温下催化醇的脱水，脱去一分子水生成烯烃；对仲醇和叔醇属于 E1 消除。卤代烷与热的氢氧化钾乙醇溶液反应则按 E2 消除机理进行。根据扎伊采夫规则，两种方法在可能生成异构体时，都会优先生成取代更多的烯烃。

12. Industrial Importance and Testing for Unsaturation | 工业重要性与不饱和度的检验

of naphtha or ethane. They are converted into polymers, ethanol, detergents, antifreeze and a wide array of organic intermediates. In the laboratory, unsaturation in alkenes is rapidly identified by two simple chemical tests: decolourisation of orange‑ O₂ precipitate (oxidation). Both tests are reliable for the presence of a C=C double bond.

烯烃是石油化学工业中最重要的原料之一。乙烯和丙烯由石脑油或乙烷蒸汽裂解大规模生产。它们可转化为聚合物、乙醇、洗涤剂、防冻剂以及各种有机中间体。在实验室中，通过两种简单的化学试验可快速鉴别烯烃的不饱和性：橙黄色溴水褪色（亲电加成）以及紫色酸性高锰酸钾溶液褪色并生成棕色 MnO₂ 沉淀（氧化）。两种试验都可靠地指示 C=C 双键的存在。

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