Alkenes | 烯烃

📚 Alkenes | 烯烃

Alkenes are a fascinating family of hydrocarbons that contain at least one carbon-carbon double bond (C=C). This double bond makes alkenes unsaturated and gives them a rich chemistry dominated by addition reactions. In GCSE CIE Chemistry, you need to understand their structure, naming, characteristic reactions, and how they are produced and used. This guide covers all the key points for exam success.

烯烃是一类迷人的碳氢化合物，分子中至少含有一个碳碳双键（C=C）。这个双键使烯烃不饱和，并赋予它们以加成反应为主的丰富化学性质。在 GCSE CIE 化学课程中，你需要掌握它们的结构、命名、特征反应以及如何生产和利用它们。本指南将覆盖所有考试要点，助你成功。

1. Introduction to Alkenes | 烯烃简介

Alkenes are unsaturated hydrocarbons, meaning they contain fewer hydrogen atoms than the corresponding alkanes. The defining feature is the presence of a C=C double bond, which is a region of high electron density and is responsible for the chemical reactivity of alkenes.

烯烃是不饱和烃，这意味着它们所含的氢原子数比相应的烷烃少。其标志性特征是存在一个C=C双键，这个区域电子密度高，是烯烃化学反应活性的原因。

Because alkenes are unsaturated, they can undergo addition reactions, where atoms or groups add across the double bond to form a saturated molecule. This contrasts with alkanes, which typically undergo substitution reactions.

由于烯烃不饱和，它们可以发生加成反应，即原子或基团加到双键上形成饱和分子。这与烷烃不同，烷烃通常发生取代反应。

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2. General Formula and Homologous Series | 通式与同系物

Alkenes form a homologous series with the general formula:

烯烃构成同系物，其通式为：

CₙH₂ₙ

In this formula, n represents the number of carbon atoms. For example, when n = 2, the formula is C₂H₄ (ethene); when n = 3, it is C₃H₆ (propene); and when n = 4, it is C₄H₈ (butene).

在此公式中，n代表碳原子数。例如，当n=2时，分子式为C₂H₄（乙烯）；当n=3时，为C₃H₆（丙烯）；当n=4时，为C₄H₈（丁烯）。

Each member of the series differs from the next by a CH₂ unit. They all share similar chemical properties due to the C=C functional group, but physical properties such as boiling points increase gradually with increasing molecular size.

该同系物中相邻成员之间相差一个CH₂单元。由于都具有C=C官能团，它们化学性质相似，但物理性质如沸点随分子增大而逐渐升高。

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3. Naming Alkenes | 烯烃的命名

The names of alkenes follow the IUPAC system, based on the corresponding alkane with the suffix -ane replaced by -ene. The first four straight-chain alkenes are:

烯烃的命名遵循IUPAC体系，以相应的烷烃为基础，将词尾-ane改为-ene。前四个直链烯烃为：

• ethene (C₂H₄) | 乙烯
• propene (C₃H₆) | 丙烯
• but-1-ene and but-2-ene (C₄H₈) | 丁-1-烯和丁-2-烯

For butene, the position of the double bond must be indicated by a number. In but-1-ene, the double bond starts at the first carbon; in but-2-ene, it starts at the second carbon. This gives rise to position isomerism.

对于丁烯，必须用数字标明双键的位置。在丁-1-烯中，双键始于第一个碳原子；在丁-2-烯中，双键始于第二个碳原子。这就产生了位置异构现象。

When drawing displayed formulae, show all atoms and bonds. Always ensure that each carbon atom forms four bonds total.

在绘制结构式时，要表示出所有原子和键。始终确保每个碳原子总共形成四个键。

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4. Structure and Bonding of the C=C Double Bond | 碳碳双键的结构与成键

The carbon atoms in the C=C double bond are joined by one sigma (σ) bond and one pi (π) bond. The sigma bond is formed by the end-on overlap of orbitals, while the pi bond results from the sideways overlap of p orbitals above and below the plane of the molecule.

C=C双键中的碳原子由一个σ键和一个π键相连。σ键由轨道端向重叠形成，而π键则由p轨道在分子平面上方和下方侧面重叠而成。

This arrangement restricts rotation around the double bond and makes alkenes more reactive than alkanes. The pi bond is relatively weak and easily broken, allowing addition reactions to occur.

这种结构限制了双键的旋转，并使烯烃比烷烃更具反应性。π键相对较弱，容易断裂，从而能够发生加成反应。

In ethene, the molecule is planar with bond angles of approximately 120° around each carbon atom. All six atoms lie in the same plane.

在乙烯分子中，分子呈平面结构，每个碳原子周围的键角约为120°。所有六个原子都位于同一平面内。

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5. Unsaturation: The Bromine Water Test | 不饱和性：溴水测试

Alkenes can be distinguished from alkanes using the bromine water test. When orange-brown bromine water is shaken with an alkene, the solution rapidly turns colourless. This is because the bromine adds across the C=C double bond in an addition reaction.

烯烃可通过溴水测试与烷烃区分。将橙黄色的溴水与烯烃一起振荡，溶液迅速变为无色。这是因为溴在加成反应中加到C=C双键上。

The equation for the reaction with ethene is:

与乙烯反应的方程式为：

C₂H₄ + Br₂ → C₂H₄Br₂

The product is 1,2-dibromoethane, a colourless compound. In contrast, alkanes do not react with bromine water under normal conditions; the orange-brown colour remains.

产物为1,2-二溴乙烷，是一种无色化合物。相反，烷烃在通常条件下不与溴水反应；橙黄色不会褪去。

This test is a simple and effective way to confirm the presence of unsaturation in an organic compound. Note that UV light can cause alkanes to react slowly with bromine via substitution, but this is not observed in the standard bromine water test without UV.

该测试是确认有机化合物中不饱和存在的一种简单、有效的方法。注意，紫外光可以引发烷烃与溴发生缓慢的取代反应，但在没有紫外光的标准溴水测试中观察不到这一现象。

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6. Addition Reaction with Hydrogen (Hydrogenation) | 与氢气的加成反应（氢化）

Alkenes react with hydrogen gas in the presence of a nickel catalyst at about 150 °C to form the corresponding alkane. This is an addition reaction known as hydrogenation.

烯烃在约150 °C和镍催化剂存在下与氢气反应，生成相应的烷烃。这是一种加成反应，称为氢化。

For example, ethene reacts with hydrogen to produce ethane:

例如，乙烯与氢反应生成乙烷：

C₂H₄ + H₂ → C₂H₆

The hydrogen molecule adds across the double bond, saturating the hydrocarbon. This process is used industrially in the hardening of vegetable oils to make margarine. Unsaturated oils are hydrogenated to increase their melting points and convert them into solid spreads.

氢分子加到双键上，使烃达到饱和。该工艺在工业上用于植物油的硬化以制造人造黄油。不饱和油通过氢化提高熔点，转变为固态涂抹脂。

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7. Addition Reaction with Halogens (Halogenation) | 与卤素的加成反应（卤化）

Alkenes react with halogens such as chlorine and bromine to form dihalogenoalkanes. The reaction is rapid at room temperature and requires no catalyst. The general pattern is that the two halogen atoms add to the two carbon atoms of the double bond.

烯烃与氯、溴等卤素反应，生成二卤代烷。该反应在室温下迅速进行，无需催化剂。一般模式是两个卤素原子分别加到双键的两个碳原子上。

With ethene and bromine, the product is 1,2-dibromoethane:

乙烯与溴反应生成1,2-二溴乙烷：

C₂H₄ + Br₂ → C₂H₄Br₂

Similarly, ethene reacts with chlorine to give 1,2-dichloroethane:

类似地，乙烯与氯反应生成1,2-二氯乙烷：

C₂H₄ + Cl₂ → C₂H₄Cl₂

The mechanism involves the polarisation of the halogen molecule as it approaches the electron-rich double bond, leading to heterolytic fission. However, you do not need to describe the mechanism at GCSE level; simply learn the overall equation and conditions.

机理涉及卤素分子靠近富电子双键时发生极化，进而导致异裂。不过，GCSE阶段不需要描述机理，只需记住总方程式和条件。

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8. Addition Reaction with Water (Hydration) | 与水的加成反应（水化）

Alkenes can react with steam (water in the gaseous state) to produce alcohols. This is called hydration and requires an acid catalyst, typically phosphoric acid (H₃PO₄), high temperature (around 300 °C) and high pressure (about 60–70 atm).

烯烃可以与水蒸气（气态水）反应生成醇。这称为水化反应，需要酸催化剂（通常为磷酸H₃PO₄）、高温（约300 °C）和高压（约60–70 atm）。

For example, ethene reacts with steam to form ethanol:

例如，乙烯与水蒸气反应生成乙醇：

C₂H₄ + H₂O → C₂H₅OH

This is one of the two main industrial methods for manufacturing ethanol. The other is fermentation of sugars. The hydration of ethene gives a continuous process producing high-purity ethanol, but it relies on non-renewable crude oil as the source of ethene.

这是工业上生产乙醇的两种主要方法之一。另一种是糖的发酵。乙烯水化是一种连续工艺，可生产高纯度乙醇，但它依赖于不可再生的原油作为乙烯的来源。

The reaction is reversible, so conditions are chosen to shift the equilibrium towards the product and achieve a reasonable rate.

该反应是可逆的，因此选择的条件是为了使平衡向产物方向移动并获得合适的反应速率。

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9. Addition Reaction with Hydrogen Halides | 与氢卤酸的加成反应

Alkenes react with hydrogen halides such as hydrogen chloride (HCl), hydrogen bromide (HBr) and hydrogen iodide (HI) to form halogenoalkanes. The reaction is an addition reaction that occurs at room temperature.

烯烃与氯化氢(HCl)、溴化氢(HBr)和碘化氢(HI)等氢卤酸反应，生成卤代烷。该反应为加成反应，室温下即可进行。

For example, ethene reacts with hydrogen bromide to produce bromoethane:

例如，乙烯与溴化氢反应生成溴乙烷：

C₂H₄ + HBr → C₂H₅Br

With unsymmetrical alkenes such as propene, two possible products can form because the hydrogen and halogen can add in different orientations. The major product is often the one where the hydrogen atom attaches to the carbon with more hydrogen atoms already present (Markovnikov’s rule). However, detailed knowledge of this selectivity is not required for GCSE CIE; simply be aware that with propene, the addition of HBr can yield both 1-bromopropane and 2-bromopropane.

对于不对称烯烃如丙烯，由于氢和卤素的加成方向不同，可能生成两种产物。主要产物通常是氢原子加到已有较多氢原子的碳上的产物（马氏规则）。不过，GCSE CIE不要求详细了解这种选择性；只需知道丙烯与HBr加成可同时生成1-溴丙烷和2-溴丙烷即可。

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10. Addition Polymerisation | 加成聚合

Alkenes can undergo addition polymerisation, where many small alkene molecules (monomers) join together to form a long-chain polymer. The C=C double bonds open up and link carbon atoms together in a chain.

烯烃可以发生加成聚合，许多小的烯烃分子（单体）连接在一起形成长链聚合物。C=C双键打开，将碳原子连接成链。

The polymerisation of ethene produces poly(ethene), commonly known as polythene:

乙烯聚合生成聚（乙烯），俗称聚乙烯：

n C₂H₄ → -(-CH₂-CH₂-)-ₙ

Poly(ethene) is a versatile plastic used for plastic bags, bottles and containers. Similarly, propene can be polymerised to form poly(propene), which is used for ropes, carpets and medical equipment.

聚乙烯是一种用途广泛的塑料，用于塑料袋、瓶子和容器。类似地，丙烯可以聚合成聚丙烯，用于绳索、地毯和医疗设备。

Under high pressure and with a catalyst, polymer chains of varying lengths are produced. The properties of the polymer can be controlled by changing the reaction conditions. Addition polymers are typically saturated and chemically unreactive, making them durable.

在高压和催化剂作用下，生成不同长度的聚合物链。通过改变反应条件可以控制聚合物的性质。加成聚合物通常是饱和的、化学性质不活泼，因此经久耐用。

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11. Production of Alkenes by Cracking | 通过裂化生产烯烃

Alkenes are primarily produced by the cracking of longer-chain hydrocarbons from crude oil. Cracking involves breaking large alkane molecules into smaller, more useful hydrocarbons, including alkenes and short-chain alkanes.

烯烃主要通过裂化原油中的长链烃来生产。裂化是将大分子烷烃断裂成更小、更有用的烃，包括烯烃和短链烷烃。

There are two main types of cracking: catalytic cracking and thermal cracking. In catalytic cracking, heavy fractions are heated with a catalyst (often zeolites) at about 500 °C, producing a high proportion of branched alkanes and aromatic compounds. Thermal cracking uses high temperatures (up to 900 °C) and high pressure to produce shorter-chain alkenes such as ethene.

裂化主要有两种类型：催化裂化和热裂化。在催化裂化中，重质馏分与催化剂（通常为沸石）一起加热至约500 °C，产生高比例的支链烷烃和芳香族化合物。热裂化使用高温（高达900 °C）和高压，产生如乙烯等短链烯烃。

For example, the cracking of decane (C₁₀H₂₂) can yield ethene and octane:

例如，癸烷(C₁₀H₂₂)的裂化可生成乙烯和辛烷：

C₁₀H₂₂ → C₈H₁₈ + C₂H₄

Cracking is economically vital because it converts low-demand heavy fractions into high-demand products such as petrol, alkenes for plastics and hydrogen for the Haber process. The alkenes produced serve as crucial feedstocks for the chemical industry.

裂化在经济上至关重要，因为它将需求量低的重质馏分转化为需求量高的产品，如汽油、用于塑料的烯烃以及哈伯法制氨所需的氢气。所生成的烯烃是化学工业的关键原料。

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