📚 Addition reactions of the alkenes | 烯烃的加成反应
Alkenes are unsaturated hydrocarbons containing at least one C=C double bond. The double bond consists of a strong σ bond and a weaker π bond. Because the π electrons are exposed and less tightly held, alkenes readily undergo addition reactions in which the double bond opens up and two atoms or groups add across the two carbon atoms.
烯烃是含有至少一个C=C双键的不饱和烃。双键由一个较强的σ键和一个较弱的π键组成。由于π电子暴露在外且结合较松，烯烃容易发生加成反应，双键打开，两个原子或基团分别加到两个碳原子上。
1. Why alkenes undergo addition | 为什么烯烃发生加成反应
The carbon-carbon double bond is an area of high electron density. The π bond lies above and below the plane of the molecule and is much weaker than the σ bond. In an addition reaction, the relatively weak π bond breaks while the stronger σ bond remains intact. This allows two new σ bonds to form, making the product more stable.
碳碳双键是一个电子密度较高的区域。π键位于分子平面的上方和下方，强度远低于σ键。在加成反应中，相对较弱的π键断裂，而较强的σ键保持完整。这样能形成两个新的σ键，使产物更加稳定。
Because the C=C bond is unsaturated, addition converts an alkene into a saturated compound. The reaction is characteristic of compounds containing double or triple bonds. Unlike substitution reactions, addition reactions do not produce a second by-product when a simple molecule adds across the double bond.
由于C=C键不饱和，加成反应将烯烃转化为饱和化合物。该反应是含有双键或三键化合物的特征反应。与取代反应不同，当简单分子加成到双键上时，加成反应不会产生第二种副产物。
2. Electrophilic addition mechanism | 亲电加成机理
Most alkene addition reactions follow an electrophilic addition mechanism. An electrophile is an electron-pair acceptor attracted to the high electron density of the double bond. In the first step, the electrophile accepts a pair of electrons from the π bond, forming a bond to one carbon atom and leaving a carbocation intermediate on the other carbon atom.
大多数烯烃加成反应遵循亲电加成机理。亲电试剂是接受电子对的物种，被双键的高电子密度吸引。第一步，亲电试剂接受π键的一对电子，与一个碳原子成键，另一个碳原子上形成碳正离子中间体。
In the second step, a nucleophile or negative ion quickly attacks the carbocation to complete the addition. For example, in the reaction with HBr, the H-Br bond polarises, and the hydrogen acts as the electrophile while the bromide ion acts as the nucleophile. An alkene such as ethene is attacked by H⁺ first, forming a carbocation, and then Br⁻ bonds to the positive carbon.
第二步，亲核试剂或负离子迅速进攻碳正离子，完成加成。例如，在与HBr的反应中，H-Br键发生极化，氢作为亲电试剂，溴离子作为亲核试剂。乙烯等烯烃首先被H⁺进攻形成碳正离子，然后Br⁻与带正电的碳成键。
The movement of electron pairs in this mechanism is shown using curly arrows. One arrow starts from the π bond towards the electrophile, and a second arrow starts from the lone pair on the nucleophile towards the carbocation.
该机理中电子对的移动用弯曲箭头表示。一个箭头从π键指向亲电试剂，另一个箭头从亲核试剂上的孤对电子指向碳正离子。
3. Hydrogenation: adding H₂ | 加氢反应
Alkenes react with hydrogen gas in the presence of a nickel catalyst at about 150 °C. This is an addition reaction that converts an alkene into an alkane. For example, ethene reacts with hydrogen to form ethane:
烯烃在镍催化剂存在下、约150 °C时与氢气反应。该加成反应将烯烃转化为烷烃。例如，乙烯与氢气反应生成乙烷：
CH₂=CH₂ + H₂ → CH₃CH₃
The reaction is used in the food industry to harden vegetable oils by converting unsaturated fats into saturated fats. Hydrogenation is also used in organic synthesis to reduce double bonds. The nickel catalyst provides a surface on which the hydrogen and alkene are adsorbed, weakening the H-H and π bonds.
该反应用于食品工业中，通过将不饱和脂肪转化为饱和脂肪来硬化植物油。氢化也用于有机合成中还原双键。镍催化剂提供表面，使氢气和烯烃吸附其上，削弱H-H键和π键。
4. Halogenation: adding Br₂ or Cl₂ | 卤素加成
Alkenes react rapidly with halogens such as bromine or chlorine at room temperature. The reaction with bromine is widely used as a test for unsaturation. When bromine water, which is orange-brown, is shaken with an alkene, it is decolourised, forming a colourless dibromoalkane. For example:
烯烃在室温下与溴或氯等卤素迅速反应。与溴的反应广泛用于检验不饱和性。橙色溴水与烯烃一起振荡后会褪色，生成无色的二溴代烷。例如：
CH₂=CH₂ + Br₂ → CH₂BrCH₂Br
No UV light or catalyst is required, unlike the substitution reaction of alkanes with halogens. As a bromine molecule approaches the electron-rich double bond, it becomes polarised. The bromine atom closer to the double bond develops a partial positive charge and acts as the electrophile, while the other bromine atom leaves as a bromide ion.
与烷烃和卤素的取代反应不同，该反应不需要紫外光或催化剂。当溴分子接近富电子的双键时，溴分子发生极化。靠近双键的溴原子带上部分正电荷，作为亲电试剂，另一个溴原子以溴离子形式离开。
5. Reaction with hydrogen halides | 与卤化氢反应
Alkenes react with hydrogen halides such as HBr, HCl and HI to form halogenoalkanes. The reaction is usually carried out at room temperature by passing the dry hydrogen halide gas into the alkene. With a symmetrical alkene such as ethene, only one product is possible:
烯烃与HBr、HCl和HI等卤化氢反应生成卤代烷。反应通常在室温下将干燥卤化氢气体通入烯烃中进行。对于乙烯等对称烯烃，只可能生成一种产物：
CH₂=CH₂ + HBr → CH₃CH₂Br
With an unsymmetrical alkene, more than one product may be possible. The major product is predicted by Markovnikov’s rule. For example, propene reacts with HBr to give mainly 2-bromopropane rather than 1-bromopropane.
对于不对称烯烃，可能生成多种产物。主要产物由马氏规则预测。例如，丙烯与HBr反应主要生成2-溴丙烷而非1-溴丙烷。
CH₃CH=CH₂ + HBr → CH₃CHBrCH₃
6. Hydration: adding steam | 水合反应：与水蒸气加成
Alkenes react with steam to form alcohols. The reaction requires an acid catalyst, usually concentrated phosphoric acid, H₃PO₄, supported on silica, a temperature of about 300 °C and a pressure of about 60 atm. For example, ethene reacts with steam to form ethanol:
烯烃与水蒸气反应生成醇。反应需要酸催化剂，通常为负载在二氧化硅上的浓磷酸H₃PO₄，温度约300 °C，压力约60 atm。例如，乙烯与水蒸气反应生成乙醇：
CH₂=CH₂ + H₂O(g) ⇌ CH₃CH₂OH
This is an important industrial route to ethanol. The reverse reaction, elimination of water from ethanol, can occur under different conditions. Hydration of an unsymmetrical alkene also follows Markovnikov’s rule, so the hydroxyl group attaches to the carbon with fewer hydrogen atoms.
这是工业上生产乙醇的重要途径。在相反条件下，乙醇可发生脱水消除反应。不对称烯烃的水合反应也遵循马氏规则，因此羟基加到氢原子较少的碳原子上。
7. Markovnikov’s rule | 马氏规则
When an unsymmetrical alkene reacts with an unsymmetrical reagent such as HBr or H₂O, two products are often possible. Markovnikov’s rule states that the hydrogen atom adds to the carbon atom that already has the greater number of hydrogen atoms, while the halogen or hydroxyl group adds to the carbon with fewer hydrogen atoms.
当不对称烯烃与不对称试剂（如HBr或H₂O）反应时，通常可能生成两种产物。马氏规则指出，氢原子加到已连有较多氢原子的碳原子上，而卤素或羟基加到氢原子较少的碳原子上。
For example, propene reacts with HBr to give mainly 2-bromopropane rather than 1-bromopropane:
例如，丙烯与HBr反应主要生成2-溴丙烷而非1-溴丙烷：
CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (major)
CH₃CH=CH₂ + HBr → CH₃CH₂CH₂Br (minor)
Markovnikov’s rule is useful for predicting the major organic product in electrophilic addition reactions of unsymmetrical alkenes.
马氏规则可用于预测不对称烯烃亲电加成反应中的主要有机产物。
8. Carbocation stability and Markovnikov explanation | 碳正离子稳定性与马氏规则解释
The electrophilic addition mechanism goes through a carbocation intermediate. Carbocations can be primary, secondary or tertiary depending on how many alkyl groups are attached to the positively charged carbon. Alkyl groups release electron density, so the stability order is tertiary > secondary > primary > methyl.
亲电加成机理经过碳正离子中间体。根据带正电碳上连接的烷基数，碳正离子可分为伯、仲、叔。烷基能释放电子密度，因此稳定性顺序为叔 > 仲 > 伯 > 甲基。
In the addition of HBr to propene, attack by H⁺ can form either a primary carbocation or a secondary carbocation. The secondary carbocation is more stable and forms faster, so the major product comes from the more stable intermediate. This explains why the bromine atom attaches to the central carbon to give 2-bromopropane.
在丙烯与HBr加成中，H⁺进攻可形成伯碳正离子或仲碳正离子。仲碳正离子更稳定且生成更快，因此主要产物来自更稳定的中间体。这解释了溴原子连接到中间碳原子上生成2-溴丙烷的原因。
A more stable carbocation has more alkyl groups donating electron density towards the positive centre. This stabilisation lowers the activation energy for its formation, so the pathway leading to the more stable carbocation is favoured.
更稳定的碳正离子有更多烷基向正电中心提供电子密度。这种稳定作用降低了其生成的活化能，因此生成更稳定碳正离子的途径更受青睐。
9. Testing for unsaturation | 不饱和性检验
The decolourisation of bromine water is a simple chemical test for the presence of a carbon-carbon double bond. Alkenes decolourise orange-brown bromine water rapidly at room temperature without UV light. Alkanes do not decolourise bromine water under these conditions.
溴水褪色是检验碳碳双键存在的简单化学方法。烯烃在室温无紫外光条件下能迅速使橙棕色溴水褪色。烷烃在这些条件下不能使溴水褪色。
This test can distinguish an alkene from an alkane. The bromine adds across the double bond to form a colourless dibromo compound. In contrast, alkanes react with bromine only by a free-radical substitution mechanism, which requires ultraviolet light and produces a different colour change.
该检验可区分烯烃与烷烃。溴加成到双键上，生成无色的二溴化合物。相比之下，烷烃只能通过自由基取代机理与溴反应，该反应需要紫外光，并产生不同的颜色变化。
10. Summary of addition reactions | 加成反应总结
The table below summarises the main addition reactions of alkenes, their reagents, conditions and products.
下表总结了烯烃的主要加成反应、试剂、条件和产物。
| Reaction | Reagent | Conditions | Product |
|---|---|---|---|
| Hydrogenation | H₂ | Ni catalyst, 150 °C | Alkane |
| Halogenation
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