📚 The Alkenes | 烯烃
ond is the functional group responsible for the characteristic addition reactions of alkenes, which makes them important starting materials in organic synthesis and in the petrochemical industry.
烯烃是一类含有至少一个碳碳双键 (C=C) 的不饱和烃。双键是决定烯烃特征加成反应的官能团，使烯烃成为有机合成和石化工业中重要的起始原料。
This article works through structure and bonding, the general formula CₙH₂ₙ, the homologous series and IUPAC naming, geometric isomerism, physical properties, electrophilic addition, oxidation, addition polymerisation, preparation and industrial importance, and finally the exam points and common mistakes that cost marks most often.
本文依次讲解结构与成键、通式 CₙH₂ₙ、同系物与 IUPAC 命名、几何异构、物理性质、亲电加成、氧化反应、加聚反应、制备方法与工业意义，最后总结最常考的要点与最容易丢分的错误。
1. Structure and Bonding | 结构与成键
In ethene (C₂H₄), each carbon atom uses three sp² hybrid orbitals to form three sigma (σ) bonds: one to the other carbon atom and two to hydrogen atoms. The remaining unhybridised 2p orbital on each carbon overlaps sideways to form a pi (π) bond above and below the plane of the molecule.
在乙烯 (C₂H₄) 中，每个碳原子使用三个 sp² 杂化轨道形成三个 σ 键：一个与另一个碳原子成键，两个与氢原子成键。每个碳上剩余的一个未杂化的 2p 轨道在分子平面的上下方侧向重叠，形成一个 π 键。
The double bond therefore consists of one σ bond and one π bond. The π bond is weaker than the σ bond because the sideways overlap of the p orbitals is less effective and the π electrons are more exposed, further from the nuclei. The carbon-carbon double bond is shorter and stronger than a single C–C bond. The bond angle around each carbon is about 120° and the arrangement around the double bond is planar.
因此，双键由一个 σ 键和一个 π 键组成。π 键比 σ 键弱，因为 p 轨道的侧向重叠效率较低，而且 π 电子离核较远、暴露程度更高。碳碳双键比单键 C–C 更短、更强。每个碳周围的键角约为 120°，双键周围的原子呈平面排列。
| Bond | 键 | Length / pm | 键长 | Enthalpy / kJ mol⁻¹ | 键焓 |
| C–C | 154 | 347 |
| C=C | 134 | 612 |
| C≡C | 120 | 837 |
Note that the C=C bond enthalpy (612 kJ mol⁻¹) is less than twice the C– er of the two, which is exactly why alkenes react by addition rather than by substitution.
注意 C=C 的键焓 (612 kJ mol⁻¹) 小于 C–C 键焓的两倍 (2 × 347 = 694 kJ mol⁻¹)。差值约 80 kJ mol⁻¹，说明 π 键是两者中较弱的，这正是烯烃发生加成反应而不是取代反应的原因。
cted because the π bond would have to break for the two ends of the molecule to twist relative to one another. This restricted rotation gives rise to geometric isomerism, which is discussed in section 4.
围绕 C=C 键的旋转受到限制，因为要让分子两端相对扭转，就必须先断裂 π 键。这种旋转受限导致了几何异构现象，将在第 4 节讨论。
2. General Formula and the Homologous Series | 通式与同系物
general formula is CₙH₂ₙ. Each member of the homologous series differs from the next by a –CH₂– y show similar chemical properties because they all contain the same C=C functional group.
对于含有一个双键的无环烯烃，其通式为 CₙH₂ₙ。同系物中相邻成员相差一个 –CH₂– 单元；由于都含有相同的 C=C 官能团，它们表现出相似的化学性质。
Examples include ethene (C₂H₄), propene (C₃H₆), but-1-ene and but-2-ene (C₄H₈), and pent-1-ene (C₅H₁₀). As the chain length increases, boiling points increase because the molecules have a larger surface area of contact and stronger van der Waals forces between them.
例子包括乙烯 (C₂H₄)、丙烯 (C₃H₆)、丁-1-烯和丁-2-烯 (C₄H₈) 以及戊-1-烯 (C₅H₁₀)。随着碳链增长，分子间接触面积增大、范德华力增强，因此沸点升高。
ee of unsaturation, i.e. two fewer hydrogen atoms than the corresponding alkane. For a hydrocarbon containing x double bonds and no rings, the formula becomes CₙH₂ₙ₊₂₋₂ₓ. For example, a diene such as buta-1,3-diene has the formula C₄H₆.
每个 C=C 单元对应一个不饱和度，即比相应的烷烃少两个氢原子。对于含 x 个双键且无环的烃，通式变为 CₙH₂ₙ₊₂₋₂ₓ。例如双烯如丁-1,3-二烯的分子式为 C₄H₆。
Cycloalkanes share the formula CₙH₂ₙ with alkenes, so C₄H₈ could be but-1-ene, but-2-ene, 2-methylpropene or cyclobutane. Determining the structure therefore requires chemical tests as well as the molecular formula.
环烷烃与烯烃共享通式 CₙH₂ₙ，所以 C₄H₈ 可能是丁-1-烯、丁-2-烯、2-甲基丙烯或环丁烷。因此确定结构不仅需要分子式，还需要化学检验。
3. Nomenclature and Structural Isomerism | 命名与结构异构
at the double bond carbons receive the lowest possible numbers, use the suffix -ene with the position number of the first double-bond carbon, and name substituents with their own locants.
烯烃的 IUPAC 命名规则：找出包含 C=C 键的最长碳链；编号时使双键碳获得尽可能小的编号；用后缀 -ene 并标出双键第一个碳的位置编号；取代基则各自标明位置。
- CH₂=CH₂ — ethene | 乙烯
- CH₃CH=CH₂ — propene | 丙烯
- CH₂=CHCH₂CH₃ — but-1-ene | 丁-1-烯
- CH₃CH=CHCH₃ — but-2-ene | 丁-2-烯
- CH₂=C(CH₃)₂ — 2-methylpropene | 2-甲基丙烯
Structural isomerism in alkenes arises in three ways: position isomerism (the double bond sits between different pairs of carbons, as in but-1-ene and but-2-ene), chain isomerism (branching of the carbon skeleton, as in but-1-ene and 2-methylpropene) and functional group isomerism (cycloalkanes with the same molecular formula).
烯烃的结构异构有三种来源：位置异构（双键位于不同的碳原子之间，如丁-1-烯与丁-2-烯）、碳链异构（碳骨架的分支方式不同，如丁-1-烯与 2-甲基丙烯）以及官能团异构（分子式相同的环烷烃）。
4. Geometric (cis-trans) Isomerism | 几何（顺反）异构
d and each double-bond carbon carries two different groups. If either carbon bears two identical groups, no geometric isomers exist. For example, propene and 2-methylpropene show no cis-trans isomerism.
当围绕 C=C 键的旋转受限，且每个双键碳上连接的两个基团互不相同时，就会出现几何异构。若任一碳上连有两个相同的基团，则不存在几何异构体。例如丙烯和 2-甲基丙烯都没有顺反异构。
But-2-ene has two geometric isomers. In cis-but-2-ene the two methyl groups are on the same side of the double bond; in trans-but-2-ene they are on opposite sides.
丁-2-烯有两个几何异构体。在顺-丁-2-烯中，两个甲基位于双键同侧；在反-丁-2-烯中，两个甲基位于双键异侧。
cis-but-2-ene: CH₃ groups on the same side | trans-but-2-ene: CH₃ groups on opposite sides
The E/Z system is used when the groups are not obviously the same. Priority is assigned by the Cahn-Ingold-Prelog rules: higher atomic number wins, and if the first atoms are identical the comparison moves outwards along the chain. If the two higher-priority groups are on the same side the isomer is Z (zusammen, together); if on opposite sides it is E (entgegen, opposite).
当基团不容易直接比较时，使用 E/Z 命名系统。优先级按 Cahn-Ingold-Prelog 规则确定：原子序数大者优先；若第一个原子相同，则沿碳链继续向外比较。若两个高优先级基团位于同侧，该异构体为 Z（zusammen，一起）；若位于异侧则为 E（entgegen，相反）。
Geometric isomers have different physical properties: cis-but-2-ene boils at about 3.7 °C and has a small dipole moment, whereas trans-but-2-ene is almost non-polar and boils at about 0.9 °C, but has the higher melting point because its molecules pack more efficiently in the solid lattice.
几何异构体的物理性质不同：顺-丁-2-烯沸点约为 3.7 °C，具有较小偶极矩；反-丁-2-烯几乎无极性，沸点约为 0.9 °C，但由于分子在固态晶格中堆积更紧密，其熔点更高。
5. Physical Properties | 物理性质
Alkenes are non-polar or only weakly polar molecules. They are insoluble in water because they cannot form hydrogen bonds with water molecules, but they dissolve readily in non-polar organic solvents such as hexane, benzene and tetrachloromethane.
烯烃分子为非极性或弱极性分子。由于不能与水分子形成氢键，它们不溶于水，但易溶于己烷、苯、四氯化碳等非极性有机溶剂。
Boiling points rise with increasing relative molecular mass because the temporary dipoles that produce van der Waals forces become larger and the contact area increases. At room temperature, ethene, propene and the butenes are gases; pentenes and higher alkenes are liquids. All alkenes are less dense than water.
沸点随相对分子质量增大而升高，因为产生范德华力的瞬时偶极增大，分子间接触面积也增大。室温下，乙烯、丙烯和丁烯是气体；戊烯及更高级的烯烃是液体。所有烯烃的密度都小于水。
| Alkene | 烯烃 | Formula | 分子式 | Mᵣ | b.p. / °C | 沸点 |
| ethene | 乙烯 | C₂H₄ | 28 | −104 |
| propene | 丙烯 | C₃H₆ | 42 | −47 |
| but-1-ene | 丁-1-烯 | C₄H₈ | 56 | −6 |
| pent-1-ene | 戊-1-烯 | C₅H₁₀ | 70 | 30 |
Branching lowers the boiling point because it makes the molecule more spherical, reducing the surface area available for intermolecular contact. This is the same trend seen in alkanes and alcohols.
支链会降低沸点，因为分子更接近球形，可供分子间接触的表面积减小。这与烷烃和醇中的规律一致。
6. Electrophilic Addition — the Core Reaction | 亲电加成——核心反应
osed above and below the molecular plane and form a region of high electron density. This attracts electrophiles (electron-pair acceptors), so the characteristic reaction of alkenes is electrophilic addition, in which the π bond breaks and two new σ bonds form.
C=C 键的 π 电子暴露在分子平面上下方，形成高电子云密度区域。这会吸引亲电试剂（电子对受体），因此烯烃的特征反应是亲电加成：π 键断裂，同时形成两个新的 σ 键。
The general mechanism has two steps. Step 1: the electrophile E⁺ attacks the π bond and forms a new bond using both π electrons, producing a carbocation intermediate and an anion. Step 2: the nucleophile Nu⁻ (or a lone pair from the anion) attacks the positively charged carbon to give the final addition product.
一般机理分两步。第一步：亲电试剂 E⁺ 进攻 π 键，用两个 π 电子形成新键，生成碳正离子中间体和阴离子。第二步：亲核试剂 Nu⁻（或来自阴离子的孤对电子）进攻带正电的碳，得到最终加成产物。
C=C + E–Nu → C–C with E and Nu added across the double bond
Carbocation stability follows the order tertiary (3°) > secondary (2°) > primary (1°) > methyl, because alkyl groups release electron density inductively and by hyperconjugation, which spreads out the positive charge. The more stable the carbocation, the faster the pathway leading to it, and this controls the regiochemistry of the reaction.
碳正离子的稳定性顺序为叔碳 (3°) > 仲碳 (2°) > 伯碳 (1°) > 甲基碳，因为烷基通过诱导效应和超共轭效应释放电子云密度，使正电荷分散。碳正离子越稳定，生成它的反应路径越快，这决定了反应的区域选择性。
7. Reaction with Bromine and Markovnikov's Rule | 与溴的反应与马氏规则
=C bond. The product is a dibromoalkane, for example ethene gives 1,2-dibromoethane. Saturated hydrocarbons such as hexane give no reaction under these conditions.
在室温、避光条件下，烯烃能迅速使溴水褪色，由橙棕色变为无色。这是检验 C=C 键的标准方法。产物是二溴代烷，例如乙烯生成 1,2-二溴乙烷。己烷等饱和烃在此条件下不反应。
The mechanism involves polarisation of the non-polar Br₂ molecule as it approaches the electron-rich π bond, forming a bromonium ion and Br⁻ in the first step. The bromide ion then attacks from the opposite face, which is why the addition is anti (the two bromine atoms end up on opposite sides of the original double bond).
机理中，非极性的 Br₂ 分子靠近富电子的 π 键时被极化，第一步生成溴鎓离子和 Br⁻。随后溴离子从背面进攻，因此该加成为反式加成（两个溴原子最终位于原双键的两侧）。
When hydrogen bromide adds to an unsymmetrical alkene such as propene, two products are possible: 1-bromopropane and 2-bromopropane. Markovnikov's rule states that the hydrogen atom adds to the carbon that already carries more hydrogen atoms, so the major product is 2-bromopropane.
当溴化氢加到丙烯这类不对称烯烃上时，可能生成两种产物：1-溴丙烷和 2-溴丙烷。马氏规则指出，氢原子加到本来连有较多氢原子的碳上，因此主要产物是 2-溴丙烷。
The explanation lies in the carbocation intermediate. Protonation at the terminal carbon gives the more stable secondary carbocation CH₃CH⁺CH₃, whereas protonation at the middle carbon would give an unstable primary carbocation. The bromide ion then attacks the secondary carbocation.
原因在于碳正离子中间体。质子加到端基碳上会生成较稳定的仲碳正离子 CH₃CH⁺CH₃；而质子加到中间碳上则会生成不稳定的伯碳正离子。随后溴离子进攻仲碳正离子。
In the presence of organic peroxides, HBr adds the other way round (anti-Markovnikov addition) by a free-radical mechanism, giving 1-bromopropane as the major product. This is a common exam trap: the peroxide route applies only to HBr, not to HCl or HI.
在有机过氧化物存在下，HBr 通过自由基机理按相反方向加成（反马氏加成），主要产物为 1-溴丙烷。这是常见的考试陷阱：过氧化物路线只适用于 HBr，不适用于 HCl 或 HI。
8. Hydrogenation, Hydration and Industrial Addition | 加氢、水合与工业加成
Hydrogenation: alkenes react with hydrogen in the presence of a nickel catalyst at about 150 °C (or platinum/palladium at room temperature) to form the corresponding alkane. The reaction is exothermic and is used industrially to harden vegetable oils into margarine.
加氢：在镍催化、约 150 °C 条件下（或在室温下用铂/钯催化），烯烃与氢气反应生成相应的烷烃。该反应放热，工业上用于将植物油氢化硬化制成人造黄油。
Hydration: ethene reacts with
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