IB OCR Chemistry: Alkenes Key Points | IB OCR 化学：烯烃考点精讲

📚 IB OCR Chemistry: Alkenes Key Points | IB OCR 化学：烯烃考点精讲

Alkenes form one of the most reactive and synthetically useful families of organic compounds. For IB and OCR chemistry students, a thorough understanding of alkene structure, bonding, isomerism, and their characteristic addition reactions is essential. This article consolidates the key learning points, reaction mechanisms, and examination tips you need to master this topic with confidence.

烯烃是最活泼、合成用途最广的有机化合物家族之一。对于 IB 和 OCR 化学学生来说，透彻理解烯烃的结构、键合、异构现象及其特征加成反应是必不可少的。本文总结了关键知识点、反应机理和应试技巧，帮助你自信掌握本专题。

1. Structure and Bonding in Alkenes | 烯烃的结构与键合

Alkenes are hydrocarbons containing at least one carbon–carbon double bond. The functional group is C=C. The carbon atoms of the double bond are sp² hybridised. Each sp² carbon forms three sigma (σ) bonds: one to a carbon and two to other atoms (C or H). The remaining unhybridised 2p orbital on each carbon overlaps sideways to form a pi (π) bond. The double bond therefore consists of one σ bond and one π bond. The π bond is weaker than the σ bond, making alkenes more reactive than alkanes. The C=C bond length is about 134 pm, shorter than a C–C single bond (154 pm). The bond angle around the sp² carbon is approximately 120°, so the double bond and its attached atoms lie in a plane.

烯烃是至少含有一个碳–碳双键的碳氢化合物。官能团是 C=C。双键上的碳原子为 sp² 杂化。每个 sp² 碳形成三个 σ 键：一个与该碳原子相连，另外两个与其他原子（C 或 H）相连。每个碳上剩余未杂化的 2p 轨道侧面重叠形成一个 π 键。因此双键由一个 σ 键和一个 π 键组成。π 键比 σ 键弱，使烯烃比烷烃更活泼。C=C 键长约 134 pm，比 C–C 单键（154 pm）短。围绕 sp² 碳的键角约为 120°，因此双键及其所连原子共面。

2. Nomenclature of Alkenes | 烯烃的命名

Alkene names follow IUPAC rules. The suffix is ‘-ene’. The parent chain is the longest continuous carbon chain that includes the C=C double bond. The chain is numbered to give the double bond the lowest possible number. This locant is placed before the ‘-ene’ suffix (e.g., pent-2-ene). Substituents are named as prefixes with their positions. For cycloalkenes, the double bond is assumed to be between C1 and C2; numbering starts from the double bond. Common alkyl groups attached to C=C include vinyl (ethenyl) and allyl (prop-2-enyl).

烯烃命名遵循 IUPAC 规则。后缀为“-烯”。主链是包含 C=C 双键的最长连续碳链。编号时使双键位置号最小。该定位号放在后缀“-烯”之前（如戊-2-烯）。取代基以前缀及位置编号命名。对于环烯，默认双键在 C1 和 C2 之间；编号从双键开始。连在 C=C 上的常见基团包括乙烯基和烯丙基。

3. Isomerism in Alkenes | 烯烃的异构现象

Alkenes exhibit structural isomerism (chain, position, and functional group) and stereoisomerism. Position isomers occur when the double bond moves along the chain (e.g., but-1-ene and but-2-ene). Functional group isomers include cycloalkanes with the same molecular formula (e.g., cyclobutane vs but-1-ene, both C₄H₈).

烯烃表现出结构异构（碳链、位置和官能团异构）和立体异构。位置异构是双键沿链移动（如丁-1-烯和丁-2-烯）。官能团异构包括相同分子式的环烷烃（如环丁烷与丁-1-烯，均为 C₄H₈）。

The restricted rotation about the C=C double bond gives rise to geometric (cis–trans or E/Z) isomerism. Cis–trans naming is used when each carbon of the double bond has two different groups, one of which must be hydrogen. If the two highest priority groups are on the same side, it is the Z (zusammen) isomer; if they are on opposite sides, it is the E (entgegen) isomer. Priority is assigned using the Cahn–Ingold–Prelog rules (higher atomic number takes precedence). E/Z is more general and applies to all alkenes with different substituents on each double‑bond carbon.

由于 C=C 双键的旋转受阻，产生了几何（顺反或 E/Z）异构。当双键每个碳上都连有两个不同基团且其中一个必为氢时，可用顺反命名。若两个优先基团在双键同侧，则为 Z（zusammen）异构体；若在异侧，则为 E（entgegen）异构体。优先次序由 Cahn–Ingold–Prelog 规则确定（原子序数大者优先）。E/Z 更通用，适用于双键碳上连有不同取代基的所有烯烃。

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

Alkenes are non-polar or very weakly polar molecules. Their physical properties resemble those of alkanes. Boiling points increase with increasing carbon chain length due to greater London dispersion forces. Branched alkenes have lower boiling points than their straight-chain isomers. Alkenes are insoluble in water but dissolve in non-polar solvents. The cis isomer is usually slightly more polar than the trans isomer and thus has a marginally higher boiling point, while the trans isomer packs better in a solid lattice, often giving a higher melting point.

烯烃是非极性或弱极性的分子，其物理性质与烷烃相似。沸点随碳链增长而升高，这是由于伦敦色散力增大。支链烯烃的沸点低于其直链异构体。烯烃不溶于水，但可溶于非极性溶剂。顺式异构体通常比反式异构体极性稍强，因而沸点略高；而反式异构体在固态晶格中排列更紧密，往往熔点更高。

5. Chemical Reactivity of Alkenes: Addition Reactions | 烯烃的化学活泼性：加成反应

The π bond is a region of high electron density and is susceptible to attack by electrophiles (electron‑deficient species). Alkenes therefore undergo electrophilic addition reactions. The double bond opens up, and two new σ bonds form as the reagent adds across the two carbon atoms. The general form is C=C + X–Y → –C–C– with X and Y attached.

π 键是电子密度高的区域，容易受到亲电试剂（缺电子物种）的进攻。因此烯烃发生亲电加成反应。双键打开，试剂加在两个碳原子上，形成两个新的 σ 键。通式为 C=C + X–Y → –C–C–（X、Y 加成上去）。

Key addition reactions include (X, Y = H, halogen, H₂O, etc.):

关键的加成反应包括（X、Y = H、卤素、H₂O 等）：

Hydrogenation: C=C + H₂ → –CH–CH– (requires a metal catalyst such as Pt, Pd, or Ni; used to convert unsaturated oils to saturated fats).

加氢：C=C + H₂ → –CH–CH–（需要金属催化剂如 Pt、Pd 或 Ni；用于将不饱和油脂转变为饱和脂肪）。
Halogenation: C=C + Br₂ → –CHBr–CHBr– (bromine water decolorisation is the classic test for unsaturation; addition of Cl₂ also occurs).

卤化：C=C + Br₂ → –CHBr–CHBr–（溴水褪色是不饱和的经典检验；也可与 Cl₂ 加成）。
Hydrogen halide addition: C=C + HX → –CH–CHX– (reactivity of HX: HI > HBr > HCl, due to bond strength; with unsymmetrical alkenes Markovnikov’s rule applies).

卤化氢加成：C=C + HX → –CH–CHX–（HX 活泼性：HI > HBr > HCl，因键强差异；不对称烯烃适用马氏规则）。
Hydration: C=C + H₂O → –CH–CHOH– (requires acid catalyst, typically concentrated H₂SO₄ or H₃PO₄; follows Markovnikov’s rule; produces alcohols industrially).

水合：C=C + H₂O → –CH–CHOH–（需酸催化，通常为浓 H₂SO₄ 或 H₃PO₄；遵循马氏规则；工业上制醇）。

6. Electrophilic Addition Mechanism | 亲电加成机理

The general mechanism proceeds in two steps. Step 1: The π electrons attack the electrophile (e.g., the slightly positive hydrogen in H–Br), forming a bond to one carbon. The other carbon becomes electron‑deficient, forming a carbocation intermediate. The heterolytic fission of the H–Br bond releases a bromide ion. Step 2: The bromide ion (nucleophile) donates its electron pair to the carbocation, forming a new C–Br σ bond. This mechanism is illustrated using curly arrows: one arrow from the double bond to the electrophilic atom, and another from the bond being broken to the leaving group (or directly from the nucleophile to the carbocation). The rate‑determining step is the formation of the carbocation.

通用机理分两步进行。第一步：π 电子进攻亲电试剂（例如 H–Br 中略带正电的氢），与一个碳形成键。另一个碳缺电子，形成碳正离子中间体。H–Br 键发生异裂，释放出溴离子。第二步：溴离子（亲核试剂）将孤对电子提供给碳正离子，形成新的 C–Br σ 键。该机理用弯箭头表示：一个箭头从双键指向亲电原子，另一个箭头从断裂的键指向离去基团（或从亲核试剂直接指向碳正离子）。决速步是碳正离子的形成。

7. Markovnikov’s Rule and Carbocation Stability | 马氏规则与碳正离子稳定性

When HX or H₂O adds to an unsymmetrical alkene, two products are theoretically possible. Markovnikov’s rule states that the hydrogen atom attaches to the carbon of the double bond that already has the greater number of hydrogen atoms (the ‘rich get richer’). In modern terms, the reaction proceeds via the more stable carbocation intermediate. Carbocation stability order: tertiary (3°) > secondary (2°) > primary (1°) > methyl. The stability is due to the positive inductive effect and hyperconjugation from alkyl groups. Thus the major product is the one formed via the more stable carbocation, which places the halogen or OH group on the more substituted carbon.

当 HX 或 H₂O 与不对称烯烃加成时，理论上有两种产物。马氏规则表述为氢原子加到含氢较多的双键碳上（“富者愈富”）。用现代术语说，反应经由更稳定的碳正离子中间体进行。碳正离子稳定性顺序：叔（3°）> 仲（2°）> 伯（1°）> 甲基。稳定性归因于烷基的正诱导效应和超共轭作用。因此主产物是经由更稳定的碳正离子生成的，卤素或 OH 基加在取代较多的碳上。

Anti‑Markovnikov addition of HBr occurs only in the presence of peroxides (radical mechanism), which is a specific exception for HBr only (not HCl or HI). Students must be able to predict and explain major products under both normal and peroxide conditions.

仅 HBr 在过氧化物存在下发生反马氏加成（自由基机理），这是 HBr 特有的例外（HCl 和 HI 不适用）。学生必须能预测并解释常规条件和过氧化物条件下的主产物。

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

Alkenes can be oxidised under different conditions, producing a range of valuable intermediates.

烯烃可在不同条件下被氧化，生成多种有用的中间体。

Mild oxidation with cold, dilute, alkaline KMnO₄: The alkene is converted to a diol (a vicinal diol, two –OH groups on adjacent carbons). The purple permanganate turns brown (MnO₂ precipitate). This reaction is also a test for unsaturation (Baeyer’s test).

用冷、稀、碱性 KMnO₄ 进行的温和氧化：烯烃转化为二醇（邻二醇，在相邻碳上各引入一个 –OH 基）。紫色的高锰酸盐变为棕色（MnO₂ 沉淀）。该反应也可检验不饱和性（拜尔试验）。
Strong oxidation with hot, concentrated, acidic KMnO₄: The double bond is cleaved (oxidative cleavage). Products depend on the substitution pattern at the double‑bond carbons. A terminal =CH₂ group is oxidised to CO₂ and H₂O. A =CH–R group gives a carboxylic acid RCOOH. A =CR₂ group yields a ketone R₂C=O. This can be used to deduce the structure of an unknown alkene from its oxidation products.

用热、浓、酸性 KMnO₄ 进行的强烈氧化：双键断裂（氧化断裂）。产物取决于双键碳上的取代情况。末端 =CH₂ 基团氧化为 CO₂ 和 H₂O。=CH–R 基团得到羧酸 RCOOH。=CR₂ 基团生成酮 R₂C=O。这可用于根据氧化产物推断未知烯烃的结构。
Ozonolysis: Ozone (O₃) cleaves the double bond to give carbonyl compounds (aldehydes and/or ketones) after reductive work‑up (e.g., Zn/H₂O or (CH₃)₂S). It is a milder and cleaner alternative for structural determination.

臭氧分解：臭氧（O₃）裂解双键，经还原后处理（如 Zn/H₂O 或 (CH₃)₂S）得到羰基化合物（醛和/或酮）。它是结构测定的更温和、更干净的方法。

9. Polymerisation of Alkenes | 烯烃的聚合

Alkenes can undergo addition polymerisation to form long‑chain polymers. The double bond opens, and monomers link together without the loss of any small molecules. The repeating unit has the same atoms as the monomer. Poly(ethene) is formed from ethene under high pressure or using a Ziegler–Natta catalyst. Poly(propene) from propene gives a useful plastic, with properties depending on the tacticity (arrangement of methyl groups). Addition polymers are non‑biodegradable and present environmental challenges, though they can be recycled or incinerated for energy recovery. Students should be able to draw the repeating unit of an addition polymer from a given monomer, and identify the monomer from a polymer section.

烯烃可发生加聚反应形成长链聚合物。双键打开，单体连接在一起而不脱除任何小分子。重复单元含有与单体相同的原子。聚乙烯由乙烯在高压下或使用齐格勒–纳塔催化剂制得。由丙烯制得的聚丙烯是一种有用的塑料，其性能取决于立构规整度（甲基排列方式）。加聚物不可生物降解并带来环境挑战，但可以回收或焚烧以回收能量。学生应能根据给定单体画出加成聚合物的重复单元，并能从聚合物片段识别出单体。

10. Addition to Unsymmetrical Alkenes and Stereochemistry | 不对称烯烃的加成与立体化学

Addition reactions can create chiral centres. If the reactant is an unsymmetrical alkene and the addition is not stereospecific, a mixture of enantiomers may result as a racemate. For example, addition of HBr to but-1-ene produces 2‑bromobutane, which is chiral; the product is a racemic mixture because the carbocation is planar and the nucleophile can attack from either side with equal probability. In addition of Br₂, the reaction proceeds via a cyclic bromonium ion, which forces anti‑addition (trans product). This stereospecificity is a crucial point for demonstrating reaction mechanism.

加成反应可能产生手性中心。如果反应物是不对称烯烃且加成不是立体专一的，则可能得到对映异构体的混合物，即外消旋体。例如，HBr 与丁-1-烯加成生成手性的 2‑溴丁烷；产物是外消旋混合物，因为碳正离子是平面的，亲核试剂可以从两侧以相等概率进攻。在 Br₂ 加成反应中，反应经过环状溴鎓离子，迫使反式加成（得到反式产物）。这种立体专一性是证明反应机理的关键点。

11. Distinguishing and Identifying Alkenes | 烯烃的鉴别与鉴定

The common chemical test for unsaturation is the bromine water test. When an alkene is shaken with orange bromine water, the colour rapidly disappears (decolourisation) as bromine adds across the double bond. A control test with an alkane gives no immediate colour change. Baeyer’s test using alkaline KMnO₄ also gives a colour change from purple to brown precipitate. Spectroscopic identification: in IR spectroscopy, C=C stretching appears around 1620–1680 cm⁻¹, and =C–H stretch absorbs just above 3000 cm⁻¹. In ¹H NMR, vinylic protons typically resonate between 4.5 and 6.5 ppm, with characteristic coupling constants.

检验不饱和的常用化学方法是溴水试验。当烯烃与橙色溴水振荡，由于溴加成到双键上，颜色迅速褪去。烷烃对照试验则不会立即变色。使用碱性高锰酸钾的拜尔试验也会发生从紫色到棕色沉淀的颜色变化。光谱鉴定：在红外光谱中，C=C 伸缩振动约在 1620–1680 cm⁻¹ 处出现，=C–H 伸缩振动在略高于 3000 cm⁻¹ 处吸收。在 ¹H NMR 中，乙烯基质子通常共振于 4.5–6.5 ppm，且有特征的耦合常数。

12. Summary and Exam Tips | 总结与应试技巧

Master alkenes by understanding the electronic structure and its consequences. Be able to draw and name alkenes, including E/Z isomers, and apply priority rules. Learn the electrophilic addition mechanism, including curly arrows and carbocation stability, to predict major products. Memorise the major reaction conditions and tests. In the exam, always show charges and lone pairs, and use correct arrows. Connect oxidation products to original structure in analysis questions, and relate polymer structures back to monomers. With systematic revision, alkenes become one of the most rewarding topics for building both conceptual understanding and problem‑solving skills.

通过理解电子结构及其影响来掌握烯烃。能够绘制和命名烯烃，包括 E/Z 异构体，并应用次序规则。学习亲电加成机理，包括弯箭头和碳正离子稳定性，以预测主产物。熟记主要反应条件和检验方法。在考试中，始终标明电荷和孤对电子，并使用正确的箭头。在分析题中将氧化产物和原始结构联系起来，并将聚合物结构回溯到单体。通过系统复习，烯烃将成为最具回报的专题之一，既能建立概念理解，又能培养解决问题的能力。

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IB OCR Chemistry: Alkenes Key Points | IB OCR 化学：烯烃 考点精讲