📚 Alkenes: Key Concepts for IB & CCEA Chemistry | 烯烃：IB与CCEA化学考点精讲
Alkenes form one of the most important and reaction-rich families of organic compounds in the IB and CCEA chemistry syllabus. Understanding their structure, bonding, isomerism, and characteristic addition reactions is essential for mastering both the core and higher-level content. This article breaks down every key concept, mechanism, and application you need to know about alkenes, from naming conventions to polymerisation, with clear explanations and paired bilingual commentary to support revision.
烯烃是IB和CCEA化学课程中最重要、反应最丰富的有机化合物家族之一。理解它们的结构、键合、异构现象以及特征性的加成反应，对于掌握核心和高阶内容至关重要。本文详细解析了烯烃每一个关键概念、反应机理和应用，从命名规则到聚合反应，搭配清晰的双语讲解，助力高效复习。
1. Introduction to Alkenes | 烯烃简介
Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C). Their general formula for non-cyclic alkenes is CnH2n. The double bond consists of a sigma (σ) bond formed by head-on overlap of sp² hybrid orbitals and a pi (π) bond formed by sideways overlap of unhybridised p orbitals. This π bond is weaker than the σ bond and is the site of high electron density, making alkenes much more reactive than alkanes.
烯烃是含有至少一个碳碳双键 (C=C) 的不饱和烃。非环状烯烃的通式为CnH2n。双键由一个sp²杂化轨道正面重叠形成的σ键和一个未杂化p轨道侧面重叠形成的π键组成。这个π键比σ键更弱，是电子密度较高的区域，使得烯烃比烷烃活泼得多。
2. Naming Alkenes (IUPAC) | 烯烃的命名 (IUPAC)
To name an alkene, identify the longest carbon chain that contains the C=C bond. Replace the ‘-ane’ ending of the corresponding alkane with ‘-ene’. Number the chain from the end nearest the double bond, and indicate the position of the double bond by the lower-numbered carbon atom involved. For example, CH₂=CHCH₂CH₃ is but-1-ene. If there are substituents, they are named as prefixes with their position numbers.
命名烯烃时，找出包含C=C双键的最长碳链。将相应烷烃的“-ane”结尾替换为“-ene”。从最靠近双键的一端开始给碳链编号，并用双键上编号较小的碳原子标明双键位置。例如，CH₂=CHCH₂CH₃ 是 1-丁烯。如果有取代基，则将它们作为前缀并标出位次。
3. Isomerism in Alkenes: Geometric (E/Z) Isomerism | 烯烃异构现象：几何异构 (E/Z)
Alkenes exhibit geometric isomerism because the C=C bond cannot rotate freely. For geometric isomers to exist, each carbon of the double bond must be attached to two different groups. The traditional cis/trans system requires at least one identical group on each carbon, while the E/Z system uses Cahn-Ingold-Prelog priority rules: assign higher priority to the atom with higher atomic number. If the two higher-priority groups are on the same side of the double bond, the configuration is Z (zusammen); if opposite, it is E (entgegen).
烯烃表现出几何异构现象，因为C=C双键不能自由旋转。要存在几何异构体，双键的每个碳原子必须连接两个不同的基团。传统的顺反体系要求每个碳至少有一个相同的基团，而E/Z体系采用Cahn-Ingold-Prelog优先规则：原子序数大的原子优先。如果两个优先基团在双键同侧，则构型为Z；如果在异侧，则为E。
4. Physical Properties of Alkenes | 烯烃的物理性质
Alkenes are non-polar or only slightly polar due to the small electronegativity difference between carbon and hydrogen. They are insoluble in water but dissolve in non-polar organic solvents. Boiling points increase with molecular mass, but are slightly lower than those of the corresponding alkanes because the π electrons produce a weaker instantaneous dipole. Branching lowers boiling points, while the rigid double bond can slightly increase melting points in symmetrical isomers.
烯烃是非极性或极弱极性的，因为碳氢之间电负性差异很小。它们不溶于水，但溶于非极性有机溶剂。沸点随分子质量增加而升高，但比对应烷烃略低，因为π电子产生的瞬时偶极较弱。支链会降低沸点，而刚性的双键在对称异构体中可略微提高熔点。
5. Electrophilic Addition Mechanism | 亲电加成机理
The most characteristic reaction of alkenes is electrophilic addition, where the π bond is attacked by an electrophile. The mechanism proceeds in two steps: first, the electrophile forms a bond to one carbon, creating a carbocation intermediate; then, a nucleophile attacks the carbocation. The reaction results in the addition of two species across the double bond, converting the sp² carbons to sp³. This mechanism explains the regio- and stereoselectivity observed in many addition reactions.
烯烃最具特征的反应是亲电加成，其中π键受到亲电试剂的进攻。机理分两步进行：首先，亲电试剂与一个碳原子成键，生成碳正离子中间体；然后，亲核试剂进攻碳正离子。反应导致两个物种加成到双键两端，将 sp² 碳转变为 sp³ 碳。这一机理解释了许多加成反应中观察到的区域和立体选择性。
6. Addition of Hydrogen Halides (HX) | 卤化氢的加成 (HX)
Alkenes react with hydrogen halides such as HBr and HCl to form halogenoalkanes. The hydrogen acts as the electrophile. With unsymmetrical alkenes, two products are possible. Markovnikov’s rule states that the hydrogen atom adds to the carbon with the greater number of hydrogen atoms already attached (i.e., the less substituted carbon), leading to the more stable carbocation intermediate. In the presence of peroxides, HBr addition follows anti-Markovnikov regiochemistry due to a free-radical mechanism.
烯烃与HBr、HCl等卤化氢反应生成卤代烷。氢作为亲电试剂。对于不对称烯烃，可能产生两种产物。马氏规则指出，氢原子加在含氢较多的碳上（即取代较少的碳），从而生成更稳定的碳正离子中间体。有过氧化物存在时，HBr 的加成遵循反马氏规律，这是因为自由基反应机理。
7. Addition of Halogens (Br₂, Cl₂) | 卤素的加成 (Br₂, Cl₂)
Alkenes decolourise bromine water or bromine in an organic solvent, providing a classic test for unsaturation. The reaction yields a vicinal dihalide. The mechanism involves the formation of a cyclic bromonium ion intermediate when using Br₂, which then undergoes backside attack by the bromide ion to give anti-addition. This stereospecificity is an important higher-level concept.
烯烃能使溴水或有机溶剂中的溴褪色，这是检验不饱和性的经典方法。反应生成邻二卤代物。使用 Br₂ 时，机理涉及环状溴鎓离子中间体的形成，然后溴离子从背面进攻，导致反式加成。这种立体专一性是重要的高阶概念。
8. Addition of Water: Hydration | 水的加成：水合反应
Alkenes can be hydrated to alcohols via electrophilic addition using steam and an acid catalyst (usually concentrated H₂SO₄ or H₃PO₄). The reaction follows Markovnikov’s rule, producing the more substituted alcohol. Industrially, ethanol is produced by the hydration of ethene. The reaction is reversible, and conditions of high temperature (300 °C) and high pressure (60-70 atm) are used to shift the equilibrium towards the product.
烯烃可以在酸催化剂（通常为浓硫酸或磷酸）存在下，与蒸汽发生亲电加成水合反应生成醇。反应遵循马氏规则，生成取代较多的醇。工业上，乙醇通过乙烯水合生产。该反应可逆，采用高温（300°C）和高压（60-70 atm）条件使平衡向产物方向移动。
9. Oxidation Reactions of Alkenes | 烯烃的氧化反应
Alkenes can be oxidised under different conditions. With cold, dilute, alkaline KMnO₄ (Baeyer’s reagent), alkenes form diols (1,2-diols) via syn addition, and the purple colour fades to a brown precipitate. Under vigorous oxidation with hot, acidified KMnO₄, the double bond is cleaved to give carbonyl compounds or carboxylic acids, depending on the substitution pattern. Ozonolysis followed by reductive work-up is a gentler method for the same purpose, yielding aldehydes or ketones.
烯烃可在不同条件下被氧化。用冷、稀的碱性高锰酸钾（贝耶尔试剂）处理，烯烃通过顺式加成生成1,2-二醇，紫色褪去并产生棕色沉淀。用热、酸化的高锰酸钾进行强烈氧化，双键断裂，根据取代情况生成羰基化合物或羧酸。臭氧分解后进行还原处理是达到同样目的的温和方法，得到醛或酮。
10. Polymerisation of Alkenes | 烯烃的聚合反应
Alkenes and substituted alkenes can undergo addition polymerisation to form long-chain polymers. In this reaction, the π bond breaks, and monomers link together without loss of any atoms. Common examples include poly(ethene) from ethene, poly(propene) from propene, and poly(chloroethene) (PVC) from chloroethene. The process is initiated by radicals, cations, or coordination catalysts. The properties of the polymer depend on the monomer structure, chain length, and branching.
烯烃及取代烯烃可发生加成聚合反应，形成长链聚合物。该反应中，π键断裂，单体连接起来而不丢失任何原子。常见例子包括由乙烯制得的聚乙烯、由丙烯制得的聚丙烯、由氯乙烯制得的聚氯乙烯（PVC）。反应由自由基、阳离子或配位催化剂引发。聚合物的性质取决于单体结构、链长和支化度。
11. Chemical Tests for Alkenes | 烯烃的化学检验
The two most common tests for alkenes are the bromine water test and the Baeyer test. Alkenes decolourise orange bromine water rapidly, while alkanes do not react. Similarly, alkenes turn purple acidified KMnO₄ colourless and form a brown precipitate with alkaline KMnO₄. These tests are specific to the presence of a carbon-carbon double bond and are widely used in qualitative organic analysis.
烯烃最常见的两种检验方法是溴水试验和贝耶尔试验。烯烃能迅速使橙色的溴水褪色，而烷烃不反应。同样，烯烃能使紫色的酸性高锰酸钾褪色，与碱性高锰酸钾生成棕色沉淀。这些试验对碳碳双键的存在具有专一性，被广泛用于有机定性分析。
12. Key Revision Tips and Common Mistakes | 重点复习提示与常见错误
When revising alkenes, pay special attention to drawing mechanisms with correct curly arrows, showing the movement of electron pairs exactly. Remember that the π electrons attack the electrophile, not the other way around. Watch for Markovnikov vs anti-Markovnikov conditions. Do not confuse geometric isomerism with optical isomerism — the restricted rotation in alkenes leads to E/Z, not chirality alone. Practice naming branched and cyclic alkenes, and always number the double bond with the lowest possible locant.
复习烯烃时，要特别注意使用正确的弯箭头画出机理，准确表示电子对的移动。记住是π电子进攻亲电试剂，而非相反。注意马氏加成和反马氏加成的条件。不要将几何异构与光学异构混淆——烯烃的受限旋转导致E/Z异构，而非单纯的手性。多练习支链和环状烯烃的命名，并始终为双键分配尽可能小的位次编号。
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