📚 AS AQA CH02 Alkanes and Alkenes – Structure, Isomerism and Reaction Mechanisms | AS AQA CH02 烷烃与烯烃——结构、同分异构与反应机理
Alkanes and alkenes are the foundation of organic chemistry in AS AQA CH02. This article explains their structural formulas, nomenclature, isomerism, key reaction mechanisms and the exam techniques needed to score full marks.
烷烃和烯烃是 AS AQA CH02 有机化学的基础。本文讲解它们的结构式、命名、同分异构、关键反应机理以及获得满分的考试技巧。
1. Homologous Series and General Formulas | 同系列与通式
Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. They contain only single C–C and C–H bonds, so every carbon atom has four sigma bonds.
烷烃是饱和烃，通式为 CₙH₂ₙ₊₂。它们只含单键 C–C 和 C–H，因此每个碳原子都有四根 σ 键。
ormula CₙH₂ₙ. They contain at least one C=C double bond made of one sigma and one pi bond.
烯烃是不饱和烃，通式为 CₙH₂ₙ。它们至少含一个 C=C 双键，由一根 σ 键和一根 π 键构成。
Homologous series: a family of compounds with the same functional group, each member differing by CH₂.
同系列： 具有相同官能团、相邻成员相差 CH₂ 的一类化合物。
2. Naming Alkanes and Alkenes | 烷烃与烯烃的命名
Use IUPAC rules: identify the longest carbon chain, number from the end nearest the double bond for alkenes, and name substituents in alphabetical order.
使用 IUPAC 规则：找出最长碳链；烯烃从离双键最近的一端开始编号；取代基按字母顺序命名。
- Alkanes: methane, ethane, propane, butane — 甲烷、乙烷、丙烷、丁烷
- Alkenes: ethene, propene, but-1-ene, but-2-ene — 乙烯、丙烯、1-丁烯、2-丁烯
For example, 2-methylbutane has a methyl group on carbon 2 of a four-carbon chain; but-2-ene has the double bond starting at carbon 2.
例如，2-甲基丁烷的 2 号碳上有一个甲基；2-丁烯的双键从 2 号碳开始。
3. Structural and Stereoisomerism | 结构异构与立体异构
Alkanes with four or more carbons show chain isomerism, such as butane and 2-methylpropane.
四个碳及以上的烷烃存在碳链异构，如丁烷和 2-甲基丙烷。
Alkenes show position isomerism (but-1-ene vs but-2-ene) and geometric E/Z isomerism when each carbon of the double bond carries two different groups.
烯烃存在位置异构（1-丁烯与 2-丁烯）以及当双键每个碳上连有两个不同基团时的几何 E/Z 异构。
4. Bonding: Sigma and Pi Bonds | 键合：σ 键和 π 键
A sigma bond forms by head-on overlap of orbitals; a pi bond forms by sideways overlap of p orbitals and restricts rotation around the double bond.
σ 键由轨道头对头重叠形成；π 键由 p 轨道侧面重叠形成，并限制双键旋转。
This restricted rotation explains why E/Z isomerism can occur in alkenes but not in alkanes.
这种旋转受限解释了为什么烯烃能出现 E/Z 异构，而烷烃不能。
5. Combustion of Alkanes | 烷烃的燃烧
mbustion produces carbon dioxide and water, releasing energy. E.g. CH₄ + 2O₂ → CO₂ + 2H₂O.
完全燃烧生成二氧化碳和水并释放能量。例如 CH₄ + 2O₂ → CO₂ + 2H₂O。
Incomplete combustion can produce carbon monoxide or soot (carbon), especially with limited oxygen.
不完全燃烧可能产生一氧化碳或炭黑（碳），尤其在氧气不足时。
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
6. Free Radical Substitution of Alkanes | 烷烃的自由基取代
Methane reacts with chlorine in UV light by a radical chain mechanism with three stages: initiation, propagation and termination.
甲烷与氯气在紫外光下通过自由基链式机理反应，分三个阶段：链引发、链增长和链终止。
Initiation: Cl₂ → 2Cl• (homolytic fission). Propagation: CH₄ + Cl• → •CH₃ + HCl; then •CH₃ + Cl₂ → CH₃Cl + Cl•. Termination: Cl• + Cl• → Cl₂ or •CH₃ + •CH₃ → C₂H₆.
链引发：Cl₂ → 2Cl•（均裂）。链增长：CH₄ + Cl• → •CH₃ + HCl；随后 •CH₃ + Cl₂ → CH₃Cl + Cl•。链终止：Cl• + Cl• → Cl₂ 或 •CH₃ + •CH₃ → C₂H₆。
Further substitution can produce a mixture of CH₂Cl₂, CHCl₃ and CCl₄, which is a limitation of this reaction.
进一步取代会生成 CH₂Cl₂、CHCl₃ 和 CCl₄ 的混合物，这是该反应的局限。
7. Electrophilic Addition of Alkenes | 烯烃的亲电加成
because the pi electrons in the C=C double bond are exposed and can attract electrophiles.
烯烃远比烷烃活泼，因为 C=C 双键中的 π 电子暴露在外，能吸引亲电试剂。
Ethene reacts with HBr: the π electrons attack the hydrogen atom in HBr, forming a carbocation and releasing Br⁻; Br⁻ then attacks the carbocation.
乙烯与 HBr 反应：π 电子进攻 HBr 中的氢，生成碳正离子并释放 Br⁻；随后 Br⁻ 进攻碳正离子。
CH₂=CH₂ + HBr → CH₃CH₂Br
With sulfuric acid, ethene forms ethyl hydrogen sulfate; with steam and a phosphoric acid catalyst, ethene forms ethanol.
乙烯与硫酸生成硫酸氢乙酯；乙烯与水蒸气在磷酸催化下生成乙醇。
8. Markownikoff's Rule and Carbocation Stability | 马氏规则与碳正离子稳定性
When HBr adds to an unsymmetrical alkene like propene, two products are possible. The major product comes from the more stable carbocation.
当 HBr 与不对称烯烃（如丙烯）加成时，可能得到两种产物。主要产物来自较稳定的碳正离子。
Carbocation stability order: tertiary > secondary > primary > methyl. This is because alkyl groups donate electron density and stabilise the positive charge.
碳正离子稳定性顺序：叔 > 仲 > 伯 > 甲基。这是因为烷基供电性分散正电荷而稳定碳正离子。
CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (major) + CH₃CH₂CH₂Br (minor)
The major product follows Markownikoff's rule: the hydrogen attaches to the carbon with more hydrogens already.
主要产物遵循马氏规则：氢加在原本含氢较多的碳上。
9. Addition Polymerisation and Uses | 加成聚合及其用途
Alkenes can undergo addition polymerisation: many monomer molecules join together without forming any side products. The double bond opens up and forms a long saturated chain.
烯烃可发生加聚反应：许多单体分子相互连接而不生成副产物。双键打开，形成长饱和链。
For example, ethene forms poly(ethene): n CH₂=CH₂ → (CH₂–CH₂)ₙ. Poly(propene), poly(chloroethene) and PTFE are common addition polymers.
例如，乙烯生成聚乙烯：n CH₂=CH₂ → (CH₂–CH₂)ₙ。聚丙烯、聚氯乙烯和聚四氟乙烯是常见的加聚产物。
10. Test for Unsaturation and Exam Tips | 不饱和性检验与考试提示
A simple test for an alkene is to shake the sample with bromine water. The orange colour disappears in the presence of an alkene because bromine adds across the double bond.
检验烯烃的简单方法是将样品与溴水混合振荡。若存在烯烃，橙色会消失，因为溴与双键发生加成。
Exam tip: always show the curly arrow starting from the pi bond or the lone pair, and clearly draw the intermediate carbocation when asked for a mechanism.
考试提示：写机理时，弯箭头必须从 π 键或孤对电子出发，并清晰画出中间体碳正离子。
Remember that free radical substitution needs UV light and produces a mixture; electrophilic addition is fast at room temperature and gives a single product when the alkene is symmetrical.
记住：自由基取代需要紫外光且得到混合物；亲电加成在室温下快速进行，当烯烃对称时只生成一种产物。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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