📚 Oxidation of Alkenes | 烯烃的氧化
Alkenes are hydrocarbons containing at least one carbon– e up of one strong σ bond and one weaker π bond. The exposed π electrons make alkenes highly susceptible to oxidation, and depending on the oxidising agent and the conditions, the products can be alcohols, carbonyl compounds, carboxylic acids, or even carbon dioxide. This article covers the key oxidation reactions of alkenes required by A-Level Cambridge Chemistry: combustion, epoxidation with peroxyacids, hydroxylation with potassium manganate(VII), oxidative cleavage, and ozonolysis, with emphasis on reaction conditions, mechanisms and the prediction of products.
烯烃是含有至少一个碳碳双键 C=C 的烃，该双键由一个强的 σ 键和一个较弱的 π 键组成。暴露在外的 π 电子使烯烃极易被氧化，根据氧化剂和反应条件的不同，产物可以是醇、羰基化合物、羧酸，甚至二氧化碳。本文涵盖 A-Level 剑桥化学要求掌握的烯烃主要氧化反应：燃烧、过氧酸环氧化、高锰酸钾羟基化、氧化性断裂以及臭氧分解，重点讲解反应条件、反应机理与产物预测。
1. Overview of Alkene Oxidation Pathways | 烯烃氧化途径概述
Oxidation of an alkene is not a single reaction with a single product; instead, the outcome depends on how powerful the oxidising agent is and on the conditions used. Mild oxidants add oxygen atoms across the π bond without breaking the carbon skeleton, whereas strong, hot oxidants can cleave the double bond completely into two carbonyl-containing fragments.
烯烃的氧化并非只生成单一产物的一步反应；其结果取决于氧化剂的强弱以及所采用的反应条件。温和的氧化剂会把氧原子加成到 π 键上而不破坏碳骨架，而强氧化剂在加热条件下则能使双键完全断裂，生成两个含羰基的片段。
There are five main pathways you must be able to recognise and distinguish: (i) complete combustion with O₂; (ii) epoxidation with peroxyacids; (iii) syn-dihydroxylation using cold, dilute, alkaline KMnO₄; (iv) oxidative cleavage with hot, concentrated, acidic KMnO₄; and (v) ozonolysis followed by either a reductive or an oxidative work-up.
你必须能够识别并区分以下五种主要途径：(i) 与 O₂ 的完全燃烧；(ii) 用过氧酸进行环氧化；(iii) 用冷、稀、碱性 KMnO₄ 进行顺式二羟基化；(iv) 用热、浓、酸性 KMnO₄ 进行氧化性断裂；(v) 臭氧分解，随后进行还原性或氧化性后处理。
2. Structure and Reactivity: Why the π Bond Is Vulnerable | 结构与活性：π键为何易被氧化
d formed by head-on overlap of sp² hybrid orbitals and a π bond formed by side-on overlap of unhybridised p orbitals. The π electrons lie above and below the plane of the molecule, so they are exposed, loosely held and easily attacked by electron-deficient species.
C=C 双键由一个由 sp² 杂化轨道"头碰头"重叠形成的 σ 键，以及一个由未杂化 p 轨道"肩并肩"重叠形成的 π 键构成。π 电子位于分子平面的上、下方，暴露在外、束缚较松，容易被缺电子物种进攻。
atoms of a C=C bond are typically at a lower oxidation state than the carbon atoms of a C– fore means an increase in the oxidation number of carbon, usually achieved either by adding oxygen atoms across the π bond or by breaking the π bond and inserting oxygen.
从氧化数来看，C=C 双键上两个碳原子的氧化数通常低于相应烷烃中 C–C 单键上的碳。例如，乙烯中碳的氧化数为 −2，而在 CO₂ 中为 +4。因此烯烃的氧化就意味着碳的氧化数升高，这通常通过向 π 键加成氧原子，或断裂 π 键并插入氧原子来实现。
3. Combustion: Complete Oxidation | 燃烧：完全氧化
When an alkene burns in a large excess of oxygen, complete combustion occurs and every carbon atom is oxidised to its highest oxidation state. The only products are carbon dioxide and water, and the reaction is strongly exothermic. The general equation for any alkene CₙH₂ₙ is shown below.
当烯烃在过量氧气中燃烧时发生完全燃烧，每个碳原子都被氧化到最高氧化数。唯一产物是二氧化碳和水，反应强烈放热。任意烯烃 CₙH₂ₙ 的通式方程如下。
CₙH₂ₙ + 3n/2 O₂ → n CO₂ + n H₂O ΔH < 0
2 CO₂ + 2 H₂O. In a limited supply of oxygen, combustion is incomplete and the products include carbon monoxide, a toxic and odourless gas, and/or solid carbon (soot). Because alkenes have a higher carbon-to-hydrogen ratio than the corresponding alkanes, their flames are more luminous and smokier.
对乙烯而言，方程式为 C₂H₄ + 3 O₂ → 2 CO₂ + 2 H₂O。在氧气供应不足时燃烧不完全，产物中会含有一氧化碳（有毒无味的气体）和／或固体碳（碳黑）。由于烯烃的碳氢比高于相应的烷烃，其火焰更明亮、更冒黑烟。
Examiners frequently ask you to calculate the volume or the mass of oxygen needed to burn a given volume of gaseous alkene; these are simple mole-ratio calculations using the equation above, with the assumption that all volumes are measured at the same temperature and pressure.
考官经常要求你计算燃烧一定体积气态烯烃所需的氧气体积或质量；只需按上述方程做简单的物质的量之比计算，并假设所有体积均在相同温度和压强下测量。
4. Epoxidation with Peroxyacids | 过氧酸的环氧化
, and 3-chloroperoxybenzoic acid (mCPBA). When a peroxyacid reacts with an alkene in an anhydrous organic solvent such as dichloromethane at room temperature, one oxygen atom is transferred to the C=C bond to give a three-membered cyclic ether called an epoxide.
过氧酸 RCO₃H 比普通羧酸多含一个氧原子，常见的有过氧乙酸 CH₃CO₃H 和间氯过氧苯甲酸（mCPBA）。当过氧酸在室温下于二氯甲烷等无水有机溶剂中与烯烃反应时，一个氧原子被转移到 C=C 双键上，生成三元环醚，即环氧化合物（epoxide）。
RCO₃H + C=C → epoxide + RCOOH
The mechanism is a single, concerted step: the oxygen atom of the peroxyacid attacks the π bond while the O–O bond breaks simultaneously through a cyclic transition state. Because no free carbocation or carbanion is formed, the addition is syn and stereospecific, meaning a cis-alkene gives a cis-epoxide and a trans-alkene gives a trans-epoxide.
该机理是一步协同反应：过氧酸的氧原子进攻 π 键，同时 O–O 键通过环状过渡态断裂。由于没有生成自由的碳正离子或碳负离子，加成是顺式且立体专一的，即顺式烯烃得到顺式环氧化合物，反式烯烃得到反式环氧化合物。
Industrially, ethene is converted into oxirane (ethylene oxide) by reaction with oxygen over a silver catalyst at about 250 °C. Epoxides are valuable intermediates because the strained three-membered ring opens readily with nucleophiles.
工业上，乙烯在约 250 °C 下与氧气在银催化剂上反应生成环氧乙烷。环氧化合物是重要的中间体，因为其张力较大的三元环容易被亲核试剂开环。
5. Epoxide Ring Opening: Anti-Dihydroxylation | 环氧开环：反式二羟基化
An epoxide can be hydrolysed by warming it with dilute acid, such as dilute sulfuric acid. Water acts as a nucleophile and attacks one of the ring carbons, opening the strained ring and giving a diol. Because the nucleophile attacks from the side opposite to the C–O bond being broken, the two OH groups end up on opposite faces of the carbon chain.
环氧化合物可以用稀酸（如稀硫酸）加热水解。水作为亲核试剂进攻环上的一个碳原子，使张力环打开并生成二醇。由于亲核试剂从被断裂的 C–O 键的反面进攻，两个 OH 基最终位于碳链的相反两侧。
This two-step sequence, epoxidation followed by acid-catalysed ring opening, is therefore an overall anti-dihydroxylation and gives the trans-diol. It contrasts sharply with the direct syn-dihydroxylation produced by cold alkaline KMnO₄. Being able to state the stereochemical difference between these two routes is a common high-mark question.
因此，"先环氧化、再酸催化开环"这两步合起来属于反式二羟基化，得到反式二醇。这与冷碱性 KMnO₄ 直接产生的顺式二羟基化形成鲜明对比。能够说明这两条路线在立体化学上的差异，是常见的高分题考点。
Note also that in acidic solution the epoxide oxygen is protonated first, which makes the ring more electrophilic and greatly accelerates the attack by water. In basic or neutral conditions the reaction is much slower.
还需注意，在酸性溶液中环氧的氧原子先被质子化，使环更缺电子，从而大大加快水的进攻；在中性或碱性条件下反应则慢得多。
6. Syn-Dihydroxylation with Cold, Alkaline KMnO₄ | 冷碱性高锰酸钾的顺式二羟基化
Shaking an alkene with cold, dilute, alkaline potassium manganate(VII) solution produces a vicinal diol, that is, a diol with the two OH groups on adjacent carbon atoms. The purple MnO₄⁻ ion is reduced to brown manganese(IV) oxide, MnO₂, so the observation is that the purple colour is decolourised and a brown precipitate forms.
将烯烃与冷、稀的碱性高锰酸钾(Ⅶ)溶液一起振荡，会生成邻二醇，即两个 OH 基位于相邻碳原子上的二醇。紫色的 MnO₄⁻ 被还原为棕色的二氧化锰 MnO₂，因此可观察到的现象是紫色褪去并生成棕色沉淀。
3 C₂H₄ + 2 MnO₄⁻ + 4 H₂O → 3 HOCH₂CH₂OH + 2 MnO₂ + 2 OH⁻
his decolourisation of KMnO₄ is a classic qualitative test for a C=C bond, alongside the decolourisation of bromine water, but it is not a clean test because other easily oxidised groups such as aldehydes and primary alcohols also decolourise it.
两个羟基是同一步从分子的同一面加成的，因此属于顺式加成，并具有立体专一性。KMnO₄ 褪色是与溴水褪色并列的经典 C=C 定性检验方法，但它并非专一性检验，因为醛、伯醇等其他易被氧化的基团也会使其褪色。
If the KMnO₄ is in excess or the mixture is warmed, the diol is oxidised further and the carbon skeleton begins to break, which links this section directly to the oxidative cleavage described next.
如果 KMnO₄ 过量或混合物被加热，二醇会被进一步氧化，碳骨架开始断裂，这就直接联系到下一节要讲的氧化性断裂。
7. Oxidative Cleavage with Hot, Acidic KMnO₄ | 热酸性高锰酸钾的氧化断裂
iol stage but cleaves the C=C bond completely, and the identity of the products depends entirely on how many hydrogen atoms were attached to each of the two alkene carbon atoms.
用稀硫酸酸化的热浓高锰酸钾是强得多的氧化体系。它不会停留在二醇阶段，而是把 C=C 双键完全断裂，产物完全取决于两个烯烃碳原子上各自连接了多少个氢原子。
- A carbon carrying two hydrogens, =CH₂, is oxidised to carbon dioxide and water.
- A carbon carrying one hydrogen, =CHR, is oxidised to a carboxylic acid, RCOOH.
- becomes a ketone, R₂C=O, because ketones resist further oxidation under these conditions.
- 带两个氢的碳，即 =CH₂，被氧化为二氧化碳和水。
- 带一个氢的碳，即 =CHR，被氧化为羧酸 RCOOH。
- 不带氢的碳，即 =CR₂，变为酮 R₂C=O，因为酮在此条件下不易被进一步氧化。
For example, propene gives ethanoic acid and carbon dioxide, while but-2-ene gives two molecules of ethanoic acid. When the double bond is inside a ring, cleavage produces a single molecule with two carboxylic acid groups: cyclohexene gives hexanedioic acid, HOOC(CH₂)₄COOH.
例如，丙烯生成乙酸和二氧化碳，而丁-2-烯生成两分子乙酸。当双键位于环内时，断裂会生成含两个羧基的单一分子：环己烯生成
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