📚 Oxidation of the Alkenes | 烯烃的氧化
Alkenes are hydrocarbons that contain at least one carbon-carbon double bond. This double bond consists of a strong sigma (σ) bond and a weaker pi (π) bond. The π bond is an area of high electron density, which makes alkenes much more reactive than alkanes. Oxidation of alkenes is a key topic in Cambridge A-Level Chemistry because it links structure, reactivity and organic analysis. Depending on the oxidising agent and the conditions, alkenes can be oxidised to diols, carbonyl compounds, carboxylic acids or even carbon dioxide.
烯烃是含有至少一个碳碳双键的烃类。该双键由一个较强的 σ 键和一个较弱的 π 键组成。π 键区域电子云密度高，因此烯烃比烷烃活泼得多。烯烃的氧化是剑桥 A-Level 化学的重要主题，因为它把结构、反应性和有机分析联系起来。根据氧化剂和条件的不同，烯烃可以被氧化为二醇、羰基化合物、羧酸甚至二氧化碳。
1. Alkenes and the Meaning of Oxidation | 烯烃与氧化的含义
π bond is formed by sideways overlap of the remaining p orbitals. Because the π electrons are held loosely above and below the molecular plane, they are easily removed or attacked, so the C=C bond is the reactive site in every oxidation reaction.
烯烃是含有一个平面 C=C 单元的不饱和烃。σ 键由 sp² 轨道头碰头重叠形成，π 键则由剩余 p 轨道肩并肩重叠形成。由于 π 电子松散地位于分子平面上下方，容易被夺走或进攻，因此 C=C 键是每一个氧化反应的活性位点。
In organic chemistry, oxidation is recognised as the gain of oxygen, the loss of hydrogen, or the loss of electrons. When an alkene is oxidised, the oxygen-to-carbon ratio in the molecule increases, and the oxidation state of at least one carbon atom rises. The extent of this change depends on how powerful the oxidising agent is and on the conditions used.
在有机化学中，氧化表现为加氧、失氢或失电子。烯烃被氧化时，分子中氧与碳的比例上升，至少有一个碳原子的氧化态升高。这种变化的程度取决于氧化剂的强弱以及所用的条件。
2. Combustion of Alkenes | 烯烃的燃烧
Like alkanes, alkenes burn in excess oxygen to form carbon dioxide and water, releasing a large amount of energy. The general equation for complete combustion of an alkene CₙH₂ₙ is shown below.
与烷烃类似，烯烃在过量氧气中燃烧生成二氧化碳和水，并释放大量能量。烯烃 CₙH₂ₙ 完全燃烧的通式如下。
CₙH₂ₙ + 3n/2 O₂ → n CO₂ + n H₂O
O₂ + 2H₂O. The reaction is highly exothermic, which is why alkenes are valuable fuels and feedstocks in the petrochemical industry.
对于乙烯，该式变为 C₂H₄ + 3O₂ → 2CO₂ + 2H₂O。该反应高度放热，这正是烯烃在石化工业中作为燃料和原料很有价值的原因。
In limited oxygen, incomplete combustion occurs. This produces carbon monoxide (CO), a toxic and odourless gas, or solid carbon in the form of soot. Alkenes tend to burn with a smokier flame than alkanes because their higher carbon-to-hydrogen ratio makes complete combustion harder to achieve.
在氧气不足时，烯烃发生不完全燃烧，生成一氧化碳（CO）这种无色有毒气体，或生成固体碳即炭黑。烯烃燃烧时通常比烷烃更容易冒黑烟，因为其较高的碳氢比使完全燃烧更难实现。
3. Mild Oxidation with Cold, Dilute KMnO₄ | 冷稀高锰酸钾的温和氧化
adding two hydroxyl groups across it, giving a diol. This is a mild addition reaction, so no carbon-carbon bond is broken. The reaction is normally carried out at about 273– a small amount of alkali is added to prevent further oxidation of the diol that is formed.
冷稀的高锰酸钾(VII)溶液通过在 C=C 键两侧加两个羟基使其氧化，生成二醇。这是一个温和的加成反应，因此没有碳碳键断裂。该反应通常在约 273–283 K 下用稀溶液进行，并加入少量碱，以防止生成的二醇被进一步氧化。
For ethene, the overall equation can be written as follows.
对于乙烯，总反应式可写为如下形式。
3 CH₂=CH₂ + 2 KMnO₄ + 4 H₂O → 3 HOCH₂CH₂OH + 2 MnO₂ + 2 KOH
In general terms the change is RCH=CHR' + [O] + H₂O → RCH(OH)–CH(OH)R' , and in neutral or alkaline solution to a brown precipitate of manganese(IV) oxide, MnO₂.
一般来说，变化可表示为 RCH=CHR' + [O] + H₂O → RCH(OH)–CH(OH)R'。紫色的 MnO₄⁻ 离子被还原：在酸性溶液中还原为几乎无色的 Mn²⁺，在中性或碱性溶液中则还原为棕色的二氧化锰 MnO₂ 沉淀。
4. The Baeyer Test and Its Limitations | 拜耳试验及其局限
The Baeyer test uses cold, dilute KMnO₄ as a qualitative test for unsaturation. A positive result is the loss of the purple colour, often accompanied by a brown precipitate when the solution is neutral or alkaline. Under the same conditions an alkane gives no visible change, so the test can be used to distinguish an alkene from an alkane.
拜耳试验用冷稀 KMnO₄ 作为检验不饱和键的定性方法。阳性结果是紫色褪去，当溶液呈中性或碱性时通常还伴随棕色沉淀。相同条件下烷烃无明显变化，因此该试验可用于区分烯烃与烷烃。
positive Baeyer test alone does not prove that a C=C bond is present. For a more reliable check, bromine water is usually preferred, because bromine is decolourised by electrophilic addition to the double bond rather than by oxidation.
然而，该试验并不专一。醛、伯醇和仲醇、炔烃甚至二氧化硫也能使该试剂褪色，因此仅凭拜耳试验阳性并不能证明存在 C=C 键。若需更可靠的检验，通常首选溴水，因为溴是被双键的亲电加成所褪色，而不是被氧化所褪色。
5. Vigorous Oxidation with Hot Concentrated Acidified KMnO₄ | 热浓酸性高锰酸钾的剧烈氧化
With hot, concentrated potassium manganate(VII) acidified with dilute sulfuric acid, the oxidation is far more vigorous. The π bond has already been destroyed, so the reagent now attacks the carbon-carbon σ bond as well, and the double bond is cleaved completely. Each of the two carbons that formed the double bond ends up in a separate molecule.
使用以稀硫酸酸化的热浓高锰酸钾(VII)时，氧化要剧烈得多。π 键早已被破坏，因此试剂此时还进攻碳碳 σ 键，双键被彻底断裂。原本构成双键的两个碳原子最终进入两个不同的分子。
The nature of each product depends on how many hydrogen atoms were attached to that carbon in the original alkene. This is the single most important pattern to memorise for exam questions on oxidative cleavage.
每个产物的性质取决于原烯烃中该碳原子上连有几个氢原子。这是应对氧化断裂考题时最需要记住的一条规律。
6. Oxidative Cleavage: Patterns of Products | 氧化断裂：产物规律
The table below summarises what each type of alkene carbon gives after vigorous oxidation. The same pattern applies whether the cleavage is carried out by hot concentrated KMnO₄ or by other strong oxidising systems.
下表总结了各类烯烃碳在剧烈氧化后所得到的产物。无论是用热浓 KMnO₄ 还是其他强氧化体系进行断裂，该规律都适用。
| Alkene carbon | 烯烃碳 | Product | 产物 |
| =CH₂ (two hydrogens | 两个氢) | style="border:1px
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