Alkenes: Essential Exam Points | 烯烃考点精讲

📚 Alkenes: Essential Exam Points | 烯烃考点精讲

In A-Level CCEA Chemistry, alkenes constitute a fundamental class of unsaturated hydrocarbons featuring at least one carbon–carbon double bond. Mastering their structure, isomerism, and characteristic electrophilic addition reactions is essential for exam success.

在A-Level CCEA化学中，烯烃是一类重要的不饱和烃，含有至少一个碳碳双键。掌握其结构、异构现象和典型的亲电加成反应是取得考试成功的关键。

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1. Introduction to Alkenes | 烯烃简介

Alkenes are hydrocarbons with the general formula CₙH₂ₙ (for one double bond). The functional group is the carbon–carbon double bond, C=C, which consists of one σ (sigma) bond and one π (pi) bond.

烯烃的通式为CₙH₂ₙ（针对一个双键）。其官能团是碳碳双键 C=C，由一个σ键和一个π键组成。

The π bond is formed by the sideways overlap of p-orbitals, resulting in a region of high electron density above and below the plane of the molecule. This makes alkenes reactive towards electrophiles.

π键由p轨道侧面重叠形成，在分子平面上方和下方形成高电子密度区域，这使得烯烃易受亲电试剂攻击。

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2. Nomenclature and Structure | 命名与结构

The IUPAC naming of alkenes follows similar rules to alkanes but with the suffix ‘-ene’ and a locant to indicate the position of the double bond. The chain is numbered to give the double bond the lowest possible number.

烯烃的IUPAC命名遵循与烷烃相似的规则，但后缀为“-ene”，并需标明双键位置的编号。主链编号应使双键具有最小的数字。

For example: CH₂=CHCH₂CH₃ is but-1-ene, CH₃CH=CHCH₃ is but-2-ene. When substituents are present, the double bond takes priority in numbering.

例如：CH₂=CHCH₂CH₃ 是1-丁烯，CH₃CH=CHCH₃ 是2-丁烯。当有取代基时，双键的位置在编号中优先。

The carbon atoms of the double bond are sp² hybridised, giving a trigonal planar geometry with bond angles of about 120°.

双键碳原子为sp²杂化，呈平面三角形几何结构，键角约为120°。

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3. Geometric (cis-trans) Isomerism | 几何（顺反）异构

Due to restricted rotation about the C=C bond, alkenes can exhibit cis-trans (geometric) isomerism when each carbon of the double bond has two different groups attached.

由于碳碳双键不能自由旋转，当每个双键碳上连有两个不同基团时，烯烃可表现出顺反（几何）异构现象。

The cis isomer has the higher-priority groups on the same side of the double bond; the trans isomer has them on opposite sides. This leads to differences in physical properties such as boiling point and polarity.

顺式异构体中，优先基团在双键的同侧；反式异构体中则在异侧。这导致了物理性质（如沸点和极性）的差异。

For example, but-2-ene exists as cis-but-2-ene and trans-but-2-ene. In the CCEA specification, you must be able to identify and draw cis-trans isomers.

例如，2-丁烯存在顺-2-丁烯和反-2-丁烯异构体。CCEA考纲要求能够识别并画出顺反异构体。

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4. Physical Properties | 物理性质

Alkenes are non-polar or weakly polar molecules, held together by van der Waals forces. Their boiling points increase with chain length but are slightly lower than those of the corresponding alkanes due to a less compact shape.

烯烃为非极性或弱极性分子，分子间以色散力结合。其沸点随碳链增长而升高，但由于形状不太紧凑，沸点略低于相应的烷烃。

Alkenes are insoluble in water but dissolve in non-polar organic solvents. The presence of the double bond introduces a slight dipole due to unsymmetrical substitution, but overall polarity is low.

烯烃不溶于水，但可溶于非极性有机溶剂。双键的存在由于不对称取代会引入微小偶极，但总体极性很低。

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5. Electrophilic Addition Mechanism | 亲电加成机理

Alkenes undergo electrophilic addition, where the π bond breaks heterolytically and two new σ bonds form. The general mechanism involves attack by an electrophile (E⁺) to form a carbocation intermediate, followed by nucleophilic attack by the counterion (Nu⁻).

烯烃发生亲电加成反应，π键异裂并形成两个新的σ键。一般机理为：亲电试剂（E⁺）进攻形成碳正离子中间体，然后抗衡离子（Nu⁻）进行亲核进攻。

For the addition of HBr to ethene: the H⁺ (electrophile) adds first to one carbon, creating a carbocation; then Br⁻ attacks the carbocation to give the product.

以乙烯与HBr的加成为例：H⁺（亲电试剂）首先加到碳原子上，形成碳正离子；然后Br⁻进攻碳正离子，得到产物。

CH₂=CH₂ + HBr → CH₃CH₂Br

The mechanism is commonly drawn using curly arrows to show electron movement: from the π bond to the electrophile, and from the nucleophile to the carbocation.

该机理通常用弯箭头表示电子转移：从π键指向亲电试剂，以及从亲核试剂指向碳正离子。

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6. Addition of Hydrogen Halides (HX) | 卤化氢加成

Alkenes react with hydrogen halides (HCl, HBr, HI) to form halogenoalkanes. The reaction occurs at room temperature by bubbling the gas through the alkene or mixing with concentrated aqueous acid.

烯烃与卤化氢（HCl、HBr、HI）反应生成卤代烷。反应在室温下通过将气体通入烯烃或与浓酸混合进行。

With unsymmetrical alkenes (e.g., propene), two regioisomeric products are possible. The major product is determined by the stability of the intermediate carbocation.

对于不对称烯烃（如丙烯），可能产生两种区域异构产物。主要产物由中间体碳正离子的稳定性决定。

The mechanism proceeds through the heterolytic fission of H–X, generating H⁺ and X⁻. The pi electrons attack H⁺ to form the most stable carbocation; then X⁻ adds to it.

机理通过H–X键异裂产生H⁺和X⁻。π电子进攻H⁺，形成最稳定的碳正离子；然后X⁻加入其中。

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

Markovnikov’s rule states that in the addition of HX to an unsymmetrical alkene, the hydrogen atom attaches to the carbon with the greater number of hydrogen atoms already attached, i.e., the carbon that is less substituted.

马尔科夫尼科夫规则指出：HX与不对称烯烃加成时，氢原子加在含氢较多的双键碳上（即取代较少的碳）。

This is explained by carbocation stability: tertiary > secondary > primary > methyl. The more alkyl groups attached to the positively charged carbon, the more stable the carbocation through inductive and hyperconjugative effects.

这可以用碳正离子稳定性解释：叔碳正离子 > 仲碳正离子 > 伯碳正离子 > 甲基碳正离子。正电荷碳上连接的烷基越多，通过诱导效应和超共轭效应越稳定。

Thus, addition of HBr to propene yields mainly 2-bromopropane (via a 2° carbocation) rather than 1-bromopropane (via a 1° carbocation).

因此，HBr与丙烯加成主要生成2-溴丙烷（经由2°碳正离子）而不是1-溴丙烷（经由1°碳正离子）。

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8. Addition of Halogens (Br₂, Cl₂) | 卤素加成

Alkenes react readily with bromine or chlorine at room temperature to give vicinal dihalogenoalkanes. The reaction with bromine is commonly used as a test for unsaturation: the orange/brown bromine water is decolourised.

烯烃在室温下与溴或氯迅速反应生成邻二卤代烷。与溴的反应常用作不饱和性检验：橙棕色的溴水褪色。

The mechanism involves the polarisation of the halogen molecule as it approaches the electron-rich double bond. The halogen acts as the electrophile, forming a cyclic bromonium (or chloronium) ion intermediate, which is then attacked by the halide ion from the opposite side (anti addition).

机理涉及卤素分子接近富电子双键时发生极化。卤素作为亲电试剂，形成环状溴鎓（或氯鎓）离子中间体，然后卤离子从反面进攻（反式加成）。

C₂H₄ + Br₂ → CH₂BrCH₂Br

Because of the cyclic intermediate, the addition gives overall trans stereochemistry. This is important in predicting the product stereochemistry of cycloalkenes.

由于环状中间体的存在，加成总体上给出反式立体化学。这在预测环烯烃产物的立体化学中很重要。

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9. Addition of Water: Hydration | 水加成：水合反应

Alkenes can be hydrated to form alcohols. The industrial method uses steam and a phosphoric(V) acid catalyst (H₃PO₄) at high temperature and pressure to produce ethanol from ethene.

烯烃可通过水合反应生成醇。工业上采用高温高压下，以磷酸（V）（H₃PO₄）为催化剂，由乙烯和水蒸汽制备乙醇。

CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH

In the laboratory, alkenes can be hydrated by cold concentrated sulfuric acid (H₂SO₄) to form alkyl hydrogensulfates, which are then hydrolysed by water and heating to yield the alcohol. This is an important route for synthesising alcohols from alkenes and follows Markovnikov’s rule.

在实验室中，烯烃可与冷浓硫酸（H₂SO₄）反应生成硫酸氢酯，再经水和加热水解得到醇。这是由烯烃合成醇的重要途径，并遵循马氏规则。

The mechanism proceeds with protonation of the double bond to form the most stable carbocation, then attack by water and deprotonation.

机理为双键质子化形成最稳定的碳正离子，然后水进攻并去质子化。

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10. Hydrogenation | 加氢反应

Alkenes react with hydrogen (H₂) in the presence of a metal catalyst (Ni, Pt, or Pd) to form alkanes. This is an addition reaction used in the food industry to convert unsaturated vegetable oils into solid margarines.

烯烃在金属催化剂（Ni、Pt或Pd）存在下与氢气（H₂）反应生成烷烃。该加成反应用于食品工业中将不饱和植物油转化为固态人造黄油。

The reaction involves adsorption of both reactants onto the catalyst surface, weakening the H–H and C=C bonds, allowing hydrogen atoms to add to the carbon atoms. The catalyst lowers the activation energy.

反应涉及两种反应物吸附在催化剂表面，削弱H–H和C=C键，使氢原子加到碳原子上。催化剂降低了活化能。

RCH=CHR + H₂ → RCH₂CH₂R (on Pt/Pd/Ni)

Hydrogenation is stereospecific: cis addition of hydrogen occurs, giving the alkane with syn stereochemistry. The enthalpy change, called hydrogenation enthalpy, can be used to compare the relative stabilities of alkenes.

加氢具有立体专一性：氢以顺式加成，得到具有同向立体化学的烷烃。其焓变称为加氢焓，可用于比较烯烃的相对稳定性。

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11. Oxidation Reactions (with KMnO₄) | 氧化反应（与高锰酸钾）

Alkenes are easily oxidised by acidified potassium manganate(VII) (KMnO₄). At room temperature, the purple solution turns colourless, while the alkene is converted to a diol (in cold, dilute, neutral or alkaline conditions) or further oxidised to carbonyl/carboxylic acids (hot, acidic conditions).

烯烃易被酸性高锰酸钾（KMnO₄）氧化。室温下，紫色溶液

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