📚 Electrophilic Addition: OCR A-Level Essentials | A-Level OCR 化学：亲电加成考点精讲
Alkenes are fundamental hydrocarbons containing at least one carbon–carbon double bond. Their characteristic reactions are electrophilic additions, in which the pi bond is attacked by an electron‑deficient species (electrophile). Understanding the mechanism, regioselectivity (Markovnikov's rule), and stereochemistry of these additions is essential for A‑Level OCR Chemistry.
烯烃是含有至少一个碳碳双键的基本碳氢化合物。它们的特征反应是亲电加成反应，其中π键受到缺电子物种（亲电试剂）的进攻。理解这些加成反应的机理、区域选择性（马氏规则）和立体化学对于A‑Level OCR化学至关重要。
1. What is Electrophilic Addition? | 什么是亲电加成？
Electrophilic addition is a reaction in which a pi bond from an alkene is broken and two new sigma bonds are formed. An electrophile, an electron‑deficient species, takes a pair of electrons from the alkene to form one bond, while a nucleophile subsequently attaches to the other carbon of the original double bond. Typical electrophiles include H⁺ (from HX or acids), Brᵟ⁺ (polarised halogen), and SO₃.
亲电加成是一种反应，其中烯烃的π键断裂并形成两个新的σ键。一个亲电试剂（缺电子物种）从烯烃获得一对电子形成一个键，同时一个亲核试剂随后连接到原来双键的另一个碳上。典型的亲电试剂包括H⁺（来自HX或酸）、Brᵟ⁺（极化的卤素）和SO₃。
2. The Pi Bond: A Source of Electrons | π键：电子之源
The carbon–carbon double bond in an alkene consists of a sigma (σ) bond and a pi (π) bond. The pi bond is formed by the sideways overlap of adjacent p orbitals and holds a pair of electrons above and below the plane of the molecule. Because these pi electrons are less tightly held than sigma electrons, the region of electron density attracts electrophiles and makes alkenes much more reactive than alkanes.
烯烃中的碳碳双键由一个sigma（σ）键和一个pi（π）键组成。π键由相邻p轨道的侧向重叠形成，在分子平面的上下方各有一对电子。由于这些π电子的束缚不如σ电子紧密，电子密度区域吸引亲电试剂，使得烯烃远比烷烃活泼。
3. Mechanism of Electrophilic Addition: General Steps | 亲电加成机理：一般步骤
The mechanism for electrophilic addition to an unsymmetrical alkene typically occurs in two steps. In the slow, rate‑determining step, the electrophile (E⁺) accepts a pair of pi electrons and forms a bond to one of the double‑bonded carbons, generating a carbocation intermediate. In the fast step, a nucleophile (Nu⁻) attacks the carbocation, forming the final saturated product. For HX addition, H⁺ is the electrophile and X⁻ is the nucleophile.
对于不对称烯烃的亲电加成，机理通常分两步进行。在慢的、决定速率的步骤中，亲电试剂（E⁺）接受一对π电子并与其中一个双键碳成键，生成碳正离子中间体。在快的步骤中，亲核试剂（Nu⁻）进攻碳正离子，形成最终的饱和产物。对于HX加成，H⁺是亲电试剂，X⁻是亲核试剂。
ttacks the carbocation to give CH₃CH₂Br. The overall reaction is CH₂=CH₂ + HBr → CH₃CH₂Br.
一个具体例子是溴化氢对乙烯的加成：第一步，H⁺进攻π键，生成CH₃CH₂⁺和Br⁻；第二步，Br⁻进攻碳正离子，得到CH₃CH₂Br。总反应为CH₂=CH₂ + HBr → CH₃CH₂Br。
4. Carbocation Stability and Its Importance | 碳正离子稳定性及其重要性
Carbocations are classified as methyl (CH₃⁺), primary (1°), secondary (2°) or tertiary (3°) according to the number of carbon atoms attached to the positively charged carbon. The order of stability is: tertiary > secondary > primary > methyl. This stability trend is crucial because it determines which carbocation intermediate is formed in an unsymmetrical alkene addition, thereby controlling the major product.
碳正离子根据与带正电荷碳相连的碳原子数目分为甲基（CH₃⁺）、伯（1°）、仲（2°）或叔（3°）碳正离子。稳定性顺序为：叔 > 仲 > 伯 > 甲基。这个稳定性趋势至关重要，因为它决定了不对称烯烃加成中形成哪种碳正离子中间体，从而控制主要产物。
5. Addition of Hydrogen Halides (HX) | 卤化氢（HX）的加成
ot require a catalyst. The overall equation for symmetrical alkenes is R₂C=CR₂ + HX → R₂HC– ucts are possible, and the proportion is governed by carbocation stability and Markovnikov's rule.
在烯烃与卤化氢（HCl、HBr、HI）的反应中，亲电试剂是质子H⁺。反应在室温下进行，不需要催化剂。对于对称烯烃，总反应为R₂C=CR₂ + HX → R₂HC–CR₂X。对于不对称烯烃，可能生成两种产物，其比例由碳正离子稳定性和马氏规则决定。
6. Markovnikov's Rule | 马氏规则
Markovnikov' more substituted carbon. As an example, propene (CH₃CH=CH₂) with HBr gives predominantly 2‑bromopropane (CH₃CHBrCH₃) rather than 1‑bromopropane (CH₂BrCH₂CH₃).
马氏规则指出，当卤化氢与不对称烯烃加成时，氢原子加到双键上已有较多氢原子的碳上（取代程度较低的碳），而卤素加到取代程度较高的碳上。例如，丙烯（CH₃CH=CH₂）与HBr主要生成2‑溴丙烷（CH₃CHBrCH₃），而不是1‑溴丙烷（CH₂BrCH₂CH₃）。
7. Why Markovnikov's Rule? Inductive Effect and Hyperconjugation | 马氏规则的解释：诱导效应与超共轭
The product selectivity is explained by the relative stability of the possible carbocation intermediates. Addition of H⁺ to the terminal carbon of propene gives a secondary carbocation (CH₃)₂CH⁺, while addition to the internal carbon gives a primary carbocation CH₃CH₂CH₂⁺. The secondary carbocation is more stable because the two alkyl groups donate electron density through the positive inductive effect (+I effect) and stabilise the positive charge via hyperconjugation. Therefore, the secondary carbocation forms faster and dominates.
产物的选择性可由可能的碳正离子中间体的相对稳定性来解释。H⁺加到丙烯的末端碳上会生成仲碳正离子（CH₃)₂CH⁺，而加到内部碳上得到伯碳正离子CH₃CH₂CH₂⁺。仲碳正离子更稳定，因为两个烷基通过正诱导效应（+I效应）提供电子密度，并通过超共轭作用稳定正电荷。因此，仲碳正离子形成得更快，占主导地位。
8. Addition of Halogens: Bromination | 卤素加成：溴化
When bromine water (Br₂(aq)) is added to an alkene, the orange colour disappears as an addition product is formed. The mechanism differs from that of HX because bromine does not form a simple carbocation. As the Br₂ molecule approaches the alkene, the pi electrons induce a dipole in the Br–Br bond, making one bromine atom electrophilic. The alkene's pi electrons attack this polarised bromine, forming a cyclic bromonium ion (a bridged intermediate, (CH₂)₂Br⁺) and releasing a bromide ion, Br⁻. In the second step, the bromide ion attacks the bromonium ion from the opposite face, giving only anti addition.
当溴水（Br₂(aq)）加入烯烃时，橙色褪去，同时生成加成产物。该机理与HX不同，因为溴不会形成简单的碳正离子。当Br₂分子靠近烯烃时，π电子会诱导Br–Br键产生偶极，使一个溴原子具有亲电性。烯烃的π电子进攻这个极化的溴原子，形成环状溴鎓离子（桥连中间体，(CH₂)₂Br⁺），并释放出一个溴离子Br⁻。第二步中，溴离子从反面进攻溴鎓离子，只得到反式加成产物。
9. Stereochemistry of Bromination: Anti Addition | 溴化的立体化学：反式加成
The two‑step addition of bromine results in anti addition, meaning the two bromine atoms end up on opposite sides of the original double bond plane. This has important stereochemical consequences. For cyclopentene, the product is trans‑1,2‑dibromocyclopentane. For a symmetrical linear alkene with two identical substituents, anti addition may give a meso compound if the substituents allow. The test for unsaturation uses bromine water, which turns from orange to colourless in the presence of alkenes.
溴的两步加成导致反式加成，意味着两个溴原子最终位于原双键平面的两侧。这具有重要的立体化学意义。对于环戊烯，产物为反式‑1,2‑二溴环戊烷。对于具有两个相同取代基的对称直链烯烃，如果取代基允许，反式加成可能产生内消旋化合物。不饱和性检验使用溴水，在烯烃存在下由橙色变为无色。
10. Hydration of Alkenes: Adding Water | 烯烃水合：加水
Alkenes can be hydrated to form alcohols. One industrial method is direct hydration using steam and a phosphoric acid catalyst adsorbed on silica (H₃PO₄/SiO₂) at about 300 °C and 60 atm. The mechanism follows the same carbocation pathway as HX addition, with H⁺ as the initial electrophile. Water then acts as a nucleophile, attacking the carbocation, and after loss of a proton the alcohol is formed. For unsymmetrical alkenes, the OH group ends up on the more substituted carbon, consistent with Markovnikov's rule.
烯烃可以水合形成醇。一种工业方法是直接水合，使用蒸汽和吸附在硅胶上的磷酸催化剂（H₃PO₄/SiO₂），在约300 °C和60 atm下进行。其机理与HX加成遵循相同的碳正离子途径，以H⁺为起始亲电试剂。然后水作为亲核试剂进攻碳正离子，失去质子后得到醇。对于不对称烯烃，OH基加在取代程度较高的碳上，符合马氏规则。
An alternative laboratory route is indirect hydration: the alkene reacts with cold concentrated sulphuric acid to form an alkyl hydrogensulphate, which is then hydrolysed by warming with water to yield the alcohol and regenerate the acid. This two‑step sequence also follows Markovnikov's rule.
另外一种实验室路线是间接水合：烯烃与冷浓硫酸反应生成烷基硫酸氢酯，然后与温水加热水解得到醇并再生酸。这个两步步骤同样遵循马氏规则。
11. Addition of Sulphuric Acid | 硫酸加成
When an alkene is shaken with cold concentrated H₂SO₄, an alkyl hydrogensulphate is formed via electrophilic addition. The electrophile is effectively H⁺ from the acid. For example, ethene yields ethyl hydrogensulphate (CH₃CH₂OSO₃H). This reaction makes alkenes soluble in sulphuric acid. Upon subsequent addition of water and heating, the ester hydrolyses to ethanol and H₂SO₄, so this is a two‑step method for alcohol synthesis.
当烯烃与冷的浓H₂SO₄一起振荡时，通过亲电加成生成烷基硫酸氢酯。这里亲电试剂实际上是酸中的H⁺。例如，乙烯产生硫酸氢乙酯（CH₃CH₂OSO₃H）。这个反应使烯烃溶于硫酸。随后加水加热，酯水解为乙醇和H₂SO₄，所以这是合成醇的两步法。
12. Testing for Alkenes: Bromine Water Test | 烯烃检验：溴水试验
The bromine water test is a simple qualitative test for the presence of a carbon–carbon double bond. Bromine water is orange‑brown. When added to a sample containing an alkene and shaken, the colour is discharged because the bromine adds across the double bond to form a colourless dibromoalkane. This distinguishes alkenes from alkanes, which do not react under these conditions without UV light.
溴水试验是一种简单的定性检验碳碳双键存在的方法。溴水呈橙棕色。当加入含烯烃的样品并振荡时，颜色褪去，因为溴加成到双键上，生成无色的二溴烷烃。这能够区分烯烃和烷烃，因为没有紫外光时烷烃在这些条件下不反应。
| Reaction | Reagents/Conditions | Key Intermediates | Regioselectivity |
|---|---|---|---|
| Addition of HX | HCl, HBr or HI, room temp., no catalyst | Carbocation | Markovnikov |
| Addition of Br₂ | Br₂ in water, room temp. | Cyclic bromonium ion | Anti addition (stereochemistry) |
| Direct hydration | H₂O(g), H₃PO₄/SiO₂, 300 °C, 60 atm | Carbocation | Markovnikov |
| Addition of H₂SO₄ | Conc. H₂SO₄, cold | Carbocation | Markovnikov |
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