📚 Electrophilic Addition: Key Revision for IB CCEA Chemistry | IB CCEA 化学：亲电加成 考点精讲
Electrophilic addition reactions are central to the chemistry of alkenes and alkynes. In IB and CCEA specifications, understanding the mechanism, regioselectivity, and stereochemistry of these reactions is essential. This article provides a thorough revision guide, covering the electrophilic addition mechanism, Markovnikov’s rule, carbocation stability, and specific addition reactions of halogens, hydrogen halides, water, and hydrogen.
亲电加成反应是烯烃和炔烃化学的核心。在IB和CCEA课程大纲中，理解这些反应的机理、区域选择性和立体化学至关重要。本文提供全面的复习指南，涵盖亲电加成机理、马尔科夫尼科夫规则、碳正离子稳定性以及卤素、卤化氢、水和氢气的特定加成反应。
1. Introduction to Electrophilic Addition | 亲电加成简介
Electrophilic addition is a reaction in which a π bond is broken and two new σ bonds are formed. Alkenes contain a C=C double bond consisting of a stronger σ bond and a weaker π bond. The π bond is an area of high electron density, making alkenes susceptible to attack by electron‑deficient species called electrophiles. The general transformation can be summarised as: C=C + X–Y → X–C–C–Y.
亲电加成是一种断裂π键并形成两个新的σ键的反应。烯烃中的C=C双键由一个较强的σ键和一个较弱的π键组成。π键是一个高电子密度区域，使得烯烃容易受到缺电子物种（亲电试剂）的进攻。其一般转化可概括为：C=C + X–Y → X–C–C–Y。
Because the π electrons are loosely held and exposed, the reaction proceeds with the electrophile first accepting a pair of electrons from the π bond to form a new σ bond. The resulting intermediate then reacts with a nucleophile to complete the addition. This stepwise nature distinguishes it from concerted pericyclic reactions.
由于π电子结合松散且暴露在外，反应首先由亲电试剂从π键接受一对电子形成新的σ键。生成的中间体再与亲核试剂反应完成加成。这种分步进行的特点使其区别于协同的周环反应。
2. The Electrophile: Definition and Examples | 亲电试剂：定义与实例
An electrophile is a reagent that is attracted to electrons and can accept an electron pair to form a covalent bond. Common electrophiles include fully positive species such as H⁺ (from strong acids) and Br⁺ (generated in situ), as well as the slightly positive end of a polarised molecule, such as the hydrogen in HBr or the partially positive halogen in Br₂ when it is polarised by the alkene’s π cloud.
亲电试剂是指被电子吸引并能接受一对电子形成共价键的试剂。常见的亲电试剂包括带完全正电荷的物种，如H⁺（来自强酸）和Br⁺（原位生成），以及极化分子中略带正电的一端，例如HBr中的氢，或当Br₂被烯烃的π电子云极化后产生的部分正电荷的卤素原子。
In the laboratory, electrophiles can be generated using catalysts. For instance, Br₂ is non‑polar in the ground state, but when it approaches the electron‑rich double bond, an induced dipole occurs, making the nearer bromine atom electrophilic. In halogen addition, a cyclic halonium ion may form, which acts as an even more powerful electrophilic centre.
在实验室中，亲电试剂可以借助催化剂生成。例如，基态Br₂是非极性的，但当它靠近富电子的双键时，会产生诱导偶极，使得较近的那个溴原子成为亲电中心。在卤素加成中，可能形成环状卤鎓离子，它作为一个更加强大的亲电中心。
3. Mechanism of Electrophilic Addition to Alkenes | 烯烃的亲电加成机理
The mechanism involves two main steps. First, the electrophile (E⁺) is attacked by the π electrons, breaking the π bond and forming a new C–E σ bond. This leaves a carbocation intermediate on the other carbon. The carbocation is sp² hybridised and planar, with an empty p orbital. In the second step, a nucleophile (Nu⁻) donates an electron pair to the carbocation, forming the second C–Nu σ bond and completing the addition.
该机理包括两个主要步骤。首先，亲电试剂（E⁺）受到π电子的进攻，π键断裂，形成一个新的C–E σ键。这使另一个碳上留下一个碳正离子中间体。该碳正离子为sp²杂化，平面构型，含有一个空的p轨道。第二步，亲核试剂（Nu⁻）向碳正离子提供一对电子，形成第二个C–Nu σ键，完成加成。
The energy profile diagram shows two transition states and a carbocation intermediate valley. The first step is rate‑determining because it requires disruption of the π bond and formation of the less stable carbocation. The overall reaction is exothermic for most additions to alkenes.
能量曲线图显示两个过渡态和一个碳正离子中间体的势阱。第一步是决速步骤，因为它需要破坏π键并生成较不稳定的碳正离子。对大多数烯烃加成反应，总反应是放热的。
The mechanism can be summarised:
Step (i): C=C + E⁺ → E–C–C⁺ (slow)
Step (ii): E–C–C⁺ + Nu⁻ → E–C–C–Nu (fast)
机理概括如下：
步骤 (i)：C=C + E⁺ → E–C–C⁺（慢）
步骤 (ii)：E–C–C⁺ + Nu⁻ → E–C–C–Nu（快）
4. Markovnikov’s Rule and Regioselectivity | 马尔科夫尼科夫规则与区域选择性
When an unsymmetrical alkene undergoes electrophilic addition with an unsymmetrical reagent such as HBr, two regioisomeric products are theoretically possible. Markovnikov’s rule predicts the major product: the hydrogen atom (or the more electropositive part) becomes attached to the carbon of the double bond that already has the greater number of hydrogen atoms. In other words, “the rich get richer” with respect to hydrogen.
当不对称烯烃与不对称试剂（如HBr）发生亲电加成时，理论上可能生成两种区域异构体产物。马尔科夫尼科夫规则预测主要产物为：氢原子（或电正性更强的部分）加成到双键中原本含有较多氢原子的碳上。换句话说，氢上“富者愈富”。
The rule is a consequence of carbocation stability. The electrophilic addition proceeds via the more stable carbocation intermediate. For example, propene (CH₃CH=CH₂) with HBr gives mainly 2‑bromopropane (CH₃CHBrCH₃) rather than 1‑bromopropane, because the secondary carbocation intermediate is more stable than the primary one.
该规则是碳正离子稳定性的结果。亲电加成经由较稳定的碳正离子中间体进行。例如，丙烯（CH₃CH=CH₂）与HBr反应主要生成2‑溴丙烷（CH₃CHBrCH₃）而非1‑溴丙烷，因为二级碳正离子中间体比一级碳正离子更稳定。
In modern terms, Markovnikov addition is termed regioselective, as one constitutional isomer is formed preferentially. Anti‑Markovnikov addition can occur only under radical conditions (peroxide effect) with HBr, which is a separate topic.
用现代术语，马尔科夫尼科夫加成被称为区域选择性反应，因为一种构造异构体优先生成。反马尔科夫尼科夫加成仅在HBr的自由基条件下（过氧化物效应）发生，这属于独立的专题。
5. Carbocation Stability and Rearrangements | 碳正离子稳定性与重排
Carbocation stability increases with substitution: tertiary (3°) > secondary (2°) > primary (1°) > methyl. This order is explained by hyperconjugation and the inductive effect of alkyl groups, which donate electron density and stabilise the positive charge. A tertiary carbocation has three alkyl groups delocalising the charge, making it the most stable.
碳正离子的稳定性随取代度增加：叔（3°）> 仲（2°）> 伯（1°）> 甲基。这一顺序可由超共轭效应和烷基的诱导效应解释，烷基提供电子密度，使正电荷得到稳定。叔碳正离子有三个烷基分散电荷，因此最稳定。
Carbocation rearrangements can occur if a more stable carbocation can be formed through a 1,2‑shift of a hydrogen atom or an alkyl group. For example, addition of HCl to 3,3‑dimethylbut‑1‑ene initially forms a secondary carbocation, but a methyl shift generates a tertiary carbocation, leading to a rearranged product. Such rearrangements are a key mechanistic evidence for the existence of carbocations and must be considered when predicting products.
如果能通过1,2‑氢迁移或1,2‑烷基迁移形成更稳定的碳正离子，就可能发生碳正离子重排。例如，3,3‑二甲基‑1‑丁烯与HCl加成最初生成二级碳正离子，但经甲基迁移生成三级碳正离子，导致重排产物。这类重排是碳正离子存在的关键机理证据，在预测产物时必须加以考虑。
However, rearrangements do not happen if a more stable carbocation cannot be reached or if the intermediate is a cyclic halonium ion (as in bromination), which avoids the open carbocation entirely.
然而，如果无法形成更稳定的碳正离子，或者中间体是环状卤鎓离子（如溴化反应），则不会发生重排，因为这样完全避免了开链碳正离子的形成。
6. Addition of Hydrogen Halides (HX) | 卤化氢（HX）的加成
Alkenes react with hydrogen halides (HCl, HBr, HI) to form haloalkanes. The reactivity order is HI > HBr > HCl, which correlates with the H–X bond strength: the weaker the bond, the easier the electrophilic H⁺ is released. HF is not used due to its strong bond and toxicity. The reaction follows Markovnikov’s rule, yielding the more substituted haloalkane as the major product.
烯烃与卤化氢（HCl、HBr、HI）反应生成卤代烷。反应活性顺序为HI > HBr > HCl，这与H–X键的强度相关：键越弱，亲电的H⁺越容易释放。HF因其键强和毒性而不被使用。反应遵循马尔科夫尼科夫规则，主要生成取代度更高的卤代烷。
| HX | Bond Energy (kJ mol⁻¹) | Relative Rate | Example Product (from propene) |
|---|---|---|---|
| HI | 299 | Fastest | CH₃CHICH₃ |
| HBr | 366 | Medium | CH₃CHBrCH₃ |
| HCl | 431 | Slowest | CH₃CHClCH₃ |
The table above summarises the relationship between bond energy, rate, and product. Gaseous HCl requires a catalyst such as AlCl₃ for addition to less reactive alkenes, while HI and HBr add readily at room temperature.
上表总结了键能、速率与产物之间的关系。气态HCl与活性较低的烯烃加成时需要AlCl₃等催化剂，而HI和HBr在室温下即可顺利加成。
7. Addition of Halogens (X₂) | 卤素（X₂）的加成
Halogens (Cl₂ and Br₂) add to alkenes to give vicinal dihalides. The reaction takes place at room temperature, often in an inert solvent such as CCl₄. The mechanism proceeds via a cyclic halonium ion rather than a free carbocation. The alkene’s π electrons attack a polarised halogen molecule, expelling a halide ion and forming a three‑membered ring with a positively charged halogen.
卤素（Cl₂和Br₂）与烯烃加成生成邻二卤代物。反应在室温下进行，常在惰性溶剂如CCl₄中实施。该机理经由环状卤鎓离子而非游离碳正离子进行。烯烃的π电子进攻被极化的卤素分子，排出一个卤离子，形成一个带正电荷的三元环。
For example, ethene reacts with bromine: CH₂=CH₂ + Br₂ → BrCH₂–CH₂Br. The cyclic bromonium ion is then attacked by the bromide ion from the opposite face (backside attack), leading to anti addition. This stereospecificity is crucial in cyclic alkenes: cyclohexene yields trans‑1,2‑dibromocyclohexane exclusively.
例如，乙烯与溴反应：CH₂=CH₂ + Br₂ → BrCH₂–CH₂Br。环状溴鎓离子随后被溴离子从背面进攻，导致反式加成。这种立体专一性在环状烯烃中至关重要：环己烯只生成反‑1,2‑二溴环己烷。
Iodine addition is reversible, and fluorine is too violent. The bromination and chlorination reactions are widely used as tests for unsaturation, as the characteristic colour of the halogen disappears.
碘的加成是可逆的，而氟反应过于激烈。溴化和氯化反应被广泛用作不饱和性检验，因为卤素的特征颜色会褪去。
8. Addition of Water: Hydration | 水加成：水合反应
Water can be added to alkenes in the presence of a strong acid catalyst, typically concentrated sulfuric acid or phosphoric acid, to produce alcohols. The reaction is an electrophilic addition where the electrophile is effectively H⁺. The alcohol is formed with Markovnikov regioselectivity, giving the more substituted alcohol.
在强酸催化剂（通常为浓硫酸或磷酸）存在下，水可与烯烃加成生成醇。该反应为亲电加成，有效亲电试剂是H⁺。醇的生成遵循马尔科夫尼科夫区域选择性，得到取代度更高的醇。
The industrial hydration of ethene to ethanol uses phosphoric acid on a solid support at high temperature and pressure: CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH. This is an important route for industrial alcohol production. For unsymmetrical alkenes, the product is the alcohol corresponding to the more stable carbocation, e.g., propene gives propan‑2‑ol.
工业上乙烯水合制乙醇使用负载在固体上的磷酸，在高温高压下进行：CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH。这是工业酒精生产的重要路线。对于不对称烯烃，产物对应于较稳定碳正离子的醇，例如丙烯生成丙‑2‑醇。
The mechanism involves protonation of the alkene to form a carbocation, followed by nucleophilic attack by water and deprotonation. Because the catalyst is regenerated, the overall process is acid‑catalysed hydration.
该机理包括烯烃的质子化形成碳正离子，接着水作为亲核试剂进攻，然后去质子化。由于催化剂得以再生，整个过程为酸催化水合反应。
9. Addition of Hydrogen: Hydrogenation | 氢气加成：氢化
Hydrogenation is the addition of H₂ across a double bond to form an alkane. This reaction requires a metal catalyst such as finely divided nickel, palladium, or platinum, which provide a surface for the activation of H₂. The addition is syn stereospecific: both hydrogen atoms add to the same face of the double bond, producing cis addition in cyclic alkenes.
氢化是H₂在双键上的加成以生成烷烃。该反应需要金属催化剂，如细粉状的镍、钯或铂，它们提供活化H₂的表面。该加成是顺式立体专一性的：两个氢原子加到双键的同一侧，在环状烯烃中产生顺式加成产物。
Hydrogenation is exothermic and is used to harden vegetable oils by saturating some of the C=C bonds in unsaturated fatty acid chains. The extent of hydrogenation can be controlled to produce semi‑solid spreads. In organic synthesis, catalytic hydrogenation also serves as a protecting strategy or to reduce alkenes selectively.
氢化是放热反应，用于通过饱和不饱和脂肪酸链中的部分C=C键来硬化植物油。控制氢化程度可生产半固态涂抹脂。在有机合成中，催化氢化也用作保护基策略或选择性还原烯烃。
A typical laboratory setup uses hydrogen gas and palladium on carbon at room temperature. The reaction is heterogeneous and does not require a polar solvent. The catalyst is recovered by filtration.
典型的实验室装置使用氢气和钯碳
Published by TutorHao | IB Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导Cancel reply