Electrophilic Addition vs Nucleophilic Substitution | 亲电加成与亲核取代对比

📚 Electrophilic Addition vs Nucleophilic Substitution | 亲电加成与亲核取代对比

In A-Level Edexcel Chemistry, understanding reaction mechanisms is crucial for predicting products and explaining reactivity. Two fundamental types of polar mechanisms are electrophilic addition and nucleophilic substitution, which dominate the reactions of alkenes and halogenoalkanes respectively. This article provides a detailed comparison of these mechanisms, highlighting their definitions, conditions, stereochemistry, and common pitfalls.

在A-Level Edexcel化学中，理解反应机理对于预测产物和解释反应性至关重要。两种基本的极性机理类型是亲电加成和亲核取代，分别主导了烯烃和卤代烷的反应。本文详细比较了这两种机理，重点阐述它们的定义、条件、立体化学和常见误区。

1. Overview | 概述

Electrophilic addition is the characteristic reaction of unsaturated compounds like alkenes, where an electrophile attacks the electron-rich π‑bond, leading to addition across the double bond. Nucleophilic substitution, on the other hand, occurs in saturated compounds such as halogenoalkanes, where a nucleophile replaces a leaving group at a saturated carbon.

亲电加成是不饱和化合物（如烯烃）的特征反应，亲电试剂进攻富电子的π键，导致双键上的加成。而亲核取代发生在饱和化合物（如卤代烷）中，亲核试剂取代饱和碳原子上的离去基团。

2. Definition and Core Concepts | 定义与核心概念

Electrophilic addition involves the attack of an electron-deficient species (electrophile) on a region of high electron density, typically the π‑bond of an alkene. The double bond breaks, and the electrophile and a nucleophilic part of the reagent add to the carbon atoms, resulting in a saturated product. In contrast, nucleophilic substitution features an electron-rich species (nucleophile) attacking an electron-deficient carbon atom that is bonded to a good leaving group. The leaving group departs, and the nucleophile forms a new bond, replacing the leaving group.

亲电加成涉及缺电子物种（亲电试剂）进攻高电子密度区域，通常是烯烃的π键。双键断裂，亲电试剂与试剂中的亲核部分分别加到碳原子上，得到饱和产物。相反，亲核取代的特征是富电子物种（亲核试剂）进攻与好的离去基团相连的缺电子碳原子。离去基团离去，亲核试剂形成新键，取代离去基团。

Key to electrophilic addition is the concept of a carbocation intermediate (in many cases), whereas nucleophilic substitution can proceed via two distinct pathways: SN₁ (unimolecular, forming a carbocation) or SN₂ (bimolecular, concerted). This leads to different stereochemical outcomes.

亲电加成的关键概念是碳正离子中间体（在许多情况下），而亲核取代可以通过两种不同路径进行：SN₁（单分子，生成碳正离子）或SN₂（双分子，协同）。这导致了不同的立体化学结果。

3. Types of Reactants Involved | 反应物类型

Electrophilic addition reactions require an unsaturated substrate (alkene or alkyne) and an electrophilic reagent. Common electrophiles include H⁺ (from acids like HBr), Br₂ (polarised by the alkene), H₂SO₄, and interhalogens like ICl. The π‑electrons of the double bond act as the nucleophilic centre.

亲电加成反应需要不饱和底物（烯烃或炔烃）和亲电试剂。常见亲电试剂包括H⁺（来自HBr等酸），Br₂（被烯烃极化），H₂SO₄，以及ICl等卤间化合物。双键的π电子作为亲核中心。

Nucleophilic substitution requires a saturated substrate with a polar carbon–leaving group bond, such as halogenoalkanes, alcohols (after protonation), or acyl chlorides. Nucleophiles are electron-rich species like OH⁻, CN⁻, NH₃, and H₂O, which can donate a pair of electrons. The leaving group is typically a halide ion (Cl⁻, Br⁻, I⁻) or water.

亲核取代需要含有极性碳-离去基团键的饱和底物，如卤代烷、醇（质子化后）或酰氯。亲核试剂是富电子物种，如OH⁻、CN⁻、NH₃和H₂O，能提供一对电子。离去基团通常是卤离子（Cl⁻、Br⁻、I⁻）或水。

4. Mechanism: Step-by-Step Comparison | 机理分步对比

Electrophilic addition of HBr to ethene: Step 1 – The π‑electrons attack the partially positive hydrogen of HBr, forming a C–H bond and generating a carbocation (more stable secondary carbocation via Markovnikov’s rule) while bromide ion is released. Step 2 – The bromide ion acts as a nucleophile and rapidly attacks the carbocation to form bromoethane. This is a two-step ionic mechanism.

HBr与乙烯的亲电加成：第一步 – π电子进攻HBr中部分正电的氢，形成C–H键并生成碳正离子（根据马氏规则形成更稳定的二级碳正离子），同时释放溴离子。第二步 – 溴离子作为亲核试剂快速进攻碳正离子，生成溴乙烷。这是一个两步离子机理。

Nucleophilic substitution: For primary halogenoalkanes, the SN₂ mechanism dominates. The nucleophile attacks the carbon from the opposite side of the leaving group in a concerted process, forming a transition state. The leaving group departs simultaneously, leading to inversion of configuration. For tertiary halogenoalkanes, SN₁ is favoured: first, the leaving group departs to form a planar carbocation; then the nucleophile attacks from either side, leading to racemisation.

亲核取代：对于伯卤代烷，SN₂机理占主导。亲核试剂从离去基团的背面进攻碳原子，经过一个协同的过渡态，离去基团同时离去，导致构型翻转。对于叔卤代烷，更倾向于SN₁机理：首先，离去基团离去形成平面碳正离子；然后亲核试剂从任一侧进攻，导致外消旋化。

CH₂=CH₂ + HBr → CH₃CH₂Br

CH₃CH₂Br + NaOH → CH₃CH₂OH + NaBr (SN₂)

5. Role of Reagent: Electrophile vs Nucleophile | 试剂角色：亲电试剂与亲核试剂

An electrophile is an electron-pair acceptor. It must have a positive charge or a partial positive charge to attract the π‑electrons. In addition reactions, the electrophile is often generated by polarisation (e.g., Br₂ is polarised when approaching the alkene, forming Brᵟ⁺–Brᵟ⁻). A nucleophile is an electron-pair donor. It must have a lone pair or negative charge to attack an electron-deficient carbon. The strength of a nucleophile depends on charge, electronegativity, and steric hindrance.

亲电试剂是电子对接受体。它必须带有正电荷或部分正电荷以吸引π电子。在加成反应中，亲电试剂常通过极化产生（例如，Br₂在靠近烯烃时极化形成Brᵟ⁺–Brᵟ⁻）。亲核试剂是电子对给予体。它必须具有孤对电子或负电荷以攻击缺电子碳。亲核试剂的强度取决于电荷、电负性和空间位阻。

In electrophilic addition, the π‑bond itself is a nucleophile, while the reagent provides the electrophile. In nucleophilic substitution, the substrate carbon is electrophilic due to the polar bond, and the reagent is the nucleophile.

在亲电加成中，π键本身是亲核试剂，而试剂提供亲电试剂。在亲核取代中，由于极性键，底物碳具有亲电性，试剂是亲核试剂。

6. Reaction Conditions and Solvents | 反应条件与溶剂

Electrophilic additions are often carried out at room temperature, with the alkene mixed with the reagent. For

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