📚 Mechanism of nucleophilic substitution in halogenoalkanes | 卤代烷的亲核取代反应机理
Halogenoalkanes (also known as haloalkanes or alkyl halides) are organic compounds where one or more hydrogen atoms in an alkane have been replaced by halogen atoms. The carbon–halogen bond is polar, making halogenoalkanes susceptible to attack by nucleophiles. This article explores the mechanisms of nucleophilic substitution reactions (SN1 and SN2) in detail, including stereochemical outcomes and factors affecting the reaction pathway.
卤代烷（又称卤代烃或烷基卤化物）是烷烃分子中的一个或多个氢原子被卤素原子取代后形成的有机化合物。碳-卤键具有极性，使得卤代烷容易受到亲核试剂的进攻。本文详细探讨亲核取代反应（SN1 和 SN2）的机理，包括立体化学结果和影响反应途径的因素。
1. The Polar Carbon–Halogen Bond | 极性的碳-卤键
In a halogenoalkane, the carbon atom bonded to the halogen carries a partial positive charge (δ+) due to the greater electronegativity of the halogen. This polar bond is prone to heterolytic fission, where the bonding electrons are taken away by the halogen, forming a halide ion.
在卤代烷中，由于卤素原子的电负性较高，与卤素相连的碳原子带有部分正电荷（δ+）。这种极性键容易发生异裂，此时成键电子对被卤素带走，形成卤离子。
The polarity of the bond makes the carbon atom electrophilic and susceptible to attack by nucleophiles – species that are electron-rich and can donate a pair of electrons. Common nucleophiles include OH⁻, CN⁻, NH₃ and H₂O.
该键的极性使碳原子具有亲电性，容易受到亲核试剂的进攻——亲核试剂是富含电子、能够提供电子对的物种。常见的亲核试剂包括 OH⁻、CN⁻、NH₃ 和 H₂O。
2. Overview of Nucleophilic Substitution | 亲核取代反应概述
ficient carbon, leading to the replacement of the halogen atom. The general equation is: R–X + Nu⁻ → R–Nu + X⁻, where X is a halogen and Nu is the nucleophile.
当亲核试剂提供电子对给缺电子的碳原子时，即发生亲核取代，卤素原子被取代。一般反应式为：R–X + Nu⁻ → R–Nu + X⁻，其中 X 是卤素，Nu 是亲核试剂。
Two distinct mechanisms exist: SN2 (bimolecular nucleophilic substitution) and SN1 (unimolecular nucleophilic substitution). The choice of mechanism depends on the structure of the halogenoalkane, the nature of the nucleophile and the solvent.
存在两种不同的机理：SN2（双分子亲核取代）和 SN1（单分子亲核取代）。究竟按哪种机理进行取决于卤代烷的结构、亲核试剂的性质以及溶剂。
3. The SN2 Mechanism – Bimolecular Substitution | SN2 机理 – 双分子取代
The SN2 mechanism is a concerted process: bond formation and bond breaking occur simultaneously in a single step. The nucleophile attacks the carbon from the opposite side of the leaving group (backside attack), pushing off the halogen as a halide ion.
SN2 机理是一个协同过程：键的生成和键的断裂在同一基元步骤中同时发生。亲核试剂从离去基团的背面进攻碳原子，将卤素以卤离子形式推离。
A transition state is formed in which the carbon atom is pentacoordinate – he departing halogen. The rate law is second-order: rate = k[halogenoalkane][nucleophile].
反应形成一个五配位碳的过渡态，碳原子同时与进入的亲核试剂和离去的卤素部分成键。其速率定律为二级：速率 = k[卤代烷][亲核试剂]。
CH₃CH₂OH + Br⁻. The reaction proceeds via a single transition state with no intermediate.
例如溴乙烷与氢氧根离子的水解反应：CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻。反应通过单一过渡态进行，没有中间体。
4. Stereochemistry of SN2 – Walden Inversion | SN2 的立体化学 – 瓦尔登翻转
SN2 reactions at a chiral carbon proceed with complete inversion of configuration. Because the nucleophile attacks from the backside, the tetrahedral arrangement is 'umbrella-flipped'. This change is known as Walden inversion.
在手性碳上进行的 SN2 反应会伴随构型的完全翻转。由于亲核试剂从背面进攻，四面体排布会发生“伞形翻转”，这一过程被称为瓦尔登翻转。
If the reactant has an R configuration, the product will have an S configuration (provided the priority order is unchanged). For instance, (R)-2-bromobutane when treated with OH⁻ yields (S)-butan-2-ol via SN2.
如果反应物为 R 构型，则产物为 S 构型（前提是优先次序不变）。例如，(R)-2-溴丁烷与 OH⁻ 反应通过 SN2 得到 (S)-丁-2-醇。
5. Factors Affecting SN2 Reactions | 影响 SN2 反应的因素
Structure of the halogenoalkane: Steric hindrance is the key factor. Primary halogenoalkanes react fastest in SN2, followed by secondary; tertiary halogenoalkanes are virtually unreactive because the bulky alkyl groups block the backside approach.
卤代烷的结构：位阻是关键因素。伯卤代烷在 SN2 中反应最快，其次为仲卤代烷；叔卤代烷由于庞大的烷基阻碍了背面进攻，几乎不发生反应。
Leaving group ability: A good leaving group is a weak base that can stabilise the negative charge. The order for halogens is I⁻ > Br⁻ > Cl⁻ > F⁻. Iodine is the best leaving group in this series.
离去基团的能力：好的离去基团是能稳定负电荷的弱碱。卤素离子的顺序为 I⁻ > Br⁻ > Cl⁻ > F⁻。碘是该系列中最优的离去基团。
Nucleophile strength: Strong, concentrated nucleophiles favour SN2. OH⁻ and CN⁻ are stronger than H₂O; a negatively charged nucleophile is generally more reactive.
亲核试剂强度：强而浓的亲核试剂有利于 SN2。OH⁻ 和 CN⁻ 的强度大于 H₂O；带负电的亲核试剂通常反应性更高。
Solvent effects: Polar aprotic solvents (e.g. propanone, CH₃COCH₃) enhance SN2 by solvating the cation but leaving the nucleophile relatively unsolvated and thus more reactive.
溶剂效应：极性非质子溶剂（如丙酮 CH₃COCH₃）可增强 SN2 反应，因为它们溶剂化阳离子而不溶剂化亲核试剂，使其更活泼。
6. The SN1 Mechanism – Unimolecular Substitution | SN1 机理 – 单分子取代
The SN1 mechanism proceeds in two steps. First, the carbon–halogen bond breaks heterolytically to form a planar carbocation intermediate and a halide ion. This is the slow, rate-determining step.
SN1 机理分两步进行。首先，碳-卤键异裂，生成平面形的碳正离子中间体与卤离子。这是慢的速率控制步骤。
ate law depends only on the concentration of the halogenoalkane: rate = k[halogenoalkane].
在第二步快步骤中，亲核试剂从平面的任一侧进攻碳正离子，生成取代产物。速率定律仅取决于卤代烷的浓度：速率 = k[卤代烷]。
Br + OH⁻ → (CH₃)₃COH + Br⁻. The first step forms the stable tertiary carbocation (CH₃)₃C⁺.
例如 2-溴-2-甲基丙烷的水解：(CH₃)₃CBr + OH⁻ → (CH₃)₃COH + Br⁻。第一步生成稳定的叔碳正离子 (CH₃)₃C⁺。
7. Stereochemistry of SN1 – Racemisation | SN1 的立体化学 – 外消旋化
If the starting halogenoalkane is chiral at the reacting carbon, an SN1 reaction leads to racemisation – a mixture of both enantiomers. The planar carbocation can be attacked from either face with equal probability, producing both R and S products.
若起始卤代烷在反应碳上具有手性，SN1 反应会导致外消旋化——生成两种对映体的混合物。平面的碳正离子可从两个面以等概率被进攻，产生 R 和 S 产物。
In practice, a small excess of inversion may occur due to ion-pair effects, but the major outcome is loss of optical activity to give a racemic mixture.
实际上，由于离子对效应，可能会产生微过量的翻转构型，但主要结果是旋光性丧失，得到外消旋混合物。
8. Factors Affecting SN1 Reactions | 影响 SN1 反应的因素
Structure of the halogenoalkane: SN1 favours tertiary and secondary halogenoalkanes because they form more stable carbocations. The order of carbocation stability is tertiary > secondary > primary > methyl. Primary and methyl substrates rarely react via SN1.
卤代烷的结构：SN1 有利于叔卤代烷和仲卤代烷，因为它们能生成更稳定的碳正离子。碳正离子稳定性顺序为：叔 > 仲 > 伯 > 甲基。伯卤代烷和甲基卤代烷很少按 SN1 进行。
Leaving group ability: The same trend applies as in SN2 – a better leaving group (e.g. I⁻) accelerates the formation of the carbocation.
离去基团的能力：与 SN2 相同——更好的离去基团（如 I⁻）可加速碳正离子的生成。
Nucleophile: SN1 depends little on the nucleophile strength because the nucleophile is not involved in the rate-determining step. Even weak nucleophiles like H₂O suffice.
亲核试剂：SN1 对亲核试剂强度的依赖性很小，因为亲核试剂不参与速率控制步骤。即使像 H₂O 这样的弱亲核试剂也足够。
Solvent effects: Polar protic solvents (e.g. water, ethanol) stabilise both the carbocation and the leaving halide ion through solvation, greatly enhancing SN1 rates.
溶剂效应：极性质子溶剂（如水、乙醇）通过溶剂化作用稳定碳正离子和离去卤离子，大幅提高 SN1 速率。
9. Comparing SN1 and SN2 – A Summary | SN1 与 SN2 对比 – 总结
In SN2, the reaction is concerted and bimolecular; rate = k[halogenoalkane][nucleophile]. In SN ion proceeds via a carbocation intermediate and is unimolecular; rate = k[halogenoalkane].
在 SN2 中，反应是协同且双分子的，速率 = k[卤代烷][亲核试剂]。在 SN1 中，反应经过碳正离子中间体，是单分子的，速率 = k[卤代烷]。
SN2 leads to complete inversion of configuration, whereas SN1 gives racemisation. SN2 is favoured by primary substrates, strong nucleophiles and polar aprotic solvents. SN1 is favoured by tertiary substrates, weak nucleophiles and polar protic solvents.
SN2 导致完全的构型翻转，而 SN1 产生外消旋化。SN2 受伯卤代烷、强亲核试剂和极性非质子溶剂促进；SN1 受叔卤代烷、弱亲核试剂和极性质子溶剂促进。
A reaction profile for SN2 shows a single transition state, while SN1 has two steps with a carbocation intermediate in an energy well between two transition states.
SN2 的反应进程图中只有一个过渡态，而 SN1 有两个步骤，碳正离子中间体处于两个过渡态之间的能量低谷中。
10. Key Examples and Applications | 关键实例与应用
Hydrolysis of halogenoalkanes: Warming a halogenoalkane with aqueous sodium hydroxide yields an alcohol. Primary halogenoalkanes follow SN2; tertiary halogenoalkanes follow SN1. E.g. CH₃CH₂CH₂Br + NaOH(aq) → CH₃CH₂CH₂OH + NaBr.
卤代烷的水解：将卤代烷与氢氧化钠水溶液加热可得到醇。伯卤代烷按 SN2 进行；叔卤代烷按 SN1 进行。例如 CH₃CH₂CH₂Br + NaOH(aq) → CH₃CH₂CH₂OH + NaBr。
Reaction with cyanide ions: Heating a halogenoalkane with potassium cyanide in ethanol extends the carbon chain by one atom, forming a nitrile. This is typically SN2 for primary substrates. CH₃CH₂Br + CN⁻ → CH₃CH₂CN + Br⁻.
与氰根离子的反应：卤代烷与氰化钾的乙醇溶液加热反应，碳链增长一个碳原子，生成腈。对于伯卤代烷通常按 SN2 进行。CH₃CH₂Br + CN⁻ → CH₃CH₂CN + Br⁻。
Reaction with ammonia: Excess concentrated ammonia in ethanol under pressure converts halogenoalkanes into amines. Primary halogenoalkanes react via SN primary amine, though further substitution can occur. CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br.
与氨的反应：在压力下用乙醇中过量的浓氨可将卤代烷转化为胺。伯卤代烷通过 SN2 反应生成伯胺，但可能发生进一步取代。CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br。
These reactions illustrate how the choice of nucleophile and conditions can control the functional group transformation, a central theme in synthetic organic chemistry.
这些反应说明了如何通过选择亲核试剂和条件来控制官能团转化，这是有机合成化学的一个核心主题。
Published by TutorHao | Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图