Nucleophilic Substitution in Halogenoalkanes | 卤代烷的亲核取代机理

📚 Nucleophilic Substitution in Halogenoalkanes | 卤代烷的亲核取代机理

Halogenoalkanes are organic compounds in which one or more hydrogen atoms of an alkane have been replaced by halogen atoms. They are important because the polar C–X bond makes them reactive towards nucleophiles, leading to substitution or elimination. In this article, we focus on nucleophilic substitution, comparing the two principal mechanisms: SN1 and SN2.

卤代烷是烷烃中一个或多个氢原子被卤素原子取代后形成的有机化合物。由于C–X键具有极性，它们对亲核试剂非常活泼，可发生取代或消除反应。本文重点讨论亲核取代，比较两种主要机理：SN1与SN2。

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1. What are halogenoalkanes? | 什么是卤代烷？

Halogenoalkanes contain at least one halogen atom (F, Cl, Br or I) bonded to an sp³ hybridised carbon atom. They are classified as primary, secondary or tertiary according to the number of alkyl groups attached to the carbon bearing the halogen.

卤代烷中至少有一个卤素原子（F、Cl、Br或I）与sp³杂化碳原子相连。根据与卤素相连的碳原子所连的烷基数，卤代烷可分为伯、仲、叔三种类型。

A primary halogenoalkane has the form RCH₂X, a secondary one has R₂CHX, and a tertiary one has R₃CX, where R represents an alkyl group and X represents a halogen atom.

伯卤代烷的形式为RCH₂X，仲卤代烷为R₂CHX，叔卤代烷为R₃CX，其中R代表烷基，X代表卤素原子。

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2. The polar carbon–halogen bond | 碳-卤极性键

Halogens are more electronegative than carbon, so the electron pair in the C–X bond is pulled towards the halogen. This produces a polar bond with the carbon atom carrying a partial positive charge (δ⁺) and the halogen carrying a partial negative charge (δ⁻).

卤素的电负性大于碳，因此C–X键中的电子对会被拉向卤素。这样形成的极性键使碳原子带部分正电荷（δ⁺），卤素带部分负电荷（δ⁻）。

The electron-deficient carbon is therefore susceptible to attack by electron-rich species called nucleophiles. The polarity of the bond is the key to understanding why halogenoalkanes undergo nucleophilic substitution so readily.

这个缺电子的碳原子因此容易受到富电子物种——亲核试剂的进攻。C–X键的极性是理解卤代烷为何容易发生亲核取代的关键。

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3. Nucleophiles: electron-pair donors | 亲核试剂：电子对给体

A nucleophile is an electron-pair donor that is attracted to positive or partially positive centres. Common nucleophiles in A-Level chemistry include OH⁻, CN⁻, NH₃ and H₂O.

亲核试剂是电子对给体，会被正电荷或部分正电荷中心吸引。A-Level化学中常见的亲核试剂包括OH⁻、CN⁻、NH₃和H₂O。

Nucleophiles usually have a lone pair of electrons or a negative charge. In a substitution reaction, the nucleophile donates its electron pair to form a new covalent bond with the electron-poor carbon atom.

亲核试剂通常带有孤对电子或负电荷。在取代反应中，亲核试剂提供电子对，与缺电子的碳原子形成新的共价键。

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4. General mechanism and leaving group | 一般机理与离去基团

In nucleophilic substitution, the nucleophile attacks the δ⁺ carbon atom by donating its lone pair. At the same time, the C–X bond breaks heterolytically, meaning the bonding pair of electrons moves onto the halogen atom, producing a halide ion, X⁻.

在亲核取代中，亲核试剂通过提供孤对电子进攻δ⁺碳原子。与此同时，C–X键发生异裂，成键电子对转移到卤素原子上，生成卤离子X⁻。

The halide ion is called the leaving group. The ability of a halide to leave follows the order I⁻ > Br⁻ > Cl⁻ > F⁻. This is because the C–I bond is weakest and breaks most easily, while the C–F bond is strongest and breaks least easily.

卤离子称为离去基团。卤离子离去能力的顺序为I⁻ > Br⁻ > Cl⁻ > F⁻。这是因为C–I键最弱、最容易断裂，而C–F键最强、最难断裂。

Nu⁻ + R–X → R–Nu + X⁻

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5. SN2 mechanism: bimolecular substitution | SN2机理：双分子取代

SN2 stands for substitution nucleophilic bimolecular. It occurs in a single step: the nucleophile attacks from the side opposite the leaving group, while the C–X bond breaks simultaneously. A single transition state is formed in which the carbon is partially bonded to both the nucleophile and the leaving group.

SN2表示双分子亲核取代。反应一步完成：亲核试剂从离去基团的反面进攻，同时C–X键断裂。反应中形成一个单一的过渡态，碳原子在其中与亲核试剂和离去基团都部分成键。

The transition state can be represented as [Nu···C···X]‡, where the dotted lines show partial bonds. The rate equation is second order overall because both the halogenoalkane and the nucleophile appear in the rate expression.

过渡态可表示为[Nu···C···X]‡，虚线表示部分成键。速率方程的总级数为二级，因为卤代烷和亲核试剂都出现在速率表达式中。

rate = k[R–X][Nu⁻]

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6. SN2 stereochemistry: inversion | SN2立体化学：构型翻转

Because the nucleophile enters from the back side and the leaving group departs from the front, the three remaining groups on carbon are forced to invert their arrangement. This is often called Walden inversion, and it is similar to an umbrella turning inside out.

由于亲核试剂从背面进攻，离去基团从正面离去，碳原子上其余三个基团的排布被迫发生翻转。这一过程常称为瓦尔登翻转，就像雨伞被风吹翻过来一样。

For example, if a single enantiomer of a secondary halogenoalkane undergoes SN2, the product has the opposite configuration at the carbon centre.

例如，如果仲卤代烷的单一对映异构体发生SN2反应，产物在碳中心的手性构型会发生反转。

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7. SN1 mechanism: unimolecular substitution | SN1机理：单分子取代

SN1 stands for substitution nucleophilic unimolecular. It occurs in two steps: first, the C–X bond breaks heterolytically to form a carbocation and a halide ion. This slow step is the rate-determining step.

SN1表示单分子亲核取代。反应分两步进行：首先C–X键异裂，生成碳正离子和卤离子。这个慢步骤是决速步骤。

R–X → R⁺ + X⁻   (slow)

In the second step, the carbocation reacts rapidly with the nucleophile to form the product.

第二步中，碳正离子与亲核试剂快速反应生成产物。

R⁺ + Nu⁻ → R–Nu   (fast)

The rate equation is first order overall because only the halogenoalkane appears in the rate expression.

速率方程的总级数为一级，因为速率表达式中只出现卤代烷。

rate = k[R–X]

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8. SN1 stereochemistry: racemisation | SN1立体化学：外消旋化

The carbocation intermediate is planar, with the positive carbon having an empty p orbital. The nucleophile can attack from either side of this plane with equal probability.

碳正离子中间体是平面型的，带正电荷的碳原子具有空的p轨道。亲核试剂可以从该平面的两侧以相等的概率进攻。

If the original halogenoalkane is a single enantiomer, the two possible attack routes lead to a mixture of both enantiomers of the product, usually in equal amounts. This mixture is called a racemic mixture or racemate.

如果起始卤代烷是单一对映异构体，两条进攻路径会生成两种对映异构体的混合物，通常为等量。这种混合物称为外消旋混合物或外消旋体。

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9. Comparing SN1 and SN2 | SN1与SN2比较

The two mechanisms differ in molecularity, number of steps, rate law, stereochemistry and preferred substrate. The table summarises the main contrasts.

两种机理在分子数、步骤数、速率方程、立体化学和偏好底物方面均不同。下表总结了主要区别。

Feature | 特点	SN1	SN2
Steps | 步骤	Two | 两步	One | 一步
Rate equation | 速率方程	rate = k[R–X] | 一级	rate = k[R–X][Nu⁻] | 二级
Preferred substrate | 偏好底物	Tertiary > secondary > primary | 叔 > 仲 > 伯	Primary > secondary > tertiary | 伯 > 仲 > 叔
Stereochemistry | 立体化学	Racemisation | 外消旋化	Inversion | 翻转

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10. Factors affecting mechanism | 影响机理的因素

The structure of the halogenoalkane has a major effect on the mechanism. Tertiary halogenoalkanes favour SN1 because they can form relatively stable tertiary carbocations. Primary halogenoalkanes favour SN2 because the carbon centre is less sterically hindered and more accessible to back-side attack.

卤代烷的结构对机理有重要影响。叔卤代烷倾向于SN1，因为它们能形成相对稳定的叔碳正离子。伯卤代烷倾向于SN2，因为其碳中心位阻较小，更容易发生背面进攻。

The strength of the nucleophile also matters. Strong nucleophiles such as OH⁻ and CN⁻ favour SN2, whereas weak nucleophiles such as H₂O often react by SN1 because the rate-determining step does not involve the nucleophile.

亲核试剂的强度也很关键。OH⁻和CN⁻等强亲核试剂有利于SN2；而H₂O等弱亲核试剂往往通过SN1反应，因为决速步骤不涉及亲核试剂。

Good leaving groups speed up both mechanisms by weakening the C–X bond, but SN1 is especially dependent on the leaving group because C–X bond breaking is the rate-determining step.

好的离去基团通过削弱C–X键加速两种机理，但SN1对离去基团的依赖性特别强，因为C–X键断裂是决速步骤。

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11. Common reactions: hydrolysis, cyanide, ammonia | 常见反应：水解、氰化物、氨

Reaction with aqueous hydroxide ions is called hydrolysis. Warming a halogenoalkane with aqueous NaOH under reflux produces an alcohol and a halide ion.

与氢氧根离子的反应称为水解。将卤代烷与NaOH水溶液在回流条件下加热

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