Halogenoalkanes | 卤代烷

📚 Halogenoalkanes | 卤代烷

Halogenoalkanes are a family of organic compounds in which one or more hydrogen atoms of an alkane have been replaced by halogen atoms such as fluorine, chlorine, bromine or iodine. They are highly useful intermediates in organic synthesis and are a central topic in Cambridge A Level Chemistry because their reactions illustrate key ideas about bond polarity, nucleophiles, mechanisms and structure-reactivity relationships.

卤代烷是一类有机化合物，其中烷烃的一个或多个氢原子被氟、氯、溴或碘等卤素原子取代。它们是有机合成中非常有用的中间体，也是剑桥 A Level 化学的核心主题，因为它们的反应可以很好地说明键的极性、亲核试剂、反应机理以及结构与反应活性之间的关系。

1. Definition and General Formula | 定义与通式

A halogenoalkane, also called a haloalkane, can be represented by the general formula R-X, where R is an alkyl group and X is a halogen atom. Monohalogenoalkanes, with one halogen atom per molecule, have the general formula CₙH₂ₙ₊₁X.

卤代烷，也称为卤代烃，可以用通式 R-X 表示，其中 R 是烷基，X 是卤素原子。每分子含一个卤素原子的一卤代烷通式为 CₙH₂ₙ₊₁X。

Common examples include CH₃Cl chloromethane, CH₃CH₂Br bromoethane and CH₃CHICH₃ 2-iodopropane. The carbon atom bonded to the halogen is often labelled as the α-carbon in reaction mechanisms.

常见的例子包括 CH₃Cl 氯甲烷、CH₃CH₂Br 溴乙烷和 CH₃CHICH₃ 2-碘丙烷。与卤素键合的碳原子在反应机理中常被标记为 α-碳。

2. Classification: Primary, Secondary and Tertiary | 分类：伯、仲、叔卤代烷

Halogenoalkanes are classified according to the number of alkyl groups attached to the carbon atom that carries the halogen. A primary halogenoalkane has the structure RCH₂X, a secondary has R₂CHX and a tertiary has R₃CX.

卤代烷根据与卤素相连的碳原子上所连烷基数目进行分类。伯卤代烷的结构为 RCH₂X，仲卤代烷为 R₂CHX，叔卤代烷为 R₃CX。

Type	Structure	Example
Primary	RCH₂X	CH₃CH₂Br
Secondary	R₂CHX	CH₃CHBrCH₃
Tertiary	R₃CX	(CH₃)₃CBr

This classification is important because primary, secondary and tertiary halogenoalkanes often follow different reaction mechanisms and show very different rates in substitution and elimination reactions.

这种分类非常重要，因为伯、仲、叔卤代烷往往遵循不同的反应机理，并且在取代反应和消除反应中表现出差异很大的反应速率。

3. Physical Properties and Bond Polarity | 物理性质与键的极性

The carbon-halogen bond is polar because halogen atoms are more electronegative than carbon. The carbon atom carries a partial positive charge δ⁺ and the halogen carries a partial negative charge δ⁻, making the carbon atom susceptible to attack by nucleophiles.

碳-卤键具有极性，因为卤素原子的电负性大于碳。碳原子带部分正电荷 δ⁺，卤素带部分负电荷 δ⁻，这使碳原子容易受到亲核试剂的进攻。

Boiling points of halogenoalkanes increase with increasing chain length because larger molecules have stronger London dispersion forces. For a given alkyl group, boiling points increase down Group 17 from fluoroalkanes to iodoalkanes because the larger halogen atoms have more electrons and stronger van der Waals forces.

卤代烷的沸点随碳链增长而升高，因为分子越大，伦敦色散力越强。对于相同烷基，沸点从氟代烷到碘代烷沿第 17 族向下升高，因为较大的卤素原子有更多电子，范德华力更强。

Halogenoalkanes are only slightly soluble in water but dissolve readily in organic solvents. Their polarity is not sufficient to form strong hydrogen bonds with water, so they remain largely immiscible with aqueous solutions.

卤代烷在水中溶解度很小，但易溶于有机溶剂。它们的极性不足以与水形成强氢键，因此与水溶液基本不相溶。

4. Nucleophilic Substitution: The Key Reaction | 亲核取代：核心反应

A nucleophile is an electron-pair donor that is attracted to positive or partially positive centres. Common nucleophiles include OH⁻, CN⁻, NH₃ and H₂O. In a nucleophilic substitution reaction, a nucleophile replaces the halogen atom in a halogenoalkane.

亲核试剂是电子对给体，被正电荷或部分正电荷中心吸引。常见的亲核试剂包括 OH⁻、CN⁻、NH₃ 和 H₂O。在亲核取代反应中，亲核试剂取代卤代烷中的卤素原子。

RX + Nu⁻ → RNu + X⁻

The carbon-halogen bond breaks heterolytically, meaning both bonding electrons go to the halogen, forming a halide ion. The nucleophile donates an electron pair to form a new covalent bond to carbon.

碳-卤键发生异裂，即成键电子对全部被卤素带走，生成卤离子。亲核试剂提供一对电子，与碳形成新的共价键。

5. SN2 Mechanism in Detail | SN2 机理详解

The SN2 mechanism is a one-step bimolecular process. The nucleophile attacks the δ⁺ carbon from the side opposite the halogen, forming a new bond while the carbon-halogen bond breaks at the same time.

SN2 机理是一步双分子过程。亲核试剂从卤素相反的侧面进攻 δ⁺ 碳，在碳-卤键断裂的同时形成新键。

CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻

The transition state has both the nucleophile and the leaving group partially bonded to carbon. Because the reaction is bimolecular, its rate depends on the concentrations of both the halogenoalkane and the nucleophile.

过渡态中亲核试剂和离去基团都与碳部分成键。由于该反应是双分子反应，其速率取决于卤代烷和亲核试剂两者的浓度。

Rate = k[halogenoalkane][Nu⁻]

SN2 reactions occur most readily with primary halogenoalkanes because the central carbon is less sterically hindered. If the carbon is chiral, an SN2 reaction leads to inversion of configuration.

SN2 反应最易发生在伯卤代烷中，因为中心碳的空间位阻较小。如果碳是手性碳，SN2 反应会导致构型翻转。

6. SN1 Mechanism in Detail | SN1 机理详解

The SN1 mechanism is a two-step unimolecular process. In the first, slow step the carbon-halogen bond breaks heterolytically to form a carbocation and a halide ion. In the second, fast step the nucleophile attacks the planar carbocation.

SN1 机理是两步单分子过程。第一步慢步骤中，碳-卤键异裂生成碳正离子和卤离子。第二步快步骤中，亲核试剂进攻平面型碳正离子。

Step 1: (CH₃)₃CBr → (CH₃)₃C⁺ + Br⁻

Step 2: (CH₃)₃C⁺ + OH⁻ → (CH₃)₃COH

Because the first step is rate-determining and involves only the halogenoalkane, the overall rate depends only on the concentration of the halogenoalkane.

由于第一步是决速步骤并且只涉及卤代烷，总反应速率只取决于卤代烷的浓度。

Rate = k[halogenoalkane]

SN1 reactions are favoured by tertiary halogenoalkanes because tertiary carbocations are stabilised by the positive inductive effect of three alkyl groups. The planar carbocation can be attacked from either side, so SN1 reactions often produce a racemic mixture if the starting material is chiral.

SN1 反应有利于叔卤代烷，因为叔碳正离子受三个烷基的正诱导效应稳定。平面型碳正离子可以从两侧被进攻，因此如果起始物是手性的，SN1 反应通常生成外消旋混合物。

7. Comparing SN1 and SN2 | SN1 与 SN2 的比较

Feature	SN1	SN2
Steps	Two	One
Rate equation	k[RX]	k[RX][Nu⁻]
Preferred substrate	Tertiary	Primary
Stereochemistry	Racemisation	Inversion
Intermediate	Carbocation	Transition state only

In general, primary halogenoalkanes react by SN2, tertiary halogenoalkanes react by SN1, and secondary halogenoalkanes can follow either pathway depending on the conditions and the strength of the nucleophile.

一般来说，伯卤代烷按 SN2 反应，叔卤代烷按 SN1 反应，而仲卤代烷可能根据条件和亲核试剂强度走任一路径。

8. Hydrolysis with Aqueous Alkali | 与碱水溶液的水解反应

When a halogenoalkane is heated under reflux with aqueous sodium hydroxide, the halogen is replaced by an OH group and an alcohol is formed. This is a nucleophilic substitution reaction and is often used to compare the reactivity of different halogenoalkanes.

卤代烷与氢氧化钠水溶液加热回流时，卤素被 OH 基取代，生成醇。这是一个亲核取代反应，常用于比较不同卤代烷的反应活性。

CH₃CH₂Br + NaOH(aq) → CH₃CH₂OH + NaBr

The rate of hydrolysis increases in the order primary < secondary < tertiary for a given halogen. This trend is explained by the increasing stability of carbocations in SN1 and by decreasing steric hindrance at the α-carbon as substitution increases for SN1 pathways.

对于同一种卤素，水解速率按伯 < 仲 < 叔顺序增大。该趋势可用 SN1 中碳正离子稳定性增加以及随取代程度增加 α-碳位阻变化来解释。

For a given alkyl group, the rate of hydrolysis increases in the order C-F < C-Cl < C-Br < C-I. This is because the carbon-halogen bond enthalpy decreases down the group, so the bond breaks more easily in iodoalkanes than in fluoroalkanes.

对于相同烷基，水解速率按 C-F < C-Cl < C-Br < C-I 顺序增大。这是因为碳-卤键的键焓沿卤素族向下减小，碘代烷中的键比氟代烷更容易断裂。

9. Reaction with Cyanide and Ammonia | 与氰化物和氨的反应

Heating a halogenoalkane with potassium cyanide in ethanol under reflux produces a nitrile. This is a useful way to extend the carbon chain by one carbon atom.

卤代烷与氰化钾在乙醇中加热回流生成腈。这是使碳链增加一个碳原子的有用方法。

CH₃CH₂Br + KCN → CH₃CH₂CN + KBr

Nitriles can be hydrolysed to carboxylic acids or reduced to amines, so this reaction is an important route in organic synthesis. The CN⁻ ion acts as a nucleophile and attacks the δ⁺ carbon of the halogenoalkane.

腈可以水解生成羧酸或还原生成胺，因此该反应是有机合成中的重要路线。CN⁻ 离子作为亲核试剂进攻卤代烷的 δ⁺ 碳。

When a halogenoalkane is heated with excess ammonia in ethanol in a sealed tube, a primary amine is formed. The ammonia molecule acts as a nucleophile and replaces the halogen.

当卤代烷与过量的乙醇氨溶液在密封管中加热时，生成伯胺。氨分子作为亲核试剂取代卤素。

CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br

An excess of ammonia is used to reduce the chance of the primary amine reacting further to form secondary and tertiary amines.

使用过量氨是为了降低伯胺继续反应生成仲胺和叔胺的可能性。

10. Elimination Reactions to Form Alkenes | 消除反应生成烯烃

When a halogenoalkane is heated with hot ethanolic potassium hydroxide, an elimination reaction occurs and an alkene is formed. The hydroxide ion acts as a base, removing a hydrogen atom from a carbon adjacent to the carbon carrying the halogen.

当卤代烷与热的乙醇氢氧化钾溶液加热时，发生消除反应生成烯烃。氢氧根离子作为碱，从与卤素相邻的碳上夺取一个氢原子。

CH₃CH₂CH₂Br + KOH(ethanolic, heat) → CH₃CH=CH₂ + KBr + H₂O

The conditions determine whether substitution or elimination occurs. Aqueous NaOH at moderate temperature favours substitution to form an alcohol, while hot ethanolic KOH favours elimination to form an alkene.

反应条件决定是发生取代还是消除。中等温度下的 NaOH 水溶液有利于取代生成醇，而热的乙醇 KOH 溶液有利于消除生成烯烃。

When elimination can produce more than one alkene, the major product is usually the more substituted alkene. This is known as Saytzeff’s rule. For example, 2-bromobutane gives but-2-ene as the major product and but-1-ene as the minor product.

当消除反应可能生成一种以上烯烃时，主要产物通常是取代程度更高的烯烃。这称为扎伊采夫规则。例如 2-溴丁烷消除时主要生成 2-丁烯，少量生成 1-丁烯。

11. CFCs and Ozone Depletion | 氯氟烃与臭氧层破坏

Chlorofluorocarbons, or CFCs, are halogenoalkanes in which all the hydrogen atoms have been replaced by chlorine and fluorine atoms. They were widely used as refrigerants, propellants and solvents because they are very stable and non-toxic.

氯氟烃（CFC）是氢原子全部被氯和氟取代的卤代烷。它们曾被广泛用作制冷剂、推进剂和溶剂，因为它们非常稳定且无毒。

In the stratosphere, ultraviolet radiation breaks a C-Cl bond in a CFC molecule to form chlorine radicals. These radicals catalyse the breakdown of ozone, which protects the Earth from harmful UV radiation.

在平流层中，紫外线使 CFC 分子中的 C-Cl 键断裂生成氯自由基。这些自由基催化臭氧分解，而臭氧可以保护地球免受有害紫外线的伤害。

CFCl₃ → CFCl₂• + Cl•

Cl• + O₃ → ClO• + O₂

ClO• + O → Cl• + O₂

The overall reaction is O₃ + O → 2O₂, and the chlorine radical is regenerated, so one chlorine radical can destroy many ozone molecules. This is why CFCs have been largely phased out under the Montreal Protocol.

总反应为 O₃ + O → 2O₂，氯自由基被再生，因此一个氯自由基可以破坏大量臭氧分子。这就是 CFC 根据《蒙特利尔议定书》被大量淘汰的原因。

12. Summary and Exam Tips | 总结与考试提示

Halogenoalkanes undergo two main types of reaction: nucleophilic substitution and elimination. Substitution with aqueous NaOH gives alcohols, while elimination with hot ethanolic KOH gives alkenes.

卤代烷主要发生两类反应：亲核取代和消除。与 NaOH 水溶液发生取代生成醇，与热乙醇 KOH 发生消除生成烯烃。

Be clear about the difference between SN1 and SN2: SN1 is unimolecular, forms a carbocation and favours tertiary halogenoalkanes, while SN2 is bimolecular, occurs in one step and favours primary halogenoalkanes.

要清楚区分 SN1 和 SN2：SN1 是单分子反应，生成碳正离子，有利于叔卤代烷；SN2 是双分子反应，一步完成，有利于伯卤代烷。

When writing mechanisms, always show the movement of electron pairs with curly arrows and indicate the partial charges on the polar carbon-halogen bond. Pay close attention to reaction conditions, as they decide which product is formed.

书写机理时，要始终用弯箭头表示电子对的移动，并标明极性碳-卤键上的部分电荷。要特别注意反应条件，因为条件决定生成哪种产物。

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Halogenoalkanes | 卤代烷