📚 Halogenoalkanes | 卤代烷
Halogenoalkanes are a class of organic compounds in which one or more hydrogen atoms in an alkane have been replaced by halogen atoms (fluorine, chlorine, bromine or iodine). They are pivotal in A-Level Chemistry because their reactions illustrate fundamental concepts of nucleophilic substitution and elimination, and they serve as intermediates in many synthetic pathways. Understanding the mechanisms, reactivity trends and environmental impact of halogenoalkanes is essential for mastering organic chemistry.
卤代烷是一类有机化合物，其中烷烃中的一个或多个氢原子被卤素原子（氟、氯、溴或碘）取代。它们在A-Level化学中至关重要，因为它们的反应揭示了亲核取代和消除反应的基本概念，并且是许多合成路线中的关键中间体。理解卤代烷的机理、活性规律和环境影响是掌握有机化学的必要条件。
1. Introduction and Classification | 引言与分类
Halogenoalkanes have the general formula R-X, where R is an alkyl group and X is a halogen (F, Cl, Br, I). They are classified as primary (1°), secondary (2°) or tertiary (3°) based on the number of alkyl groups attached to the carbon atom that is bonded to the halogen. In a primary halogenoalkane, the carbon bearing the halogen is attached to only one other carbon (e.g., CH₃CH₂Br). In a secondary halogenoalkane, it is attached to two carbons (e.g., (CH₃)₂CHCl), and in a tertiary halogenoalkane, it is attached to three carbons (e.g., (CH₃)₃CBr).
卤代烷通式为R-X，其中R是烷基，X是卤素（F、Cl、Br、I）。根据与卤素相连的碳原子所连烷基的数目，它们可分为伯（1°）、仲（2°）和叔（3°）卤代烷。伯卤代烷中，带卤素的碳只与一个其他碳相连（如CH₃CH₂Br）；仲卤代烷中，该碳与两个碳相连（如(CH₃)₂CHCl）；叔卤代烷中，该碳与三个碳相连（如(CH₃)₃CBr）。
This classification is crucial because it determines the mechanism of nucleophilic substitution and the reactivity of the halogenoalkane.
这种分类至关重要，因为它决定了亲核取代反应的机理和卤代烷的反应活性。
2. Nomenclature | 命名法
In IUPAC nomenclature, the longest continuous carbon chain is selected, and the halogen is treated as a substituent with the prefix fluoro-, chloro-, bromo- or iodo-. The chain is numbered to give the carbon bearing the halogen the lowest possible number. If there are multiple halogens, prefixes such as di-, tri- are used and halogens are listed alphabetically. For example, CH₃CH(Cl)CH₃ is 2-chloropropane, and CH₂BrCH₂CH₂I is 1-bromo-3-iodopropane.
在IUPAC命名法中，选择最长碳链，卤素作为取代基，使用前缀氟代、氯代、溴代或碘代。给碳链编号，使连有卤素的碳位次尽可能小。若有多个卤素，则用二、三等前缀，并按字母顺序列出。例如，CH₃CH(Cl)CH₃的名称为2-氯丙烷，CH₂BrCH₂CH₂I的名称为1-溴-3-碘丙烷。
Common names are sometimes used but the systematic nomenclature is preferred for exams.
有时使用普通名称，但考试中首选系统命名法。
3. Physical Properties | 物理性质
Halogenoalkanes are generally colourless liquids or solids with characteristic odours. Their boiling points are higher than those of the corresponding alkanes because of increased molecular mass and the polarity of the carbon–halogen bond, which leads to stronger van der Waals forces and dipole–dipole interactions. For a given alkyl group, the boiling point increases from fluoro- to iodo-compounds due to the increase in size and polarisability of the halogen. Branching lowers the boiling point because it reduces the surface area available for intermolecular forces.
卤代烷通常为具有特殊气味的无色液体或固体。由于相对分子质量增加以及碳-卤键的极性，其沸点高于相应的烷烃，因为产生了更强的范德华力和偶极-偶极作用。对于给定烷基，沸点从氟代物到碘代物递增，因为卤素原子大小和极化率增大。支链会降低沸点，因为它减小了分子间接触的表面积。
Halogenoalkanes are almost insoluble in water but are soluble in organic solvents. The carbon–halogen bond is polar, with the carbon bearing a partial positive charge (δ+) and the halogen a partial negative charge (δ–), making it susceptible to attack by nucleophiles.
卤代烷几乎不溶于水，但可溶于有机溶剂。碳-卤键具有极性，碳带部分正电荷（δ+），卤素带部分负电荷（
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