📚 3.3 Halogenoalkanes: Nucleophilic Substitution, Elimination and Ozone Depletion | 卤代烷：亲核取代、消去反应与臭氧消耗
Halogenoalkanes are a core AQA A-level topic because they connect organic naming, polarity, reaction mechanisms and environmental chemistry. They contain at least one carbon-halogen bond, which makes the carbon atom susceptible to attack by nucleophiles and bases.
卤代烷是AQA A-level化学的核心主题之一，因为它串联了有机命名、极性、反应机理和环境化学。它们含有至少一个碳-卤键，使碳原子容易受到亲核试剂和碱的进攻。
1. What Are Halogenoalkanes? | 什么是卤代烷
r a single halogen substituent, the general formula is CₙH₂ₙ₊₁X, where X = F, Cl, Br or I.
卤代烷是烷烃中一个或多个氢原子被卤素原子取代后的化合物。对于单卤代物，其通式为 CₙH₂ₙ₊₁X，其中 X = F、Cl、Br 或 I。
The carbon-halogen bond is polar because the halogen is more electronegative than carbon. This gives the carbon a partial positive charge (δ+) and the halogen a partial negative charge (δ-). The δ+ carbon is the site attacked by nucleophiles.
碳-卤键具有极性，因为卤素的电负性大于碳。这使得碳带部分正电荷（δ+），卤素带部分负电荷（δ-）。带 δ+ 的碳就是亲核试剂进攻的位置。
Common examples include CH₃Cl, CH₃CH₂Br and CH₃CHClCH₃. The halogen is usually shown as X in general equations.
常见例子包括 CH₃Cl、CH₃CH₂Br 和 CH₃CHClCH₃。在一般方程中，卤素通常用 X 表示。
2. Classification: Primary, Secondary, Tertiary | 分类：一级、二级、三级
Halogenoalkanes are classified by the number of alkyl groups attached to the carbon atom that carries the halogen. A primary halogenoalkane has the halogen-bearing carbon bonded to one alkyl group, a secondary has two alkyl groups, and a tertiary has three alkyl groups.
卤代烷根据与卤素所在碳原子相连的烷基数目进行分类。一级卤代烷中，含卤碳与一个烷基相连；二级与两个烷基相连；三级与三个烷基相连。
For example, 1-chloropropane is primary, 2-chloropropane is secondary, and 2-chloro-2-methylpropane is tertiary. This classification is important because primary and tertiary halogenoalkanes often react by different mechanisms.
例如，1-氯丙烷属于一级，2-氯丙烷属于二级，2-氯-2-甲基丙烷属于三级。这种分类非常重要，因为一级和三级卤代烷通常以不同的机理反应。
3. Naming Halogenoalkanes | 卤代烷的命名
IUPAC naming uses the prefixes fluoro-, chloro-, bromo- and iodo- before the longest carbon chain. The chain is numbered to give the halogen substituent the lowest possible locant.
IUPAC 命名使用前缀 fluoro-、chloro-、bromo- 和 iodo-，加在最长的碳链名称之前。给碳链编号时，应使卤素取代基的位置编号尽可能小。
When more than one halogen is present, the prefixes are arranged alphabetically. For example, CH₃CHBrCH₂CH₃ is 2-bromobutane, and CH₂ClCH₂Cl is 1,2-dichloroethane.
当存在多个卤素时，前缀按字母顺序排列。例如，CH₃CHBrCH₂CH₃ 是 2-溴丁烷，而 CH₂ClCH₂Cl 是 1,2-二氯乙烷。
4. Physical Properties and Trends | 物理性质与变化趋势
Halogenoalkanes have higher boiling points than alkanes of similar chain length. This is because they have more electrons, leading to stronger London dispersion forces, and they also have permanent dipole-dipole interactions due to the polar carbon-halogen bond.
卤代烷的沸点高于相近链长的烷烃。这是因为它们拥有更多电子，从而产生更强的伦敦色散力，同时由于碳-卤键的极性，它们还存在永久偶极-偶极相互作用。
Boiling points increase as the chain length increases, and for the same alkyl group they generally increase down Group 7: RF < RCl < RBr < RI. The larger halogen atoms have more electrons, so London forces become stronger.
沸点随碳链增长而升高；对于相同的烷基，沸点通常沿第7族向下升高：RF < RCl < RBr < RI。卤素原子越大，电子越多，伦敦力就越强。
Most halogenoalkanes are only slightly soluble in water because they cannot form hydrogen bonds with water molecules. They are, however, soluble in organic solvents.
大多数卤代烷在水中仅微溶，因为它们无法与水分子形成氢键。但它们可溶于有机溶剂。
5. Nucleophilic Substitution: General Features | 亲核取代：总体特征
A nucleophile is an electron-pair donor. It is attracted to the δ+ carbon atom in the polar carbon-halogen bond. Common nucleophiles include OH⁻, CN⁻ and NH₃.
亲核试剂是电子对给体。它被极性碳-卤键中带 δ+ 的碳原子
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