卤代烃Haloalkane

按与卤素相连的碳原子所连烃基数分为伯（1°）、仲（2°）、叔（3°）卤代烃：CH₃CH₂CH₂Br 为伯，CH₃CHBrCH₃ 为仲，(CH₃)₃CBr 为叔。系统命名把卤素作取代基前缀，如 2-溴丙烷 2-bromopropane、2-氯-2-甲基丙烷 2-chloro-2-methylpropane。

They are classed as primary (1°), secondary (2°) or tertiary (3°) by how many alkyl groups are on the carbon bearing the halogen: CH₃CH₂CH₂Br is primary, CH₃CHBrCH₃ secondary and (CH₃)₃CBr tertiary. Systematic names use the halogen as a prefix: 2-bromopropane, 2-chloro-2-methylpropane.

C–X 键极性使碳带部分正电荷，易受亲核试剂进攻。与 NaOH 水溶液加热生成醇，与 KCN 乙醇溶液加热生成腈（碳链增加一个碳），与过量浓氨在密封管中加热生成胺。伯卤代烃多按 SN2 机理反应，叔卤代烃多按 SN1 机理。与 NaOH 的乙醇溶液加热则发生消除反应生成烯烃。

The polar C–X bond leaves carbon δ+, open to nucleophilic attack. Heated with aqueous NaOH it gives an alcohol; with KCN in ethanol a nitrile (adding a carbon to the chain); with excess concentrated ammonia in a sealed tube an amine. Primary haloalkanes react mostly by SN2, tertiary mostly by SN1. Heated with NaOH in ethanol instead, they undergo elimination to alkenes.

水解速率 C–I > C–Br > C–Cl > C–F，主要由 C–X 键能决定（C–F 最强、C–I 最弱），而不是键的极性。可用硝酸银的乙醇溶液比较：碘代烃最快生成黄色 AgI 沉淀，溴代烃生成奶油色 AgBr，氯代烃最慢生成白色 AgCl。

The rate of hydrolysis is C–I > C–Br > C–Cl > C–F, governed mainly by C–X bond enthalpy (C–F strongest, C–I weakest) rather than bond polarity. Ethanolic silver nitrate shows this: iodoalkanes give a yellow AgI precipitate fastest, bromoalkanes cream AgBr, and chloroalkanes white AgCl slowest.

卤代烃用作溶剂、制冷剂、灭火剂、麻醉剂和合成中间体，如聚四氟乙烯和聚氯乙烯的单体。氯氟烃（CFC）化学性质稳定，进入平流层后在紫外光下释放氯原子破坏臭氧层，已根据《蒙特利尔议定书》逐步淘汰，由氢氟烃（HFC）等替代。

Haloalkanes serve as solvents, refrigerants, fire extinguishers, anaesthetics and synthetic intermediates, including the monomers of PTFE and PVC. Chlorofluorocarbons (CFCs) are so stable that they reach the stratosphere, where UV light releases chlorine atoms that destroy ozone; they have been phased out under the Montreal Protocol and replaced by hydrofluorocarbons (HFCs) and others.