定义Definition
溴乙烷 CH₃CH₂Br 是无色、易挥发的液体，沸点约 38 °C，难溶于水。它是典型的伯卤代烃，在不同条件下与 OH⁻、CN⁻、NH₃ 发生亲核取代，或在醇溶液中发生消除反应，是学习卤代烃反应的代表物。
Bromoethane, CH₃CH₂Br, is a colourless, volatile liquid boiling at about 38 °C and barely soluble in water. A typical primary halogenoalkane, it undergoes nucleophilic substitution with OH⁻, CN⁻ and NH₃, or elimination in alcoholic solution, and serves as the standard example of halogenoalkane chemistry.
反应一览Summary of reactions
NaOH 水溶液加热回流：CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻（取代）；NaOH 或 KOH 的乙醇溶液加热：CH₃CH₂Br + OH⁻ → CH₂=CH₂ + H₂O + Br⁻（消除）；KCN 乙醇溶液加热回流：生成丙腈；过量氨的乙醇溶液在密封管中加热：生成乙胺。
Aqueous NaOH under reflux: CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻ (substitution). NaOH or KOH in ethanol, heated: CH₃CH₂Br + OH⁻ → CH₂=CH₂ + H₂O + Br⁻ (elimination). KCN in ethanol under reflux gives propanenitrile; excess ammonia in ethanol heated in a sealed tube gives ethylamine.
取代与消除的竞争Substitution versus elimination
水作溶剂有利于取代，乙醇作溶剂、温度较高有利于消除。伯卤代烃主要按 SN2 机理取代；叔卤代烃更易发生 SN1 和消除。
Water as solvent favours substitution, while ethanol and higher temperature favour elimination. Primary halogenoalkanes substitute mainly by SN2; tertiary ones favour SN1 and elimination.
化学性质Chemical properties
水解：CH₃CH₂Br + NaOH → CH₃CH₂OH + NaBr
Hydrolysis: CH₃CH₂Br + NaOH → CH₃CH₂OH + NaBr
与氰化钾反应：CH₃CH₂Br + KCN → CH₃CH₂CN + KBr
With potassium cyanide: CH₃CH₂Br + KCN → CH₃CH₂CN + KBr