定义Definition
脱水反应是从一个分子（分子内脱水）或两个分子之间（分子间脱水）除去一分子水的反应，如醇分子内脱水成烯烃、两分子醇分子间脱水成醚。
A dehydration reaction removes water from within one molecule (intramolecular) or between two molecules (intermolecular), e.g. alcohols to alkenes, or two alcohols to an ether.
醇的脱水Dehydration of alcohols
浓硫酸加热：乙醇 170 °C 分子内脱水成乙烯（E1/E2），140 °C 分子间脱水成乙醚（SN2）。叔醇最易脱水（E1，经碳正离子，可能重排），主要得扎伊采夫烯烃。Al₂O₃ 或沸石气相脱水是工业路线。
With hot concentrated H₂SO₄, ethanol gives ethylene at 170 °C (E1/E2) and diethyl ether at 140 °C (SN2). Tertiary alcohols dehydrate most easily (E1 via cations, possibly rearranging), favouring the Zaitsev alkene. Gas-phase dehydration over Al₂O₃ or zeolites is used industrially.
其他脱水Other dehydrations
羧酸脱水成酸酐（P₂O₅）；酰胺脱水成腈（POCl₃、SOCl₂）；浓硫酸使蔗糖炭化（夺取化合物中按 2:1 比例的 H、O）；生物体内氨基酸缩合成肽、单糖缩合成多糖都是脱水缩合。
Acids to anhydrides (P₂O₅); amides to nitriles (POCl₃, SOCl₂); concentrated H₂SO₄ chars sugar by removing H and O in a 2:1 ratio; in biology peptide and polysaccharide formation are dehydrative condensations.
脱水剂Dehydrating agents
浓硫酸、五氧化二磷、无水氯化锌、分子筛、DCC（肽合成中的缩合剂）。
Concentrated sulfuric acid, phosphorus pentoxide, anhydrous ZnCl₂, molecular sieves and DCC (a peptide coupling agent).
反应Reactions
浓硫酸或磷酸加热，或 Al₂O₃ 气相 350 °C
Hot concentrated H₂SO₄ or H₃PO₄, or Al₂O₃ in the gas phase at ~350 °C
羟基质子化成好的离去基团。
Protonation turns OH into a good leaving group.
失水生成碳正离子（E1）或协同消除（E2）。
Water leaves to give a cation (E1) or departs concertedly (E2).
失去 β-H 生成烯烃。
Loss of a β-H gives the alkene.
扎伊采夫产物为主。
Zaitsev product predominates.
反应机理图Mechanism
红色弯箭头表示一对电子的移动方向：从电子来源（孤对电子或化学键）指向电子去向。Red curly arrows show where an electron pair moves: from its source (lone pair or bond) to its destination.
浓硫酸使羟基质子化，–OH 变成好的离去基团 H₂O
Sulfuric acid protonates –OH, turning it into the good leaving group H₂O
伯醇按 E2：HSO₄⁻ 夺取 β-H，同时水离去
Primary alcohols react by E2: HSO₄⁻ takes a β-H as water leaves
乙烯；140 °C 时则生成乙醚
Ethene; at 140 °C diethyl ether forms instead
年表Timeline
- 1540
科尔杜斯用硫酸与乙醇制得乙醚
Cordus makes ether from ethanol and sulfuric acid
- 1795
荷兰化学家由乙醇脱水制得乙烯（“油生气”）
Dutch chemists make ethylene (olefiant gas) from ethanol