定义Definition
皂化反应是酯在强碱（NaOH、KOH）作用下水解，生成羧酸盐和醇的反应。油脂（甘油三酯）皂化得到高级脂肪酸钠（肥皂）和甘油。
Saponification is the hydrolysis of an ester by strong base (NaOH, KOH) to a carboxylate salt and an alcohol. Saponifying fats (triglycerides) gives sodium fatty-acid salts (soap) and glycerol.
机理与不可逆性Mechanism and irreversibility
OH⁻ 进攻酯羰基得四面体中间体，烷氧负离子离去生成羧酸；羧酸立即被碱（或烷氧负离子）夺去质子成为羧酸根，由于羧酸根不再被醇进攻，反应不可逆，可进行到底。反应为二级（对酯和 OH⁻ 各一级）。
Hydroxide adds to the ester carbonyl; alkoxide leaves to give the acid, which is immediately deprotonated to the carboxylate. Carboxylates are not attacked by alcohols, so the reaction is irreversible and goes to completion; it is second order overall.
肥皂与皂化值Soap and saponification value
肥皂分子有长疏水尾和亲水羧酸根头，在水中形成胶束包裹油污。皂化值（皂化 1 g 油脂所需 KOH 的毫克数）反映脂肪酸平均链长；加食盐使肥皂析出（盐析）。
Soap molecules have a long hydrophobic tail and a carboxylate head, forming micelles that trap grease. The saponification value (mg KOH per g fat) reflects average chain length; adding salt precipitates the soap (salting out).
历史History
用草木灰（K₂CO₃）与动物脂肪制皂的历史可追溯到古巴比伦（约公元前 2800 年）。谢弗勒尔（1823）阐明油脂是甘油与脂肪酸形成的酯。
Soap from wood ash (K₂CO₃) and animal fat goes back to ancient Babylon (~2800 BCE). Chevreul (1823) showed that fats are esters of glycerol and fatty acids.
反应Reactions
NaOH 或 KOH 水溶液或醇溶液，加热回流
Aqueous or alcoholic NaOH or KOH at reflux
OH⁻ 加成到羰基。
Hydroxide adds to the carbonyl.
烷氧负离子离去得羧酸。
Alkoxide leaves, giving the acid.
羧酸被去质子化成羧酸盐（不可逆）。
The acid is deprotonated irreversibly.
硬脂酸甘油酯皂化制肥皂。
Saponification of tristearin to soap.
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.
加成：OH⁻ 进攻羰基碳
Addition: OH⁻ attacks the carbonyl carbon
四面体中间体塌缩，赶走 CH₃O⁻（消除）
The tetrahedral intermediate collapses and expels CH₃O⁻ (elimination)
酸碱反应，生成羧酸根（不可逆）
Acid–base step gives the carboxylate (irreversible)
乙酸根 + 甲醇
Acetate + methanol
年表Timeline
- 约公元前 2800
巴比伦已有制皂记载
Soap-making recorded in Babylon
- 1791
吕布兰法使廉价纯碱推动制皂工业
The Leblanc process supplies cheap soda for soap
- 1823
谢弗勒尔阐明油脂结构
Chevreul explains the structure of fats