定义Definition
酯交换是一种酯与醇（或另一种酯、酸）在酸、碱或酶催化下交换烷氧基，生成新酯和新醇的可逆反应。
Transesterification is the reversible exchange of the alkoxy group of an ester with an alcohol (or another ester or acid) under acid, base or enzyme catalysis, giving a new ester and a new alcohol.
机理Mechanism
碱催化：醇盐 R″O⁻ 进攻酯羰基，四面体中间体放出 R′O⁻；酸催化：与费歇尔酯化类似，以醇代替水。因为反应可逆，用大大过量的醇或蒸出生成的低沸点醇推动平衡。
Base: an alkoxide attacks the ester and the tetrahedral intermediate expels the original alkoxide; acid: as in Fischer esterification with alcohol in place of water. Being reversible, it is driven by excess alcohol or by distilling off the lower-boiling alcohol.
生物柴油Biodiesel
植物油（甘油三酯）与甲醇在 NaOH 或 KOH 催化下酯交换得脂肪酸甲酯（生物柴油）和甘油。原料中游离脂肪酸多时会皂化，需先酸催化酯化预处理。
Vegetable oils transesterified with methanol over NaOH or KOH give fatty-acid methyl esters (biodiesel) and glycerol; feedstocks rich in free fatty acids saponify and need acid pre-esterification.
高分子工业Polymer industry
聚对苯二甲酸乙二醇酯（PET）可由对苯二甲酸二甲酯与乙二醇酯交换制备；PET 的醇解（糖酵解）是化学回收的途径之一。脂肪酶催化的酯交换用于手性拆分与油脂改性。
PET can be made by transesterifying dimethyl terephthalate with ethylene glycol, and glycolysis of PET is a chemical-recycling route. Lipase-catalysed transesterification is used for kinetic resolution and fat modification.
反应Reactions
NaOMe/MeOH、KOH、H₂SO₄ 或脂肪酶，加热
NaOMe/MeOH, KOH, H₂SO₄ or lipases with heating
新醇（或醇盐）进攻酯羰基。
The new alcohol (or alkoxide) attacks the ester carbonyl.
四面体中间体形成。
A tetrahedral intermediate forms.
原烷氧基离去得新酯。
The original alkoxide leaves, giving the new ester.
生物柴油生产。
Biodiesel production.
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.
甲氧负离子进攻酯羰基
Methoxide attacks the ester carbonyl
四面体中间体塌缩，赶走乙氧负离子
The tetrahedral intermediate collapses, expelling ethoxide
乙酸甲酯；每步可逆，用大量甲醇推动平衡
Methyl acetate; every step is reversible, so excess methanol drives it
年表Timeline
- 1853
达菲与帕特里克用酯交换处理植物油
Duffy and Patrick transesterify vegetable oil
- 1941
惠恩菲尔德与迪克森发明 PET
Whinfield and Dickson invent PET