对映异构体Enantiomer

一对对映体的熔点、沸点、密度、溶解度和与非手性试剂的反应完全相同。不同之处在于：它们使平面偏振光旋转的角度相等、方向相反，分别称为右旋（+）和左旋（−）；与其他手性物质（如酶、受体）作用时可能差别很大。等量对映体的混合物称为外消旋体，不旋光。

Enantiomers have identical melting and boiling points, density, solubility and reactions with achiral reagents. They differ in rotating plane-polarised light by equal angles in opposite directions, dextrorotatory (+) and laevorotatory (−), and they may behave very differently towards other chiral substances such as enzymes and receptors. An equal mixture is a racemate and shows no optical rotation.

用楔形键和虚楔键表示手性碳上基团的空间取向，画出镜像即得另一对映体。每个手性中心的构型用 CIP 规则标为 R 或 S；旋光方向（+/−）需由实验测定，与 R/S 没有固定对应关系。

Wedged and hashed bonds show the arrangement of groups at a chiral carbon; drawing the mirror image gives the other enantiomer. Each centre is labelled R or S by the CIP rules; the sign of rotation (+/−) must be measured and has no fixed link to R/S.

生物体内的氨基酸几乎都是 L 型，糖多为 D 型，许多药物只有一种对映体有效。由非手性原料合成手性分子通常得到外消旋体，需拆分或采用不对称合成。

Amino acids in living things are almost all L and sugars mostly D, and many drugs work as only one enantiomer. Making a chiral molecule from achiral starting materials usually gives a racemate, which must be resolved, or asymmetric synthesis is used.