手性中心Stereocenter

检查每个 sp³ 碳原子所连的四个基团是否各不相同。例如丁-2-醇 CH₃CH(OH)CH₂CH₃ 中 C-2 连有 H、OH、CH₃、C₂H₅，是手性中心；乳酸 CH₃CH(OH)COOH 的 C-2 和除甘氨酸外的 α-氨基酸的 α-碳也是。连有两个相同基团（如两个 CH₃）的碳不是手性中心。

Check whether each sp³ carbon carries four different groups. In butan-2-ol, CH₃CH(OH)CH₂CH₃, C-2 has H, OH, CH₃ and C₂H₅, so it is a stereocentre; so is C-2 of lactic acid, CH₃CH(OH)COOH, and the α-carbon of every α-amino acid except glycine. A carbon carrying two identical groups, such as two CH₃, is not.

一对对映异构体的熔点、沸点、溶解度和一般化学性质相同，但使平面偏振光旋转的方向相反、角度相等，又称旋光异构体。等量对映体的混合物（外消旋体）不旋光。对映体与手性环境（如酶、受体）作用时可能截然不同：沙利度胺的一种对映体有镇静作用，另一种致畸，且两者在体内会相互转化。

A pair of enantiomers has identical melting and boiling points, solubility and ordinary chemical reactions, but rotates plane-polarised light by equal angles in opposite directions, hence the name optical isomers. An equal mixture, a racemate, shows no rotation. Enantiomers can behave very differently in chiral surroundings such as enzymes and receptors: one enantiomer of thalidomide is a sedative and the other causes birth defects, and the two interconvert in the body.

含 n 个手性中心的分子最多有 2ⁿ 个立体异构体；若分子内有对称面，会出现内消旋体，异构体数目减少，如酒石酸只有 3 种。每个手性中心的构型用 CIP 规则标记为 R 或 S：按原子序数确定四个基团的优先次序，把最低优先的基团指向后方，其余三个从高到低为顺时针则为 R，逆时针为 S。

A molecule with n stereocentres has at most 2ⁿ stereoisomers; an internal mirror plane gives a meso form and fewer isomers, so tartaric acid has only three. Each centre is labelled R or S by the CIP rules: rank the four groups by atomic number, point the lowest away from the viewer, and if the other three run clockwise from highest to lowest the centre is R, anticlockwise S.