定义Definition
费歇尔投影式是把手性分子画成十字形的二维表示法：横线表示伸向观察者的键，竖线表示远离观察者的键，交点为手性碳。主链竖直，氧化态最高的碳（醛基等）写在最上方。
A Fischer projection draws a chiral molecule as a cross: horizontal lines are bonds towards the viewer, vertical lines bonds away, and the intersection is the stereocentre. The main chain runs vertically with the most oxidised carbon at the top.
操作规则Manipulation rules
在纸面内旋转 180° 构型不变；旋转 90° 或离开纸面翻转则变成对映体；交换任意两个基团一次构型翻转，交换两次不变。判断 R/S：最低优先基团在竖线上时直接读，在横线上时读出结果取反。
Rotating 180° in the plane keeps the configuration; rotating 90° or flipping out of plane gives the enantiomer; one swap of two groups inverts, two swaps restore. For R/S, read directly if the lowest priority is vertical, and reverse the answer if it is horizontal.
由来与应用Origin and use
埃米尔·费歇尔于 1891 年研究糖类立体化学时提出。现在主要用于糖、氨基酸的结构表示和 D/L 判断。
Introduced by Emil Fischer in 1891 for sugar stereochemistry; still used for sugars and amino acids and for assigning D/L.
局限Limitations
表示的是重叠式构象，不反映分子的真实优势构象；对环状结构不直观（改用哈沃斯式或椅式）。
It depicts an eclipsed conformation, not the real preferred one, and is awkward for rings (use Haworth or chair drawings).
年表Timeline
- 1891
费歇尔提出投影式
Fischer introduces the projection
- 1902
费歇尔获诺贝尔化学奖
Fischer receives the Nobel Prize