定义Definition
CIP 顺序规则是确定手性中心、双键等立体单元上取代基优先次序的一套规则，用于给出 R/S、E/Z 等立体描述符。
The Cahn–Ingold–Prelog rules rank substituents on stereogenic units such as stereocentres and double bonds, providing the basis for R/S and E/Z descriptors.
基本规则Basic rules
①比较直接相连原子的原子序数，大者优先（同位素按质量数，D > H）；②若相同，依次比较下一层原子组成的集合，找到第一个不同点；③双键、三键视为该原子重复连接（C=O 当作 C 连两个 O，O 连两个 C）。
(1) Higher atomic number of the directly attached atom wins (isotopes by mass, D > H); (2) on a tie, compare the sets of atoms one bond further out until the first difference; (3) double and triple bonds count as duplicated atoms (C=O treated as C bearing two O, O bearing two C).
R/S 判断Assigning R/S
把优先级最低的基团指向远离观察者，其余三个按 1 → 2 → 3 顺时针为 R（rectus），逆时针为 S（sinister）。
With the lowest-priority group pointing away, a clockwise 1 → 2 → 3 sequence is R (rectus), anticlockwise S (sinister).
由来Origin
罗伯特·康、克里斯托弗·英戈尔德（1951）和弗拉基米尔·普雷洛格（1956、1966 修订）提出，后来为 IUPAC 采纳并于 1982、2013 年进一步完善（处理复杂分子的更高级规则）。
Proposed by Robert Cahn and Christopher Ingold (1951) with Vladimir Prelog (1956, revised 1966), adopted by IUPAC and refined in 1982 and 2013 with higher-level rules for complex cases.
年表Timeline
- 1951
康与英戈尔德提出初版
Cahn and Ingold’s first version
- 1956
普雷洛格加入，形成 CIP 体系
Prelog joins, forming the CIP system
- 1966
CIP 规则全面修订
Comprehensive revision
- 1975
普雷洛格获诺贝尔化学奖
Prelog receives the Nobel Prize