定义Definition
SN1 反应是单分子亲核取代：底物先慢速解离出离去基团生成碳正离子（决速步），亲核试剂再快速与碳正离子结合。速率只与底物浓度有关。
The SN1 reaction is unimolecular nucleophilic substitution: the substrate slowly ionises to a carbocation (rate-determining), which a nucleophile then captures rapidly. The rate depends only on substrate concentration.
特征Characteristics
一级动力学 v = k[RX]；叔 > 仲 ≫ 伯（烯丙基、苄基也快）；极性质子溶剂通过稳定离子过渡态大大加速；手性底物主要得到外消旋产物（常有少量构型翻转，因离子对屏蔽离去一侧）；常伴随 E1 消除和碳正离子重排。
First-order kinetics v = k[RX]; 3° > 2° ≫ 1° (allylic and benzylic also fast); polar protic solvents accelerate greatly by stabilising the ionic TS; chiral substrates racemise (often with slight inversion from ion-pair shielding); E1 elimination and rearrangements compete.
溶剂效应的定量Quantifying solvent effects
温斯坦–格伦沃尔德方程 lg(k/k₀) = mY，以叔丁基氯在 80% 乙醇中的溶剂解为标准定义溶剂离子化能力 Y。
The Grunwald–Winstein equation lg(k/k₀) = mY uses tert-butyl chloride solvolysis in 80 % ethanol to define solvent ionising power Y.
实例与意义Examples and significance
叔丁基溴水解、糖苷的酸催化水解（经氧鎓离子）、萜类生物合成中焦磷酸酯离去生成碳正离子。
tert-Butyl bromide hydrolysis, acid-catalysed glycoside hydrolysis (via oxocarbenium ions), and terpene biosynthesis where pyrophosphate departs to form cations.
反应Reactions
叔或稳定化碳正离子底物，极性质子溶剂，弱亲核试剂
Tertiary or stabilised substrates, polar protic solvents, weak nucleophiles
C–X 键异裂生成碳正离子（决速步）。
Heterolysis to the carbocation (rate-determining).
亲核试剂从平面两侧进攻。
Nucleophile attacks either face of the planar ion.
若亲核试剂为溶剂，最后失去质子。
If solvent is the nucleophile, deprotonation follows.
叔丁基氯水解。
Hydrolysis of tert-butyl chloride.
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.
C–Br 键异裂，生成平面碳正离子（决速步）
Heterolysis gives a planar carbocation (rate-determining)
水从平面任一侧进攻
Water attacks either face
另一分子水夺去质子
A second water removes the proton
叔丁醇
tert-Butanol
年表Timeline
- 1935
休斯与英戈尔德提出 SN1 机理
Hughes and Ingold propose the SN1 mechanism
- 1948
温斯坦–格伦沃尔德方程
Grunwald–Winstein equation
- 1960s
奥拉在超强酸中直接观测碳正离子（1994 年诺贝尔奖）
Olah observes carbocations directly in superacids (Nobel 1994)