定义Definition
E2 反应是双分子消除：碱夺取 β-H、C=C 形成、离去基团离去在一步中协同完成。速率与底物和碱的浓度都成正比。
The E2 reaction is bimolecular elimination: base removal of the β-H, C=C formation and departure of the leaving group occur in one concerted step. The rate is proportional to both substrate and base concentrations.
反式共平面要求Anti-periplanar requirement
被夺取的 H 与离去基团须处于反式共平面（二面角 180°），使 C–H σ 轨道与 C–X σ* 轨道最佳重叠。因此 E2 是立体专一的：(1R,2S)- 与 (1R,2R)-1,2-二苯基-1-溴丙烷分别给出 E- 与 Z-烯烃。环己烷中 H 和 X 都必须处于直立键（如薄荷基氯只能得到一种烯烃）。
The H and the leaving group must be anti-periplanar (180°) for best overlap of σ(C–H) with σ*(C–X), making E2 stereospecific: diastereomeric 1-bromo-1,2-diphenylpropanes give E or Z alkenes. In cyclohexanes both must be axial (menthyl chloride gives a single alkene).
区域选择Regioselectivity
小碱（EtO⁻）得扎伊采夫烯烃，大位阻碱（t-BuO⁻、LDA）得霍夫曼烯烃；离去基团差（F、NMe₃⁺）时过渡态偏碳负离子性质，也倾向霍夫曼产物。
Small bases (EtO⁻) give Zaitsev alkenes; bulky bases (t-BuO⁻, LDA) give Hofmann alkenes; poor leaving groups (F, NMe₃⁺) shift the TS toward carbanion character and also favour Hofmann products.
证据Evidence
二级动力学、明显的一级氘同位素效应（kH/kD ≈ 3–8）、立体专一性。
Second-order kinetics, a substantial primary deuterium KIE (kH/kD ≈ 3–8) and stereospecificity.
反应Reactions
强碱（NaOEt、t-BuOK、DBU），加热；仲、叔卤代烃
Strong bases (NaOEt, t-BuOK, DBU) with heat; secondary and tertiary halides
碱、H、C、C、X 排成反式共平面。
Base, H, C, C and X align anti-periplanar.
一步协同断键、成键。
One concerted step breaks and forms the bonds.
扎伊采夫、E 型为主。
Mainly the Zaitsev E-alkene.
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.
碱夺取 β-H，同时形成 C=C 并赶走 Br⁻；H 与 Br 反式共平面
The base takes the β-H while C=C forms and Br⁻ leaves; H and Br anti-periplanar
一步完成，速率 = k[底物][碱]
One step; rate = k[substrate][base]
年表Timeline
- 1927
英戈尔德提出双分子消除
Ingold proposes bimolecular elimination
- 1948
克里斯托尔、克拉姆研究 E2 立体化学
Cristol and Cram study E2 stereochemistry