定义Definition
消除反应是从分子相邻的两个原子上各除去一个原子或基团，形成双键（或三键）的反应，最常见的是从卤代烃脱去 HX、从醇脱去 H₂O 生成烯烃。主要机理有 E1、E2 和 E1cB。
An elimination reaction removes an atom or group from each of two adjacent atoms to form a double (or triple) bond — commonly loss of HX from haloalkanes or H₂O from alcohols to give alkenes. The main mechanisms are E1, E2 and E1cB.
机理谱Mechanistic spectrum
E2：一步协同，速率 = k[底物][碱]，需反式共平面；E1：先离去成碳正离子再失 β-H，一级动力学；E1cB：先失去酸性 β-H 成碳负离子再离去（需吸电子基稳定）。三者构成一个连续谱。
E2: concerted, rate = k[substrate][base], anti-periplanar; E1: ionisation first then β-H loss, first order; E1cB: β-H removal first to a carbanion, then loss of the leaving group (needs an acceptor). They form a continuum.
区域规则Regiochemistry
扎伊采夫规则：一般生成取代较多、更稳定的烯烃；大位阻碱（t-BuOK）或差离去基团（–NMe₃⁺、–F）时生成取代较少的霍夫曼烯烃。
Zaitsev: usually the more substituted, more stable alkene; bulky bases (t-BuOK) or poor leaving groups (–NMe₃⁺, –F) give the less substituted Hofmann alkene.
与取代的竞争Competition with substitution
强碱、高温、位阻大的底物和碱有利于消除；强亲核弱碱（I⁻、RS⁻）、低温有利于取代。叔卤代烃与强碱主要 E2，与弱亲核试剂在质子溶剂中 SN1/E1 混合。
Strong bases, heat and hindered substrates or bases favour elimination; good nucleophiles that are weak bases (I⁻, RS⁻) and low temperature favour substitution. Tertiary halides give E2 with strong bases and SN1/E1 mixtures in protic solvents.
反应Reactions
强碱（醇钠、t-BuOK）加热；或酸催化醇脱水
Strong base (alkoxides, t-BuOK) with heat, or acid-catalysed dehydration of alcohols
碱夺取 β-H（E2、E1cB）或离去基团先离去（E1）。
Base removes the β-H (E2, E1cB) or the leaving group departs first (E1).
形成 π 键。
The π bond forms.
扎伊采夫产物为主。
Zaitsev product predominates.
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
- 1875
扎伊采夫提出消除规则
Zaitsev formulates his elimination rule
- 1851
霍夫曼消除
Hofmann elimination
- 1940s
英戈尔德系统阐明 E1/E2
Ingold systematises E1/E2