📚 Cis/Trans and E/Z Isomers | 顺反异构与E/Z异构
Geometric isomerism is a fundamental concept in organic chemistry: it describes molecules that have exactly the same connectivity but differ in the spatial arrangement of their groups around a double bond or a ring. In IB Chemistry and A Level Chemistry, understanding cis/trans and E/Z isomers is essential for predicting physical properties, naming compounds correctly and reasoning about reactivity.
几何异构是有机化学的基本概念：它描述连接性完全相同，但双键或环周围基团空间排列不同的分子。在 IB 化学和 A Level 化学中，理解顺反异构与 E/Z 异构对于预测物理性质、正确命名化合物以及推理反应性都至关重要。
This article keeps the original theme of the topic and expands it step by step, from the origin of restricted rotation, through the Cahn-Ingold-Prelog priority rules, to the physical consequences and the classic exam traps.
本文保留原有主题并逐步展开：从旋转受阻的成因，到 Cahn-Ingold-Prelog 优先级规则，再到对物理性质的影响以及经典考试陷阱。
1. Introduction to Stereoisomerism | 立体异构简介
Stereoisomers share the same molecular formula and atom connectivity but differ in the three-dimensional orientation of their atoms. This category includes both optical isomers (enantiomers and diastereomers) and geometric isomers (cis/trans and E/Z).
立体异构体具有相同的分子式和原子连接顺序，但原子的三维取向不同。这一类别包括光学异构体（对映异构体和非对映异构体）以及几何异构体（顺反和 E/Z）。
A useful way to organise the field is to separate configurational isomers from conformational isomers. Configurational isomers - including geometric isomers - can only be interconverted by breaking and reforming covalent bonds. Conformers, such as the staggered and eclipsed forms of ethane, interconvert freely by rotation about a single bond and are not treated as separate isomers in examination contexts.
一个有用的整理方式是区分“构型异构体”和“构象异构体”。包括几何异构体在内的构型异构体，只能通过断裂并重新形成共价键才能互相转化。而构象异构体（如乙烷的交错式与重叠式）通过单键旋转即可自由互变，在考试中不作为独立异构体处理。
Geometric isomerism arises when there is restricted rotation, typically in alkenes due to the π bond, or in cyclic alkanes where the ring locks the positions of substituents. In both cases the molecule is held in a fixed arrangement, and the relative positions of the substituents become permanent features of the structure.
几何异构的出现源于旋转受阻：典型情况是烯烃因 π 键而无法自由旋转，或环烷烃因环的闭合锁定了取代基的位置。在这两种情况下，分子被固定在特定排布中，取代基的相对位置便成为结构的永久特征。
C=C rotation barrier ≈ 270 kJ mol⁻¹   vs C–C rotation barrier ≈ 12–20 kJ mol⁻¹
2. Why Rotation Is Restricted | 旋转为何受阻
In an alkene, each doubly bonded carbon is sp² hybridised with bond angles close to 120°. The two p orbitals overlap sideways to form a π bond above and below the molecular plane. Rotating one end of the double bond would destroy this sideways overlap, which costs far more energy than is available at room temperature. The result is a rigid, planar arrangement that makes cis and trans forms genuinely different compounds.
在烯烃中，每个双键碳都是 sp² 杂化，键角接近 120°。两个 p 轨道侧面重叠，在分子平面的上下方形成 π 键。若旋转双键的一端，这种侧面重叠会被破坏，所需能量远超室温可提供的范围。结果是分子呈刚性平面排布，使顺式与反式成为真正不同的化合物。
re the ring itself prevents the substituents from swapping sides. Carbonyl C=O and nitrile C≡ ups do not show this behaviour because the carbon carries only one non-oxygen substituent.
旋转受阻并不限于 C=C。它同样出现在 C=N 键（肟和亚胺）、N=N 键（偶氮化合物）以及取代环中——环本身阻止了取代基换边。羰基 C=O 和腈基 C≡N 则不会出现这种现象，因为该碳上只有一个非氧取代基。
There is also an essential structural condition: each doubly bonded carbon must carry two different groups ydrogens on one carbon, and 2-methylbut-2-ene has two identical methyl groups on the same carbon. Neither shows geometric isomerism, because swapping the two identical groups produces an identical molecule.
还有一个必要的结构条件：每个双键碳必须连接两个不同的基团。丁-1-烯 CH₂=CHCH₂CH₃ 的一个碳上连有两个氢，2-甲基丁-2-烯的同一个碳上连有两个相同的甲基。二者都没有几何异构，因为交换两个相同基团后得到的分子与原来完全相同。
Requirement: Ca(X)(Y)=Cb(Z)(W) with X ≠ Y and Z ≠ W
3. The Cahn-Ingold-Prelog Priority Rules | Cahn-Ingold-Prelog 优先级规则
The E/Z system was introduced because cis/trans becomes ambiguous once a doubly bonded carbon carries two substituents that are neither hydrogen nor identical. The Cahn-Ingold-Prelog (CIP) rules give every substituent an unambiguous rank.
E/Z 体系之所以被引入，是因为当双键碳上连接的两个取代基既不是氢也不相同时，顺反命名就变得含糊。Cahn-Ingold-Prelog（CIP）规则为每个取代基给出无歧义的排序。
Rule 1 - Compare the atoms directly attached. her priority. Br (Z = 35) beats Cl (Z = 17), which beats O (Z = 8), which beats C (Z = 6).
规则 1 —— 比较直接相连的原子。 原子序数较大的原子优先级更高。Br（Z = 35）优先于 Cl（Z = 17），Cl 优先于 O（Z = 8），O 优先于 C（Z = 6）。
Rule 2 - If there is a tie, look outward. Move one atom away from the double bond and compare the sets of attached atoms in order of decreasing atomic number. For example, –CH₂CH₃ beats –CH₃ because the first carbon of the ethyl group is attached to (C, H, H), while the methyl carbon is attached to (H, H, H).
规则 2 —— 若出现平手，则向外看。 从双键向外推移一个原子，并按原子序数由大到小比较其连接的原子集合。例如 –CH₂CH₃ 优先于 –CH₃，因为乙基的碳连接 (C, H, H)，而甲基的碳只连接 (H, H, H)。
Rule 3 - Duplicate atoms for multiple bonds. A double bond counts as two bonds to that atom, and a triple bond as three. In –CHO the carbon is treated as being attached to (O, O, H); in –COOH it is treated as attached to (O, O, O).
规则 3 —— 重键按“复制原子”处理。 双键视为与该原子连接两次，三键视为三次。–CHO 中的碳按连接 (O, O, H) 处理；–COOH 中的碳按连接 (O, O, O) 处理。
Rule 4 - Isotopes and lone pairs. A heavier isotope has higher priority, so –CH₂D beats –CH₂H in deuterium-labelled work. A lone pair is always the lowest priority.
规则 4 —— 同位素与孤对电子。 较重的同位素优先级更高，因此在氘代研究中 –CH₂D 优先于 –CH₂H。孤对电子永远是优先级最低的。
4. Assigning E and Z Labels Step by Step | E/Z 标记的逐步判定
The labels come from German: Z stands for zusammen (together) and E stands for entgegen (opposite). They describe where the two high-priority groups sit with respect to the double bond.
这两个标记来自德语：Z 意为 zusammen（在一起），E 意为 entgegen（相反）。它们描述两个高优先级基团相对于双键所处的位置。
Step 1: Identify the doubly bonded carbons and check that each carries two different groups.
第一步： 找出双键两端碳，并确认每个碳都连有两个不同的基团。
Step 2: Apply the CIP rules to rank the two groups on the left-hand carbon as high (1) and low (2).
第二步： 用 CIP 规则对左侧碳上的两个基团排序，标为高优先级 (1) 与低优先级 (2)。
Step 3: Repeat the same ranking for the right-hand carbon. Rank each carbon independently - never mix them.
第三步： 对右侧碳重复同样的排序。每个碳必须独立排序，切勿混在一起。
Step 4: Compare the two high-priority groups. If they lie on the same side of the double bond, the isomer is Z. If they lie on opposite sides, the isomer is E.
第四步： 比较两个高优先级基团。若它们位于双键同侧，该异构体为 Z；若位于异侧，则为 E。
High-priority groups together → Z   | High-priority groups opposite → E
Worked example: 2-bromobut-2-ene, CH₃–CBr=CH– the same side, the molecule is (Z)-2-bromobut-2-ene; if they are on opposite sides, it is the (E) isomer.
示例：2-溴丁-2-烯，CH₃–CBr=CH–CH₃。左侧碳连 Br 和 CH₃，故 Br 优先级更高（Z = 35 对 6）。右侧碳连 H 和 CH₃，故 CH₃ 优先级更高。若图中 Br 与 CH₃ 在同侧，该分子为 (Z)-2-溴丁-2-烯；若在异侧，则为 (E) 异构体。
5. Cis/Trans versus E/Z: What Is the Difference? | 顺反与 E/Z 的区别
The cis/trans notation is the older and simpler language. It works unambiguously only when each doubly bonded carbon carries one hydrogen (or one identical reference group). In that case, cis means the two reference groups are on the same side and trans means they are on opposite sides.
顺反命名是更早、更简单的语言。只有当每个双键碳都连有一个氢（或一个相同的参照基团）时，它才无歧义。此时 cis 表示两个参照基团在同侧，trans 表示在异侧。
orks even when there is no hydrogen on the double bond, as in 1-bromo-1-chloropropene, CH₃– t be applied consistently because no common reference group exists, but E/Z can always be assigned.
E/Z 标记则是通用的。即使双键上没有氢，例如 1-溴-1-氯丙-1-烯 CH
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