📚 Stereoisomerism | 立体异构
Isomers are compounds with the same molecular formula but different arrangements of atoms. Stereoisomerism is the branch of isomerism in which molecules have the same structural formula but a different spatial arrangement of atoms. For A-Level Chemistry, you must be able to describe, recognise and draw two main types: geometric (cis-trans and E/Z) isomerism and optical isomerism.
异构体是指分子式相同但原子排列不同的化合物。立体异构是异构现象的一种，分子具有相同的结构式，但原子在空间中的排列方式不同。在 A-Level 化学中，你需要能够描述、识别并绘制两种主要类型：几何异构（顺反和 E/Z）和光学异构。
1. What Is Stereoisomerism? | 什么是立体异构？
Stereoisomers are molecules that share the same structural formula but differ in how their atoms or groups are arranged in three-dimensional space. This is different from structural isomerism, where atoms are connected in a different order.
立体异构体是指结构式相同、但原子或基团在三维空间中排列方式不同的分子。这与结构异构不同，结构异构是原子的连接顺序不同。
In structural isomers, the molecular skeleton changes; in stereoisomers, the skeleton remains the same, but the spatial orientation changes. This means stereoisomers often have very similar chemical properties but can have very different biological behaviour.
在结构异构体中，分子骨架发生改变；在立体异构体中，骨架保持不变，但空间取向不同。这意味着立体异构体通常具有非常相似的化学性质，但可能具有非常不同的生物学行为。
2. Types of Stereoisomerism | 立体异构的类型
Cambridge A-Level Chemistry focuses on two classes of stereoisomerism: geometric isomerism (also called cis-trans or E/Z isomerism) and optical isomerism (enantiomerism). Both arise from restricted movement or from chirality.
剑桥 A-Level 化学重点关注两类立体异构：几何异构（也称顺反或 E/Z 异构）和光学异构（对映异构）。两者分别源于受限运动或手性。
- Geometric isomerism occurs in alkenes and some cyclic compounds where rotation is restricted.
- 几何异构发生在旋转受限制的烯烃和某些环状化合物中。
- Optical isomerism occurs in molecules that contain a chiral centre, usually a carbon atom bonded to four different groups.
- 光学异构发生在含有手性中心的分子中，通常是一个碳原子连接四个不同的基团。
3. Geometric Isomerism: Cis-Trans | 几何异构：顺反异构
Geometric isomerism is found in molecules with a carbon-carbon double bond because the pi bond prevents rotation around the double bond. If each carbon of the double bond carries two different groups, two distinct spatial arrangements exist.
几何异构存在于含有碳碳双键的分子中，因为 π 键阻止了双键的旋转。如果双键的每个碳原子连接两个不同的基团，就会存在两种不同的空间排列。
For example, in but-2-ene, CH₃CH=CHCH₃, the two methyl groups can be on the same side of the double bond (cis) or on opposite sides (trans).
例如，在丁-2-烯 CH₃CH=CHCH₃ 中，两个甲基可以位于双键的同一侧（顺式），也可以位于相反侧（反式）。
cis-trans naming is only possible when each carbon of the double bond has one group that is the same as a group on the other carbon. If all four substituents are different, the E/Z system must be used.
顺反命名仅在双键的每个碳原子上都有一个与另一个碳原子上相同的基团时才适用。如果四个取代基都不同，则必须使用 E/Z 标记法。
4. E/Z Notation and Cahn-Ingold-Prelog Rules | E/Z 标记法与 CIP 顺序规则
The E/Z system is a more general way to describe geometric isomers. It uses the Cahn-Ingold-Prelog (CIP) priority rules to assign the higher priority group on each carbon of the double bond.
E/Z 体系是一种更通用的描述几何异构体的方法。它使用 Cahn-Ingold-Prelog（CIP）顺序规则来确定双键每个碳原子上优先级较高的基团。
The first rule is simple: the atom directly attached to the double-bond carbon with the higher atomic number has higher priority. For example, in CH₃CH=CHBr, bromine (atomic number 35) has higher priority than hydrogen (atomic number 1).
第一条规则很简单：与双键碳直接相连的原子原子序数越大，优先级越高。例如，在 CH₃CH=CHBr 中，溴（原子序数 35）的优先级高于氢（原子序数 1）。
If the two atoms are the same, move along the chain until a point of difference is found. For example, a -CH₂CH₃ group has higher priority than -CH₃ because the first carbon is the same, but the second carbon has C and H compared with H and H.
如果两个原子相同，就沿着链继续比较，直到找到第一个差异点。例如，-CH₂CH₃ 的优先级高于 -CH₃，因为第一个碳相同，但第二个碳连接的是 C 和 H，而 -CH₃ 连接的是 H 和 H。
If the two highest priority groups are on the same side of the double bond, the isomer is Z (from German zusammen, meaning together). If they are on opposite sides, the isomer is E (from German entgegen, meaning opposite).
如果两个优先级最高的基团位于双键的同一侧，该异构体为 Z（源自德语 zusammen，意为“一起”）。如果它们位于相反侧，该异构体为 E（源自德语 entgegen，意为“相反”）。
5. Conditions for E/Z Isomerism | E/Z 异构的条件
For a molecule to exhibit E/Z isomerism, there must be restricted rotation around a bond, usually a C=C double bond, and each carbon of that bond must be attached to two different groups.
分子要表现出 E/Z 异构，必须存在围绕某个键的受限旋转（通常是 C=C 双键），并且该键的每个碳原子必须连接两个不同的基团。
For example, ethene (CH₂=CH₂) has no geometric isomers because each carbon carries two identical hydrogen atoms. Likewise, propene (CH₃CH=CH₂) has no geometric isomers because one carbon still carries two identical hydrogen atoms.
例如，乙烯（CH₂=CH₂）没有几何异构体，因为每个碳原子连接两个相同的氢原子。同样，丙烯（CH₃CH=CH₂）也没有几何异构体，因为其中一个碳原子仍连接两个相同的氢原子。
Cyclic alkanes can also show cis-trans isomerism because the ring restricts rotation. In 1,2-dimethylcyclohexane, the two methyl groups can be on the same side or on opposite sides of the ring.
环烷烃也可以表现出顺反异构，因为环限制了旋转。在 1,2-二甲基环己烷中，两个甲基可以在环的同一侧或相对侧。
6. Optical Isomerism and Chirality | 光学异构与手性
Optical isomerism arises when a molecule is chiral. A chiral molecule is one that cannot be superimposed on its mirror image. The most common cause of chirality is the presence of an asymmetric carbon atom, also called a chiral centre or stereocentre.
光学异构产生于分子具有手性时。手性分子是指不能与其镜像重合的分子。手性最常见的原因是存在不对称碳原子，也称为手性中心或立体中心。
An asymmetric carbon atom is usually a carbon atom bonded to four different groups. For example, in 2-bromobutane, CH₃CHBrCH₂CH₃, the second carbon is attached to H, CH₃, Br, and C₂H₅, so it is chiral.
不对称碳原子通常是连接四个不同基团的碳原子。例如，在 2-溴丁烷 CH₃CHBrCH₂CH₃ 中，第二个碳分别连接 H、CH₃、Br 和 C₂H₅，因此它是手性的。
The two possible mirror-image forms of a chiral molecule are called enantiomers. Enantiomers are non-superimposable mirror images, just like your left and right hands.
手性分子的两种可能镜像形式称为对映异构体。对映异构体是不可重合的镜像，就像你的左手和右手一样。
7. Drawing Enantiomers in 3D | 绘制对映体的三维结构
In A-Level Chemistry, you must draw optical isomers using wedge and dashed bonds to show the three-dimensional arrangement around a chiral centre. A solid wedge represents a bond coming out of the plane towards you, and a dashed wedge represents a bond going behind the plane.
在 A-Level 化学中，你必须使用楔形和虚线键来表示手性中心周围的三维排列。实心楔形表示化学键朝你伸出平面，虚线楔形表示化学键伸向平面后方。
For example, for CH₃CHBrCH₂CH₃, draw the central carbon with two normal lines for bonds in the plane, one solid wedge for the group coming towards you, and one dashed wedge for the group going away.
例如，对于 CH₃CHBrCH₂CH₃，中心碳用两条普通线表示平面内的键，一个实心楔形表示朝外的基团，一个虚线楔形表示朝内的基团。
The mirror image is drawn by reflecting the arrangement across a mirror plane. The two drawings should look like non-superimposable mirror images, not identical after rotation.
镜像异构体通过镜像平面反射排列来绘制。两个图应看起来像不可重合的镜像，而不是旋转后完全相同的图。
8. Optical Activity and Plane-Polarised Light | 旋光性与平面偏振光
Enantiomers have identical physical properties such as melting point, boiling point and solubility, but they differ in their effect on plane-polarised light. This property is called optical activity.
对映异构体具有相同的物理性质，如熔点、沸点和溶解性，但它们对平面偏振光的影响不同。这种性质称为旋光性。
When plane-polarised light passes through a solution of one enantiomer, the plane of polarisation rotates. One enantiomer rotates the plane clockwise, called the (+) or dextrorotatory form, and the other rotates it anticlockwise, called the (-) or laevorotatory form.
当平面偏振光通过一种对映异构体的溶液时，偏振平面会发生旋转。一种对映异构体使平面顺时针旋转，称为（+）或右旋体；另一种使平面逆时针旋转，称为（-）或左旋体。
The enantiomers rotate light by the same angle but in opposite directions. A polarimeter is used to measure the angle of rotation and the concentration of an optically active sample.
对映异构体使光旋转的角度相同但方向相反。旋光仪用于测量旋转角度和旋光性样品的浓度。
9. Racemic Mixtures and their
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