Stereoisomerism: Types and Characteristics | 立体异构体的类型与特征

📚 Stereoisomerism: Types and Characteristics | 立体异构体的类型与特征

In organic chemistry, isomers are compounds with the same molecular formula but different structural arrangements. Stereoisomers are a special class of isomers where atoms are connected in the same order but arranged differently in three-dimensional space. This concept is fundamental to understanding molecular behaviour in IB Chemistry HL.

在有机化学中，异构体是指具有相同分子式但结构排列不同的化合物。立体异构体是一类特殊的异构体，其原子连接顺序相同，但在三维空间中的排列方式不同。这一概念是理解IB化学HL中分子行为的基础。

Unlike structural isomers, which differ in how atoms are bonded together, stereoisomers share identical bond connectivity. Their differences arise purely from the spatial orientation of atoms or groups. This seemingly subtle distinction leads to dramatically different chemical, physical, and biological properties.

与结构异构体（原子间成键方式不同）不同，立体异构体具有完全相同的键连接方式。它们的差异纯粹源于原子或基团在空间中的取向不同。这种看似微妙的差异会导致化学、物理和生物性质产生天壤之别。

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1. Classification of Isomers | 异构体的分类

Isomers are broadly divided into two main categories: structural isomers and stereoisomers. Structural isomers differ in the connectivity of atoms, while stereoisomers share the same connectivity but differ in spatial arrangement. Stereoisomers are further classified into conformational isomers and configurational isomers.

异构体大致分为两大类：结构异构体和立体异构体。结构异构体的原子连接方式不同，而立体异构体具有相同的连接方式但在空间排列上存在差异。立体异构体又进一步分为构象异构体和构型异构体。

Configurational isomers can only interconvert by breaking and reforming covalent bonds. This category includes geometrical (cis-trans) isomers and optical isomers. Conformational isomers, on the other hand, interconvert through rotation about single bonds and are generally less stable as distinct species.

构型异构体只能通过断裂和重新形成共价键才能相互转化。该类别包括几何（顺反）异构体和光学异构体。构象异构体则可以通过绕单键旋转而相互转化，通常难以作为稳定而独立的物种存在。

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2. What Are Stereoisomers? | 什么是立体异构体？

Stereoisomers are molecules that have the same molecular formula and the same sequence of bonded atoms, but differ in the three-dimensional orientation of their atoms in space. The word “stereo” comes from Greek meaning “solid” or “three-dimensional,” emphasising the spatial nature of these differences.

立体异构体是具有相同分子式和相同原子连接顺序，但原子在三维空间中的取向不同的分子。”stereo”一词源自希腊语，意为”立体”或”三维”，强调了这些差异的空间特性。

For IB Chemistry HL, the two key types of stereoisomers you must master are geometrical (cis-trans) isomers and optical isomers. Both arise from restricted rotation or the presence of chiral centres, respectively, and each exhibits unique characteristics that affect molecular properties.

对于IB化学HL，你必须掌握的两种关键立体异构体是几何（顺反）异构体和光学异构体。它们分别源于受限旋转或手性中心的存在，各自展现出影响分子性质的独特特征。

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3. Geometrical Isomerism | 几何异构

Geometrical isomerism, also known as cis-trans isomerism, arises when there is restricted rotation around a bond, typically a carbon-carbon double bond (C=C) or in cyclic compounds. The restriction prevents the interconversion of the two forms without breaking the bond.

几何异构，也称为顺反异构，产生于键的旋转受限时，典型情况是碳碳双键（C=C）或环状化合物中。这种限制使得两种形式在不断裂化学键的情况下无法相互转化。

In a C=C double bond, the p orbitals overlap sideways to form a pi bond, which locks the molecule in a planar configuration. This prevents free rotation. When each carbon atom of the double bond bears two different substituents, two distinct isomers can exist.

在C=C双键中，p轨道侧面重叠形成π键，将分子锁定在平面构型中，从而阻止了自由旋转。当双键的每个碳原子连接两个不同的取代基时，可以存在两种不同的异构体。

cis-isomer: identical or priority groups on the same side of the double bond.
trans-isomer: identical or priority groups on opposite sides of the double bond.

Cis: 相同或优先基团在双键同侧
Trans: 相同或优先基团在双键异侧

For example, but-2-ene exists as cis-but-2-ene and trans-but-2-ene. In cis-but-2-ene, the two methyl groups are on the same side; in trans-but-2-ene, they are on opposite sides. These isomers have different boiling points, polarities, and densities.

例如，2-丁烯存在顺式-2-丁烯和反式-2-丁烯两种形式。在顺式-2-丁烯中，两个甲基位于同侧；在反式-2-丁烯中，它们位于异侧。这两种异构体具有不同的沸点、极性和密度。

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4. Conditions for Geometrical Isomerism | 几何异构的产生条件

Not every molecule with a double bond exhibits geometrical isomerism. Two essential conditions must be satisfied for cis-trans isomerism to be observed.

并非每个含有双键的分子都会表现出几何异构。要观察到顺反异构现象，必须满足两个基本条件。

• Condition 1: Restricted rotation — There must be a bond that does not allow free rotation, such as a C=C double bond or a ring structure.

  条件1：受限旋转——必须存在不允许自由旋转的键，如C=C双键或环状结构。

• Condition 2: Two different groups on each carbon — Each carbon atom involved in the restricted bond must bear two different substituents. If a carbon has two identical groups, no geometrical isomerism is possible.

  条件2：每个碳上连有两个不同基团——参与受限键的每个碳原子必须连接两个不同的取代基。如果某个碳连接两个相同基团，则不可能产生几何异构。

Take 1,2-dichloroethene as an example. Both carbon atoms have H and Cl attached, satisfying condition 2. Thus, cis-1,2-dichloroethene and trans-1,2-dichloroethene exist. In contrast, 1,1-dichloroethene has one carbon bearing two identical chlorine atoms, so it cannot exhibit geometrical isomerism.

以1,2-二氯乙烯为例。两个碳原子都连接着H和Cl，满足条件2。因此，存在顺式-1,2-二氯乙烯和反式-1,2-二氯乙烯。相比之下，1,1-二氯乙烯的一个碳原子上连接着两个相同的氯原子，因此不能表现出几何异构。

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5. The E-Z Notation System | E-Z标记系统

The cis-trans system is limited to molecules with two identical substituent pairs. For more complex molecules, particularly alkenes with three or four different substituents, the E-Z notation system is used. This system is based on the Cahn-Ingold-Prelog priority rules.

顺反体系仅适用于具有两个相同取代基对的分子。对于更复杂的分子，特别是含有三个或四个不同取代基的烯烃，需要采用E-Z标记系统。该系统基于Cahn-Ingold-Prelog优先规则。

Z-isomer (Zusammen, German for “together”): the higher-priority groups are on the same side of the double bond.

Z-异构体（Zusammen，德语意为”一起”）：较高优先级的基团位于双键的同侧。

E-isomer (Entgegen, German for “opposite”): the higher-priority groups are on opposite sides of the double bond.

E-异构体（Entgegen，德语意为”相反”）：较高优先级的基团位于双键的两侧。

The priority of substituents is determined by the atomic number of the atom directly attached to the double-bond carbon. The higher the atomic number, the higher the priority. If the first atoms are the same, you compare the next atoms along the chain until a difference is found.

取代基的优先级由与双键碳直接相连的原子的原子序数决定。原子序数越大，优先级越高。如果第一个原子相同，则沿着碳链比较下一个原子，直到找到差异为止。

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6. Physical Properties of Geometrical Isomers | 几何异构体的物理性质

Cis and trans isomers often display markedly different physical properties. The cis isomer typically has a higher boiling point and greater solubility in polar solvents, while the trans isomer tends to have a higher melting point.

顺式和反式异构体通常表现出截然不同的物理性质。顺式异构体通常具有更高的沸点和在极性溶剂中更大的溶解度，而反式异构体往往具有更高的熔点。

Property	cis-isomer | 顺式异构体	trans-isomer | 反式异构体
Dipole moment | 偶极矩	Non-zero (polar) | 非零（有极性）	Zero or small (less polar) | 为零或较小（极性较小）
Boiling point | 沸点	Generally higher | 通常较高	Generally lower | 通常较低
Melting point | 熔点	Generally lower | 通常较低	Generally higher | 通常较高
Stability | 稳定性	Less stable (steric hindrance) | 稳定性较低（位阻效应）	More stable | 稳定性更高

The higher dipole moment of cis isomers leads to stronger intermolecular forces, raising their boiling points. Trans isomers pack more efficiently in the solid state due to their symmetric shape, leading to higher melting points. Understanding these trends is essential for predicting and explaining physical data in examinations.

顺式异构体较高的偶极矩导致更强的分子间作用力，从而提高其沸点。反式异构体由于结构对称，在固态中排列更紧密，因此具有更高的熔点。理解这些规律对于在考试中预测和解释物理数据至关重要。

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7. Optical Isomerism and Chirality | 光学异构与手性

Optical isomerism is a second major type of stereoisomerism, arising from the presence of a chiral centre in a molecule. A chiral centre is a carbon atom bonded to four different groups or atoms. The molecule and its mirror image are non-superimposable, just like your left and right hands.

光学异构是立体异构的第二大类，源于分子中存在手性中心。手性中心是连接四个不同基团或原子的碳原子。该分子与其镜像不能重叠，就像你的左手和右手一样。

Optical isomers, called enantiomers, exist as pairs of mirror-image molecules. A molecule that is not superimposable on its mirror image is said to be chiral (from the Greek “cheir,” meaning hand). Molecules that are superimposable on their mirror image are called achiral.

光学异构体称为对映异构体，以互为镜像的分子对形式存在。不能与其镜像重叠的分子称为手性分子（源自希腊语”cheir”，意为”手”）。能与镜像重叠的分子称为非手性分子。

Chiral carbon = sp³ carbon with four different substituents
手性碳 = 连接四个不同取代基的sp³碳

For IB HL, you should be able to identify chiral centres in molecules, draw the two enantiomers using wedge-and-dash notation, and explain why they are non-superimposable mirror images.

对于IB HL，你应该能够识别分子中的手性中心，使用楔形-虚线表示法绘制两种对映异构体，并解释为什么它们是不可重叠的镜像。

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8. Properties of Enantiomers | 对映异构体的性质

Enantiomers share identical physical properties in an achiral environment: the same melting point, boiling point, density, and solubility. They also have identical chemical reactivity toward achiral reagents. However, they differ in two crucial aspects: their interaction with plane-polarised light and their behaviour toward other chiral species.

对映异构体在非手性环境中具有完全相同的物理性质：相同的熔点、沸点、密度和溶解度。它们与非手性试剂的化学反应活性也相同。然而，它们在两个关键方面存在差异：与平面偏振光的相互作用以及与其他手性物质的反应行为。

One enantiomer rotates plane-polarised light clockwise (dextrorotatory, designated + or d), while the other rotates it counterclockwise (laevorotatory, designated − or l). A 50:50 mixture of enantiomers, called a racemic mixture, shows no net optical rotation.

一个对映异构体使平面偏振光顺时针旋转（右旋，标记为+或d），另一个使其逆时针旋转（左旋，标记为−或l）。由对映异构体以50:50比例组成的混合物称为外消旋混合物，不显示净旋光性。

In biological systems, enantiomers can have dramatically different effects. For example, one enantiomer of limonene smells like oranges, while the other smells like lemons. Thalidomide is a tragic historical example: one enantiomer was effective as a sedative, while the other caused severe birth defects.

在生物系统中，对映异构体可能具有截然不同的效果。例如，柠檬烯的一个对映异构体闻起来像橘子，而另一个闻起来像柠檬。沙利度胺（反应停）是一个悲惨的历史例证：一种对映异构体有效作为镇静剂，而另一种则导致严重的出生缺陷。

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9. Identifying Chiral Centres | 识别手性中心

To determine whether a molecule is chiral, follow these systematic steps. First, draw the full structural formula. Second, identify all carbon atoms that are bonded to four different groups. Third, verify that the carbon has four distinct substituents — if any two groups are identical, the carbon is not chiral.

要判断一个分子是否具有手性，请遵循以下系统步骤。首先，画出完整的结构式。其次，找出所有连接四个不同基团的碳原子。第三，确认该碳确实连接着四个不同的取代基——如果有任何两个基团相同，该碳就不是手性碳。

A carbon atom in a double bond or a triple bond cannot be chiral because it is not bonded to four separate groups. Similarly, a CH₂ carbon with two hydrogen atoms can never be a chiral centre. Carbon atoms in symmetrical molecules often lack chirality even if they have four different groups.

双键或三键中的碳原子不可能是手性的，因为它没有连接四个独立的基团。类似地，带有两个氢原子的CH₂碳永远不可能是手性中心。对称分子中的碳原子即使连接四个不同基团，也往往不具有手性。

Key indicators that a molecule is NOT chiral:

• The molecule has a plane of symmetry.

  分子具有对称平面。

• It is identical to its mirror image.

  它与镜像完全相同。

• It contains no carbon atom with four different substituents.

  它不含连接四个不同取代基的碳原子。

For example, 2-bromobutane has a chiral centre at carbon 2, which bears H, CH₃, C₂H₅, and Br. It exists as two enantiomers. In contrast, 2-bromopropane has two identical methyl groups on the central carbon, so it is achiral.

例如，2-溴丁烷在碳2处有一个手性中心，该碳连接着H、CH₃、C₂H₅和Br四个不同基团，因此存在两种对映异构体。相比之下，2-溴丙烷的中心碳连接两个相同的甲基，因此是非手性的。

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10. Drawing Enantiomers: Wedge-and-Dash Notation | 绘制对映异构体：楔形-虚线表示法

In structural diagrams, the wedge-and-dash notation is used to represent three-dimensional geometry on a two-dimensional page. A solid wedge indicates a bond coming out of the plane of the page toward you. A dashed wedge indicates a bond going behind the plane of the page away from you. A straight line indicates a bond in the plane of the page.

在结构图中，楔形-虚线表示法用于在二维纸面上表达三维几何形状。实心楔形表示化学键从纸面向外伸出朝向观察者。虚线楔形表示化学键向纸面内部延伸远离观察者。直线表示化学键位于纸面平面内。

To draw a pair of enantiomers, place the chiral carbon at the centre. Arrange the four substituents around it with wedge, dash, and line bonds. Then draw the mirror image of the entire structure, which effectively inverts the wedge and dash positions. The two structures will be non-superimposable.

要绘制一对对映异构体，将手性碳放在中心位置。用楔形键、虚线键和直线键将四个取代基排列在周围。然后画出整个结构的镜像，这实际上反转了楔形和虚线的位置。两个结构将不可重叠。

We use the R/S notation (also based on Cahn-Ingold-Prelog priority rules) to label the absolute configuration of chiral centres. If the priority groups 1→2→3 are arranged clockwise, the configuration is R (rectus, right). If counterclockwise, it is S (sinister, left). The R and S enantiomers are mirror images of each other.

我们使用R/S标记系统（同样基于Cahn-Ingold-Prelog优先规则）来标注手性中心的绝对构型。如果优先基团1→2→3按顺时针排列，构型为R（rectus，右）。如果按逆时针排列，则为S（sinister，左）。R和S对映异构体互为镜像。

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11. Optical Activity and Plane-Polarised Light | 旋光性与平面偏振光

Optical activity is the ability of a chiral substance to rotate the plane of plane-polarised light. Plane-polarised light consists of light waves oscillating in a single plane. When this light passes through a solution containing a single enantiomer, the plane of polarisation rotates by a specific angle.

旋光性是手性物质使平面偏振光的偏振面发生旋转的能力。平面偏振光是在单一平面内振动的光波。当这种光通过含有单一对映异构体的溶液时，偏振面会旋转特定角度。

The observed rotation angle (α) depends on several factors: the concentration of the solution, the path length of the cell, the nature of the solvent, the temperature, and the wavelength of light used. The specific rotation [α] is a characteristic physical constant for a given enantiomer at a specified temperature and wavelength.

观测到的旋光角（α）取决于多个因素：溶液浓度、样品池光程长度、溶剂性质、温度和所用光波长。比旋光度[α]是特定对映异构体在一定温度和波长下的特征物理常数。

A racemic mixture (equimolar mixture of two enantiomers) produces no net rotation because the two enantiomers rotate light by equal and opposite amounts. This is why drug manufacturers must ensure that their products contain the desired enantiomer, not a racemic mixture.

外消旋混合物（两种对映异构体的等摩尔混合物）不产生净旋光，因为两种对映异构体使光旋转的角度相等但方向相反。这就是为什么药物制造商必须确保其产品含有所需的对映异构体，而不是外消旋混合物。

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12. Stereoisomerism in IB Chemistry HL Exams | IB化学HL考试中的立体异构

In IB Chemistry HL examinations, stereoisomerism frequently appears in both Paper 1 (multiple choice) and Paper 2 (short answer and extended response). Common assessment objectives include identifying chiral centres, predicting the number of stereoisomers, drawing cis/trans and optical isomers, and explaining differences in physical properties.

在IB化学HL考试中，立体异构经常出现在试卷1（选择题）和试卷2（简答题和扩展题）中。常见的考查目标包括：识别手性中心、预测立体异构体的数量、绘制顺反异构体和光学异构体，以及解释物理性质的差异。

Key formulas to remember: A molecule with n chiral centres can have up to 2ⁿ stereoisomers. For example, a molecule with two chiral centres can have up to 4 stereoisomers. These include pairs of enantiomers and, when possible, diastereomers (stereoisomers that are not mirror images).

需要记住的关键公式：含有n个手性中心的分子最多可以有2ⁿ个立体异构体。例如，含有两个手性中心的分子最多可以有4个立体异构体。这些异构体包括对映异构体对，以及在可能情况下的非对映异构体（不互为镜像的立体异构体）。

When answering exam questions, always explicitly mention restricted rotation for geometrical isomers and non-superimposable mirror images for optical isomers. Use correct terminology: enantiomers, racemic mixture, chiral centre, plane-polarised light, and specific rotation. Drawing structures accurately with wedge-and-dash notation is essential for full marks.

回答考试问题时，始终明确提到几何异构中的受限旋转和光学异构中的不可重叠镜像。使用正确的术语：对映异构体、外消旋混合物、手性中心、平面偏振光和比旋光度。使用楔形-虚线表示法准确绘制结构对于获得满分至关重要。

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