Optical Isomerism: Chirality and Enantiomers | 光学异构：手性与对映异构体

📚 Optical Isomerism: Chirality and Enantiomers | 光学异构：手性与对映异构体

Optical isomerism is a type of stereoisomerism in which molecules have the same molecular formula and structural formula but differ in the spatial arrangement of their atoms, leading to different interactions with plane-polarised light. It is also called chirality because the most common cause is the presence of a chiral centre, a carbon atom bonded to four different groups.

光学异构是立体异构的一种，指分子具有相同的分子式和结构式，但原子在空间中的排列不同，从而对平面偏振光产生不同影响。由于最常见的原因是一个手性碳原子连接了四个不同的基团，所以这种异构现象也常称为手性。

1. The Chiral Centre | 手性中心

In organic chemistry, a chiral centre (or stereogenic centre) is usually a saturated carbon atom (sp³ hybridised) that is bonded to four different atoms or groups. For example, in 2-hydroxypropanoic acid (lactic acid), the central carbon is attached to a hydrogen atom, a hydroxyl group, a methyl group and a carboxylic acid group. Because these four groups are all different, the carbon is asymmetric.

在有机化学中，手性中心通常是一个饱和碳原子（sp³杂化），它连接着四个不同的原子或基团。例如，在2-羟基丙酸（乳酸）中，中心碳原子连接着一个氢原子、一个羟基、一个甲基和一个羧基。由于这四个基团各不相同，这个碳原子是不对称的。

CH₃–C*(H)(OH)–COOH

The asterisk indicates the chiral centre. A molecule with one chiral centre exists as a pair of enantiomers, which are non-superimposable mirror images of each other.

星号表示手性中心。含有一个手性中心的分子会以一对对映体的形式存在，它们互为不可重叠的镜像。

2. Enantiomers and the Mirror Image Relationship | 对映体与镜像关系

Enantiomers are a pair of molecules that are mirror images of each other but cannot be superimposed, no matter how the models are rotated. The classic analogy is your left and right hands: they are mirror images, yet a left glove does not fit the right hand. Similarly, an enzyme that binds one enantiomer may not bind the other.

对映体是一对互为镜像但无论如何旋转都无法完全重叠的分子。最经典的类比是左右手：它们互为镜像，但左手套不能戴在右手上。同样，能够与一种对映体结合的酶可能无法与另一种对映体结合。

Each enantiomer has the same physical properties such as boiling point and density when measured in an achiral environment. The key difference appears when they interact with plane-polarised light or with other chiral molecules, such as enzymes.

在非手性环境中测量时，每种对映体具有相同的物理性质，如沸点和密度。关键差异出现在它们与平面偏振光或其他手性分子（如酶）相互作用时。

3. Drawing Enantiomers: 3D Representations | 绘制对映体：三维表示法

To draw optical isomers, we use 3D perspective formulae. A normal solid line means a bond in the plane of the paper. A wedge (solid triangle) points out of the paper toward the viewer, and a dashed wedge points away behind the paper. This is essential for showing the tetrahedral arrangement about the chiral centre.

绘制光学异构体时，我们使用三维立体结构式。普通实线表示位于纸平面内的键；楔形实线表示指向纸面外、朝向观察者的键；虚线楔形表示指向纸面内、远离观察者的键。这对于展示手性中心周围的四面体排布至关重要。

For example, the two enantiomers of 2-hydroxypropanoic acid can be drawn as mirror images. If you try to rotate one formula in the plane of the paper, it will never superimpose on the other. You must also be careful: rotating the molecule in 3D space may convert one drawing into another, so the absolute configuration must be assigned by a system such as Cahn–Ingold–Prelog (R/S) or by the direction of rotation of plane-polarised light (+/−).

例如，2-羟基丙酸的两种对映体可以画成镜像。如果在纸平面内旋转一个结构式，它永远无法与另一个重叠。还要注意：在三维空间中旋转分子可能会使一种画法变成另一种，因此必须用如Cahn–Ingold–Prelog（R/S）或平面偏振光旋转方向（+/−）等体系来指定绝对构型。

4. Optical Activity and the Polarimeter | 旋光性与旋光仪

Enantiomers rotate the plane of plane-polarised light by equal amounts but in opposite directions. One enantiomer is said to be dextrorotatory and is labelled (+) because it rotates light to the right (clockwise); the other is laevorotatory and labelled (−) because it rotates light to the left (anticlockwise). This rotation is measured with an instrument called a polarimeter.

对映体能使平面偏振光的偏振面旋转大小相等但方向相反的度数。其中一种对映体称为右旋体，标记为(+)，因为它使光向右（顺时针）旋转；另一种称为左旋体，标记为(−)，因为它使光向左（逆时针）旋转。这种旋转可以用称为旋光仪的仪器来测量。

Observed rotation α = [α] × l × c

In the equation, α is the observed rotation in degrees, [α] is the specific rotation, l is the path length through the sample, and c is the concentration of the solution. The specific rotation is a physical constant for a given enantiomer.

在该公式中，α为观测到的旋光度（度），[α]为比旋光度，l为样品光路长度，c为溶液浓度。比旋光度是给定对映体的物理常数。

A 50:50 mixture of the two enantiomers is called a racemic mixture or racemate. Its rotations cancel out, so the mixture shows no net optical activity.

两种对映体各占50%的混合物称为外消旋混合物或外消旋体。其旋光作用相互抵消，因此该混合物不表现出净旋光性。

5. Racemic Mixtures and Their Formation | 外消旋混合物及其形成

Racemic mixtures are formed whenever a reaction produces a chiral product from an achiral starting material without any chiral influence. For example, if butan-2-one undergoes reduction to butan-2-ol, the new chiral centre is created at a planar carbonyl carbon, and attack by the reducing agent is equally likely from either face. This gives a 50:50 mixture of the two enantiomeric alcohols.

每当一个非手性底物在没有手性影响的情况下生成手性产物时，就会形成外消旋混合物。例如，丁酮被还原为丁-2-醇时，新的手性中心是在平面羰基碳上形成的，还原剂从两面进攻的几率相等，因此得到一对对映体醇的50:50混合物。

Another common example is the addition of HCN to an aldehyde to form a cyanohydrin. The trigonal planar carbonyl group is attacked from either side with equal probability, giving equal amounts of the two enantiomers.

另一个常见例子是醛与HCN加成生成氰醇。三角平面状的羰基从两侧被进攻的几率相等，从而得到等量的两种对映体。

In the laboratory, a racemic mixture is optically inactive. You can distinguish it from a pure enantiomer using a polarimeter: the pure enantiomer rotates light, the racemate does not.

在实验室中，外消旋混合物没有旋光性。可以用旋光仪将其与纯对映体区分开来：纯对映体会使光旋转，而外消旋体不会。

6. Identifying a Chiral Centre | 判断手性中心

To decide whether a molecule can exhibit optical isomerism, look for a carbon atom that is tetrahedral and has four different substituents. A quick method is to check whether the carbon is attached to four different groups and has no plane of symmetry. If the molecule as a whole has an internal plane of symmetry, it may be meso and optically inactive even with chiral centres.

判断一个分子能否显示光学异构，需要寻找四面体结构且连接四个不同取代基的碳原子。一个快速方法是检查该碳是否连接四个不同的基团，并且整个分子没有对称面。如果整个分子具有内部对称面，即使含有手性中心，也可能是内消旋体，没有旋光性。

Be alert: a molecule with a plane of symmetry, such as 2,3-dichlorobutane in the meso form, does not rotate plane-polarised light even though it has chiral centres, because the two halves are mirror images and the rotation cancels internally.

要注意：具有对称面的分子，如meso-2,3-二氯丁烷，即使含有手性中心也不旋转平面偏振光，因为分子的两半互为镜像，旋光性在分子内部相互抵消。

7. Importance in Biological Systems | 在生物体系中的重要性

Many biologically important molecules are chiral, including amino acids and sugars. In nature, proteins are built almost exclusively from L-amino acids, and DNA helices are right-handed. Enzymes, being chiral, usually catalyse reactions of only one enantiomer of a molecule.

许多具有重要生物功能的分子是手性的，包括氨基酸和糖类。在自然界中，蛋白质几乎全部由L-氨基酸组成，DNA螺旋是右手性的。酶本身是手性的，通常只催化某一种对映体的反应。

This selectivity is crucial for pharmaceutical action. A chiral drug’s two enantiomers may have very different effects in the body. The classic example is thalidomide: the (+)-enantiomer was an effective sedative, while the (−)-enantiomer was responsible for severe birth defects. Because the drug was used as a racemic mixture, both effects were observed.

这种选择性对药物作用至关重要。手性药物的两种对映体在体内可能产生截然不同的效果。经典例子是沙利度胺：(+)-对映体是有效的镇静剂，而(−)-对映体则导致严重的胎儿畸形。由于该药以外消旋混合物的形式使用，因此两种效应都被观察到。

8. Separation and Analysis of Enantiomers | 对映体的分离与分析

Pure enantiomers are not easy to separate from a racemic mixture because they have identical boiling points, solubility and melting points in an achiral environment. Methods such as distillation or recrystallisation do not work. Instead, a technique called chiral resolution is used, often by converting the enantiomers into diastereomers using a chiral resolving agent, which can then be separated by fractional crystallisation.

纯粹的对映体很难从外消旋混合物中分离，因为在非手性环境中它们具有相同的沸点、溶解度和熔点。蒸馏或重结晶等方法不起作用。通常采用所谓的手性拆分技术，即用手性拆分剂将对映体转化为非对映异构体，然后通过分步结晶等方法来分离。

Alternatively, chiral chromatography can be used with a stationary phase that has a chiral ligand. One enantiomer interacts more strongly with the stationary phase and moves more slowly, allowing separation.

或者，可以使用具有手性配体的固定相进行手性色谱分离。其中一种对映体与固定相相互作用更强，移动较慢，从而实现分离。

9. Common Exam Mistakes | 常见考试错误

Examiners often report that students confuse optical isomerism with geometrical isomerism. Optical isomerism arises from a tetrahedral chiral centre, whereas geometrical (E/Z) isomerism arises from restricted rotation around a C=C double bond. Also, do not say that a racemic mixture contains equal amounts of “rotating left and right molecules” – instead say it contains equal amounts of the two enantiomers, so rotations cancel.

考官常报告学生将光学异构与几何异构混淆。光学异构来源于四面体手性中心，而几何（E/Z）异构来源于C=C双键周围旋转受阻。此外，不要说外消旋混合物含有等量“左旋和右旋分子”，而应说它含有等量的两种对映体，因此旋光性相互抵消。

Another common mistake is drawing the chiral centre without showing all four bonds as different. Always label the chiral centre with an asterisk, and if asked to draw the enantiomer, draw the mirror image rather than just rotating the molecule.

另一个常见错误是画手性中心时没有把四个键画成不同。始终用星号标出手性中心，若要求画对映体，应画出镜像而不是仅仅旋转分子。

10. Summary and Revision Points | 总结与复习要点

Optical isomerism occurs when molecules have an asymmetric tetrahedral carbon bonded to four different groups. Such molecules exist as two non-superimposable mirror-image enantiomers. A pure enantiomer rotates plane-polarised light; a racemic mixture, with equal amounts of each enantiomer, is optically inactive.

光学异构发生在不对称四面体碳连接四个不同基团时。这类分子以一对不可重叠的对映体存在。纯对映体旋转平面偏振光；而含有等量两种对映体的外消旋混合物没有旋光性。

Identify chiral centres: look for sp³ carbon with four different groups.

判断手性中心：寻找四个不同基团连接的sp³碳。
Distinguish enantiomers by drawing mirror images and checking superimposability.

通过绘制镜像并检查是否可重叠来区分对映体。
Know that a racemate results from reactions at planar carbonyl groups without chiral influence.

了解外消旋体是平面羰基反应中无手性影响时形成的。
Remember biological significance: enzymes are chiral and usually act on one enantiomer only.

记住生物学意义：酶是手性的，通常只作用于一种对映体。

For AQA A-level Chemistry, you should be able to draw 3D formulae, identify chiral centres, predict the formation of racemates from carbonyl addition reactions, and explain why optical activity matters in drug action.

对于AQA A-level化学，你应该能够绘制三维立体结构式、判断手性中心、预测羰基加成反应中外消旋体的形成，并解释旋光性在药物作用中的重要性。

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Optical Isomerism: Chirality and Enantiomers | 光学异构：手性与对映异构体