📚 Optical Isomers | 光学异构体
Optical isomerism is a form of stereoisomerism in which molecules possess the same molecular formula and the same connectivity of atoms, yet they are non‑superimposable mirror images of one another. These isomers behave identically in almost all physical and chemical properties except for their interaction with plane‑polarised light and their behaviour in chiral environments. Understanding optical isomerism is essential for explaining the three‑dimensional architecture of organic molecules and the specific biological activity of many drugs, enzymes, and natural products.
光学异构现象是立体异构的一种形式，分子具有相同的分子式和相同的原子连接方式，但却互为不可重叠的镜像。这些异构体在几乎所有的物理和化学性质上都表现相同，唯独在平面偏振光中的行为以及手性环境下的反应不同。理解光学异构对于解释有机分子的三维结构以及许多药物、酶和天然产物的特定生物活性至关重要。
1. Introduction to Optical Isomerism | 光学异构现象简介
Optical isomerism was first discovered in the early 19th century when the French physicist Jean‑Baptiste Biot observed that certain organic substances, such as sugar solutions or tartaric acid crystals, could rotate the plane of polarised light. Later, Louis Pasteur famously separated crystals of sodium ammonium tartrate by hand and found that the two types of crystals rotated polarised light in opposite directions, providing the first evidence that molecular asymmetry – chirality – is responsible for optical activity.
光学异构现象最早于19世纪初由法国物理学家让‑巴蒂斯特·比奥发现，他观察到某些有机物（如糖溶液或酒石酸晶体）能使偏振光的平面发生旋转。后来，路易·巴斯德通过手工分离酒石酸铵钠晶体并发现两种晶型使偏振光向相反方向旋转，首次证明了分子不对称性——手性——是产生旋光性的原因。
2. Chirality and Chiral Centres | 手性及手性中心
A molecule is chiral if it is not superimposable on its mirror image. The most common source of chirality in organic chemistry is the presence of a carbon atom bonded to four different groups. Such a carbon atom is called a chiral centre, stereocentre, or asymmetric carbon, often marked with an asterisk (C*). If a molecule contains one chiral centre, it will exist as a pair of optical isomers known as enantiomers.
如果一个分子与它的镜像不能重叠，这个分子就是手性的。有机化学中最常见的手性来源是存在一个连接了四个不同基团的碳原子。这样的碳原子被称为手性中心、立体中心或不对称碳，常以星号（C*）标记。如果一个分子含有一个手性中心，它将以一对光学异构体即对映异构体的形式存在。
For example, in 2‑hydroxypropanoic acid (lactic acid), the central carbon is attached to –H, –OH, –CH₃, and –COOH, making it chiral. Molecules like CHFClBr are also chiral because four different substituents are attached to carbon.
例如，在2‑羟基丙酸（乳酸）中，中心碳原子连接了–H、–OH、–CH₃和–COOH，因此是手性的。像CHFClBr这样的分子同样因连接四个不同取代基而具有手性。
3. Enantiomers: Mirror Images | 对映异构体：镜像关系
Enantiomers are pairs of optical isomers that are non‑superimposable mirror images of each other. They have identical physical properties such as melting point, boiling point, and solubility, and identical chemical reactivity with achiral reagents. However, enantiomers rotate the plane of plane‑polarised light in opposite directions: one enantiomer will rotate the light clockwise (dextrorotatory, designated as + or d), while the other rotates it anticlockwise (laevorotatory, designated as − or l).
对映异构体是互为不可重叠镜像的一对光学异构体。它们具有相同的物理性质，如熔点、沸点和溶解度，以及与无手性试剂的相同化学反应活性。但对映异构体使平面偏振光的偏振面旋转方向相反：一种对映体使光顺时针旋转（右旋，以+或d表示），另一种则使光逆时针旋转（左旋，以−或l表示）。
It is crucial to understand that the direction of optical rotation (+) or (−) does not directly tell us the actual spatial arrangement of the groups around the chiral centre; this is determined by the R/S naming system, which is independent of the sign of rotation.
必须理解，旋光方向（+）或（−）并不能直接告诉我们手性中心周围基团的实际空间排列；这是由R/S命名系统决定的，与旋光符号无关。
4. Plane‑Polarised Light and Optical Activity | 平面偏振光与旋光性
Ordinary light vibrates in all planes perpendicular to its direction of travel. When it passes through a polarising filter, only light waves vibrating in a single plane emerge; this is called plane‑polarised light. An optically active substance is one that can rotate the plane of this polarised light. The ability to rotate plane‑polarised light is known as optical activity and is measured using a polarimeter.
普通光在垂直于传播方向的所有平面上振动。当它通过一个偏振滤光片时，只有一个平面上振动的光波能够通过，这就是平面偏振光。具有光学活性的物质能够使此偏振光的平面发生旋转。旋转平面偏振光的能力称为旋光性，通过旋光仪来测量。
The rotation occurs because the two enantiomers interact differently with the electric field of the polarised light, with one slowing the left‑handed circularly polarised component more than the right, leading to a net rotation of the resultant plane.
旋转的发生是因为两种对映体与偏振光电场的作用方式不同，其中一种使左旋圆偏振分量的速度减慢得比右旋分量更多，从而导致合成平面的净旋转。
5. Measuring Optical Rotation: The Polarimeter | 旋光性的测量：旋光仪
A polarimeter consists of a light source (usually a sodium lamp producing D‑line light at 589 nm), a polarising filter, a sample tube to hold the chiral substance in solution, an analysing filter, and a scale to read the angle of rotation. When the sample tube is empty or filled with an achiral solvent, the light passes through and is extinguished at 0°. A chiral sample rotates the plane, so the analyser must be turned by an angle α (observed rotation) to restore the dark field.
旋光仪由光源（通常为产生589 nm D线光的钠灯）、一个偏振滤光片、盛装手性物质溶液的样品管、一个检偏滤光片以及读取旋转角度的刻度盘组成。当样品管为空或装有无手性溶剂时，光直接通过，在0°时消光。手性样品使偏振面旋转，因此必须将检偏器转动一个角度α（观察旋光度）才能恢复暗场。
The observed rotation α depends on the concentration of the sample, the length of the tube, the temperature, and the wavelength of light used. To standardise comparison, chemists use specific rotation, symbolised as [α].
观察旋光度α取决于样品浓度、样品管长度、温度以及所用光的波长。为了便于标准化比较，化学家使用比旋光度，符号为[α]。
6. Specific Rotation and its Calculation | 比旋光度及其计算
The specific rotation of a chiral compound is defined by the equation:
手性化合物的比旋光度由下式定义：
[α]ᵀ λ = α / (l × c)
where α is the observed rotation (in degrees), l is the path length of the sample tube (in dm), and c is the concentration of the solution (in g mL⁻¹). The temperature T (usually 20°C) and the wavelength λ (usually the sodium D line) are specified as [α]D²⁰. Specific rotation is a characteristic physical constant for a pure enantiomer.
其中α为观察旋光度（度），l为样品管长度（dm），c为溶液浓度（g mL⁻¹）。温度T（通常20°C）和波长λ（通常为钠D线）在[α]D²⁰中指定。比旋光度是纯对映体的一个特征物理常数。
If a sample is not enantiomerically pure, the observed rotation will differ from the literature value of the pure enantiomer, allowing the enantiomeric excess (ee) to be calculated:
如果样品不是对映体纯的，测得的旋光度将与纯对映体文献值不同，从而可以计算对映体过量（ee）：
ee(%) = (|[α]observed| / |[α]pure|) × 100%
7. Racemic Mixtures and Resolution | 外消旋混合物与外消旋体的拆分
A racemic mixture (racemate) contains equal amounts of both enantiomers of a chiral compound. Because the rotations of the two enantiomers cancel each other out, a racemic mixture is optically inactive. Racemates are often produced in reactions where there is no chiral influence, such as in the SN1 reaction at a chiral centre or in laboratory syntheses starting from achiral reagents.
外消旋混合物（外消旋体）含有等量的手性化合物的两种对映体。由于两个对映体的旋光相互抵消，外消旋混合物没有光学活性。在手性影响缺失的反应中，如在手性中心的SN1反应中，或使用无手性试剂的实验室合成中，常会生成外消旋体。
Separating a racemic mixture into its individual enantiomers is called resolution. Common methods include using chiral resolving agents (forming diastereomeric salts that have different solubilities), chiral chromatography, and enzymatic resolution.
将外消旋混合物分离成单一对映体的过程称为拆分。常见方法包括使用手性拆分剂（形成溶解性不同的非对映异构体盐）、手性色谱法和酶法拆分。
8. Drawing Enantiomers: Fischer Projections | 绘制对映异构体：费歇尔投影式
Fischer projections are a convenient two‑dimensional representation of chiral molecules. The chiral carbon is placed at the centre of a cross; the horizontal lines represent bonds that project out of the plane toward the viewer, while the vertical lines represent bonds that project away from the viewer. By convention, the carbon chain is drawn vertically with the most oxidised carbon at the top.
费歇尔投影式是表示手性分子的一种方便二维画法。手性碳位于十字交叉的中心；水平线代表朝向观察者伸出平面的键，垂直线代表背向观察者伸入平面的键。按照惯例，碳链竖画，氧化态最高的碳位于顶端。
When drawing enantiomers using Fischer projections, simply swapping any two groups on the chiral centre generates the opposite enantiomer. If two swaps are made, the original configuration is restored. Fischer projections are particularly useful for comparing configurations of sugars and amino acids.
当用费歇尔投影式绘制对映体时，只需交换手性中心上的任意两个基团，即可获得相反的对映体。如果交换两次，则恢复原来的构型。费歇尔投影式在比较糖类和氨基酸的构型时尤其有用。
9. The Cahn–Ingold–Prelog (CIP) Priority Rules | Cahn–Ingold–Prelog (CIP) 顺序规则
To assign the absolute configuration of a chiral centre unambiguously, the Cahn–Ingold–Prelog (CIP) system uses a set of priority rules. The atoms directly attached to the chiral centre are ranked according to atomic number: the higher the atomic number, the higher the priority. If two atoms have the same atomic number, we move to the next set of atoms until a difference is found.
为了明确确定手性中心的绝对构型，Cahn–Ingold–Prelog (CIP) 系统使用了一组优先规则。直接连接在手性中心上的原子按原子序数排序：原子序数越大，优先顺序越高。如果两个原子具有相同的原子序数，我们继续向下一组原子比较，直至找到差异。
d as if the carbon is attached to two oxygen atoms by single bonds. Isotopes are prioritised by mass number, with heavier isotopes having higher priority.
多重键被视同该原子与同种原子以等数量的单键相连。例如，羰基C=O被视作碳原子与两个氧原子以单键连接。同位素按质量数排序，较重的同位素优先顺序更高。
10. Assigning R/S Configuration | R/S 构型的确定
chiral centre are prioritised (1 > 2 > 3 > 4), the molecule is oriented so that the lowest‑ clockwise, the absolute configuration is R (from the Latin rectus, right). If the path is anticlockwise, the configuration is S (from Latin sinister, left).
一旦手性中心上四个取代基的优先顺序得到确定（1 > 2 > 3 > 4），将分子取向使最低优先的基团（4）远离观察者。然后观察1→2→3的旋转方向。若为顺时针，绝对构型为R（来自拉丁文 rectus，右）；若为逆时针，则构型为S（来自拉丁文 sinister，左）。
When using Fischer projections, if the lowest‑ an be read directly. If the lowest group is on a horizontal line, the observed direction must be inverted to obtain the correct R/S assignment.
在使用费歇尔投影式时，如果最低优先基团位于竖直线上（指向纸后），可直接读取1→2→3的旋转方向。若最低基团位于水平线上，观察到的方向必须反转才能获得正确的R/S归属。
11. Diastereomers and Meso Compounds | 非对映异构体和内消旋化合物
Stereoisomers that are not mirror images of each other are called diastereomers. Unlike enantiomers, diastereomers have different physical properties (melting point, boiling point, solubility) and can be separated by conventional techniques such as fractional crystallisation or distillation. Molecules with two or more chiral centres can give rise to diastereomers as well as enantiomers.
互为非镜像的立体异构体被称为非对映异构体。与对映体不同，非对映异构体具有不同的物理性质（熔点、沸点、溶解度），可以通过分级结晶或蒸馏等常规技术分离。具有两个或更多手性中心的分子可以同时产生非对映异构体和对映体。
A meso compound is an achiral member of a set of diastereomers that contains chiral centres. Meso compounds possess an internal plane of symmetry, which makes the molecule as a whole superimposable on its mirror image. For example, tartaric acid has two chiral centres; the (2R,3S) isomer is an optically inactive meso form, while the (2R,3R) and (2S,3S) isomers are the enantiomeric pair.
内消旋化合物是一组含有手性中心的非对映异构体中的非手性成员。内消旋化合物具有内部对称面，使整个分子与其镜像可以重叠。例如，酒石酸有两个手性中心；(2R,3S) 异构体是光学惰性的内消旋体，而 (2R,3R) 和 (2S,3S) 异构体则是对映体对。
12. Optical Isomerism in Biological Systems and Pharmaceuticals | 生物系统及药物中的光学异构现象
Biological systems are highly chiral; enzymes, receptors, and many signalling molecules are composed of L‑amino acids and D‑sugars. Because the active sites of enzymes and receptors are chiral, they can distinguish between enantiomers. Consequently, the two enantiomers of a drug can exhibit drastically different pharmacological effects – one may be therapeutic while the other may be inactive or even toxic.
生物系统是高度手性的；酶、受体和许多信号分子由L‑氨基酸和D‑糖组成。由于酶和受体的活性位点具有手性，它们能够区辨对映体。因此，一种药物的两种对映体可能呈现出截然不同的药理效应——一种具有疗效，另一种可能无活性甚至有毒。
A well‑known example is thalidomide: one enantiomer (R‑thalidomide) was an effective sedative, while the other (S‑thalidomide) caused severe birth defects. This tragic case highlights why the pharmaceutical industry now rigorously tests and often markets single‑enantiomer drugs instead of racemic mixtures.
一个著名的例子是沙利度胺：一种对映体（R‑thalidomide）是有效的镇静剂，而另一种（S‑thalidomide）则导致严重的出生缺陷。这一悲剧性案例突显了为何如今制药行业严格测试并往往只销售单一对映体药物，而不使用外消旋混合物。
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