Chirality and Optical Isomerism (Edexcel A-Level Chemistry Topic 17.1) | 手性与光学异构（爱德思A-Level化学 17.1）

📚 Chirality and Optical Isomerism (Edexcel A-Level Chemistry Topic 17.1) | 手性与光学异构（爱德思A-Level化学 17.1）

Chirality is a central idea in organic chemistry that explains how molecules with the same structural formula can exist as non-superimposable mirror images. This topic is essential for Edexcel A-Level Chemistry Topic 17.1 and links closely to reaction mechanisms and pharmaceutical chemistry.

手性是有机化学中的一个核心概念，用来解释为何结构式相同的分子可以存在互为镜像但不能重叠的两种形式。该主题是爱德思A-Level化学 17.1 的核心内容，并与反应机理和药物化学密切相关。

1. What is Chirality? | 什么是手性？

In chemistry, a molecule is chiral if it cannot be superimposed on its mirror image. This property usually arises when one carbon atom is bonded to four different atoms or groups, making that carbon a chiral centre or asymmetric carbon.

在化学中，如果分子不能与其镜像重合，则称其具有手性。通常当碳原子连接四个不同的原子或基团时，该碳原子称为手性中心或不对称碳原子。

A chiral molecule and its mirror image are two distinct species called enantiomers. Chirality is a type of stereoisomerism: the atoms are connected in the same order, but their arrangement in three-dimensional space differs.

手性分子与其镜像形成两种不同的物质，称为对映异构体。手性属于立体异构的一种：原子的连接顺序相同，但它们在三维空间中的排列方式不同。

2. Identifying a Chiral Centre | 如何识别手性中心

To identify a chiral centre, look for an sp³ hybridised carbon atom bonded to four different substituents. For example, in 2-bromobutane, C2 is attached to CH₃, C₂H₅, H and Br, so C2 is chiral.

要识别手性中心，需要找到与四个不同取代基相连的 sp³ 杂化碳原子。例如在 2-溴丁烷中，C2 分别连接 CH₃、C₂H₅、H 和 Br，因此 C2 是手性中心。

A carbon atom with two identical groups, such as a CH₂ or CH₃ group, cannot be a chiral centre because it does not have four different substituents.

若碳原子连有两个相同基团（如 CH₂ 或 CH₃），该碳原子不能作为手性中心，因为它没有四个不同的取代基。

Molecule	Chiral centre?	Reason
CH₃CHBrCH₂CH₃	Yes	C2 is bonded to CH₃, C₂H₅, H and Br
CH₃CH₂CH₂Br	No	No carbon is bonded to four different groups

3. Enantiomers: Mirror Image Isomers | 对映异构体：镜像异构体

Enantiomers are pairs of optical isomers that are non-superimposable mirror images of each other. They have identical physical properties such as boiling point, melting point and density, and identical chemical reactivity with non-chiral reagents.

对映异构体是互为镜像的一对光学异构体，它们不能重叠。它们具有相同的物理性质，如沸点、熔点和密度，并且与非手性试剂的化学反应性相同。

However, enantiomers rotate plane-polarised light in opposite directions and can behave very differently in chiral environments, including enzyme active sites and biological receptors.

然而，对映异构体使平面偏振光旋转的方向相反，并且在手性环境（包括酶活性位点和生物受体）中可能表现出非常不同的行为。

4. Drawing 3D Structures of Enantiomers | 绘制对映异构体的三维结构

Wedge and dash notation is used to show the tetrahedral arrangement around a chiral centre. A solid wedge represents a bond coming out of the plane of the page, while a dashed wedge represents a bond going behind the page.

楔形键和虚线键用于表示手性中心周围的四面体构型。实心楔形表示键朝纸面外，虚线楔形表示键朝纸面后方。

To draw the enantiomer, keep the same connectivity but reverse the positions of the wedge and dash bonds at the chiral centre. This creates the mirror image without rotating the whole molecule.

绘制对映异构体时，保持连接方式不变，但反转手性中心处的楔形键和虚线键位置。这样即可得到镜像结构，而无需旋转整个分子。

5. Optical Activity and Plane-Polarised Light | 光学活性与平面偏振光

A chiral molecule is optically active because it rotates the plane of plane-polarised light. The angle of rotation α depends on concentration, path length, temperature and the wavelength of light used.

手性分子具有光学活性，因为它会旋转平面偏振光的偏振面。旋转角 α 取决于浓度、光程长度、温度和所用光的波长。

Enantiomers rotate light by exactly the same angle but in opposite directions. One enantiomer is labelled (+) or d, and the other is labelled (−) or l.

对映异构体使光旋转的角度完全相同但方向相反。一个对映异构体标记为 (+) 或 d，另一个标记为 (−) 或 l。

Optical activity is measured using a polarimeter, which is a key piece of evidence for distinguishing enantiomers.

光学活性使用旋光仪测量，这是区分对映异构体的关键手段。

6. Racemic Mixtures and Optical Inactivity | 外消旋混合物与无光学活性

A racemic mixture, also called a racemate, contains equal amounts of two enantiomers. Because the optical rotations cancel, a racemic mixture is optically inactive.

外消旋混合物（也称消旋体）含有等量的两种对映异构体。由于旋光相互抵消，外消旋混合物没有光学活性。

Many laboratory reactions that produce a chiral product from an achiral or planar intermediate form a racemic mixture unless a chiral catalyst or chiral starting material is used.

除非使用手性催化剂或手性起始物，许多由非手性或平面中间体生成手性产物的实验室反应都会形成外消旋混合物。

7. Chirality in SN1 and SN2 Reactions | SN1 和 SN2 反应中的手性

In the hydrolysis of a halogenoalkane by an SN1 mechanism, the reaction proceeds via a planar carbocation intermediate. The nucleophile can attack from either side of the plane, producing equal amounts of the two enantiomers, so the product is a racemic mixture.

在卤代烃通过 SN1 机理水解时，反应经过平面碳正离子中间体。亲核试剂可以从平面两侧进攻，生成等量的两种对映异构体，因此产物为外消旋混合物。

In contrast, an SN2 mechanism involves a single back-side attack by the nucleophile. If the carbon attacked is a chiral centre, this usually causes inversion of configuration, often called Walden inversion.

相反，SN2 机理涉及亲核试剂的单一背面进攻。如果被进攻的碳是手性中心，通常会导致构型反转，常称为瓦尔登翻转。

8. Chirality in Drug Design: Thalidomide | 药物设计中的手性：沙利度胺

Enantiomers cannot be separated by ordinary physical methods because their physical properties are identical. They can be separated by chiral resolution using a chiral

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