📚 A-Level Chemistry Topic 17 & 18 Combined: A2 Organic Chemistry | A-Level 化学专题 17 与 18 合并：A2 有机化学
Topic 17 and Topic 18 in Edexcel A-Level Chemistry bring together the core A2 organic chemistry ideas: chirality, carbonyl compounds, carboxylic acids and derivatives, aromatic chemistry, nitrogen compounds, and organic synthesis. This combined revision guide walks through the key concepts, mechanisms, and exam strategies you need to master.
Edexcel A-Level 化学专题 17 和 18 汇集了 A2 有机化学的核心内容：手性、羰基化合物、羧酸及其衍生物、芳香化学、含氮化合物和有机合成。这份合并复习指南带你梳理关键概念、反应机理和考试策略。
1. Chirality and Optical Isomerism | 手性与光学异构
A chiral centre is a carbon atom bonded to four different groups. Such molecules exist as two non-superimposable mirror images called enantiomers. Enantiomers have identical physical and chemical properties except for their effect on plane-polarised light and their reactions with other chiral molecules.
手性中心是与四个不同基团相连的碳原子。这类分子存在两种不能重合的镜像，称为对映异构体。对映异构体除对平面偏振光的影响和与其他手性分子的反应外，物理和化学性质相同。
A racemic mixture contains equal amounts of two enantiomers, so it is optically inactive overall. In contrast, a single enantiomer rotates plane-polarised light by a specific angle in one direction.
外消旋混合物含有等量的两种对映异构体，因此整体没有旋光性。相反，单一对映异构体能使平面偏振光向某一方向旋转特定角度。
2. Aldehydes and Ketones | 醛和酮
Aldehydes have the carbonyl group at the end of a carbon chain, while ketones have the carbonyl group between two carbon atoms. Both are polar because the C=O bond is polar. Aldehydes and ketones cannot form strong hydrogen bonds with themselves, so their boiling points are lower than those of alcohols with similar relative molecular mass.
醛的羰基位于碳链末端，酮的羰基位于两个碳原子之间。由于 C=O 键具有极性，醛酮分子为极性分子。醛和酮分子间不能形成强氢键，因此沸点低于相对分子质量相近的醇。
A key test is that 2,4-dinitrophenylhydrazine, often called 2,4-DNPH, gives an orange precipitate with both aldehydes and ketones. Tollens reagent only oxidises aldehydes to carboxylic acids, producing a silver mirror on the inside of a clean test tube.
重要检验：2,4-二硝基苯肼（通常简写为 2,4-DNPH）与醛和酮均生成橙色沉淀。托伦试剂只将醛氧化成羧酸，并在洁净试管内壁形成银镜。
3. Nucleophilic Addition Reactions | 亲核加成反应
The carbonyl carbon is electron-deficient, so it is attacked by nucleophiles. Sodium borohydride, NaBH₄, in water reduces aldehydes to primary alcohols and ketones to secondary alcohols. The nucleophilic addition mechanism involves the hydride ion, H⁻, from NaBH₄ attacking the carbonyl carbon.
羰基碳缺电子，容易受到亲核试剂进攻。硼氢化钠 NaBH₄ 在水溶液中将醛还原为伯醇，将酮还原为仲醇。亲核加成机理包括来自 NaBH₄ 的氢负离子 H⁻ 进攻羰基碳。
Hydrogen cyanide adds across the C=O bond to form hydroxynitriles, increasing the carbon chain length by one carbon. The nucleophile is the cyanide ion, CN⁻, which attacks the carbonyl carbon, followed by protonation of the oxygen.
氢氰酸与碳氧双键加成生成羟基腈，使碳链增加一个碳原子。亲核试剂是氰根离子 CN⁻，它进攻羰基碳，随后氧被质子化。
RCHO + HCN → RCH(OH)CN
4. Carboxylic Acids and Esters | 羧酸与酯
Carboxylic acids are weak acids because they only partially dissociate in water. They form hydrogen bonds with each other, so they have relatively high boiling points and can exist as dimers in non-polar solvents.
羧酸在水中仅部分电离，属于弱酸。羧酸分子间形成氢键，因此沸点较高，在非极性溶剂中可以二聚体形式存在。
CH₃COOH ⇌ CH₃COO⁻ + H⁺
Esters are formed by reacting a carboxylic acid with an alcohol in the presence of an acid catalyst. This is a condensation reaction that releases water. Esters often have sweet, fruity smells and are used in flavourings, perfumes, and solvents.
酯由羧酸与醇在酸催化下反应生成。这是释放水的缩合反应。酯通常具有香甜的水果气味，用作香精、香料和溶剂。
5. Acyl Chlorides and Acid Anhydrides | 酰氯与酸酐
Acyl chlorides are more reactive than carboxylic acids and esters because the chlorine atom is a good leaving group. They react readily with water, alcohols, ammonia, and primary amines to form carboxylic acids, esters, amides, and N-substituted amides respectively.
酰氯比羧酸和酯更活泼，因为氯原子是良好的离去基团。酰氯很容易与水、醇、氨和伯胺反应，分别生成羧酸、酯、酰胺和 N-取代酰胺。
Acid anhydrides are often preferred in synthesis because they are less vigorous and safer to handle than acyl chlorides. For example, aspirin is made by reacting salicylic acid with ethanoic anhydride.
酸酐在合成中常被优先使用，因为它比酰氯温和、更安全。例如阿司匹林由水杨酸与乙酸酐反应制得。
6. Benzene and Aromatic Stability | 苯与芳香稳定性
Benzene is planar, with six carbon atoms forming a regular hexagon. Each carbon uses three sp² hybrid orbitals to form sigma bonds. The remaining p orbitals overlap sideways to form a delocalised π ring above and below the plane.
苯是平面分子，六个碳原子形成正六边形。每个碳原子用三个 sp² 杂化轨道形成 σ 键，剩余的 p 轨道侧向重叠，在平面上下形成离域 π 环。
The delocalisation energy makes benzene more stable than the hypothetical cyclohexatriene. Because of this stable delocalised ring, benzene undergoes electrophilic substitution rather than addition reactions. Addition would destroy the delocalised π system, so it is not favoured.
离域能使苯比假想的环己三烯更稳定。由于稳定的离域环，苯发生亲电取代而非加成反应。加成会破坏离域 π 体系，因此在能量上不利。
7. Electrophilic Substitution of Benzene | 苯的亲电取代
Common electrophilic substitution reactions of benzene include nitration, halogenation, Friedel-Crafts alkylation, and Friedel-Crafts acylation. Each requires a strong electrophile generated with a catalyst.
苯的常见亲电取代反应包括硝化、卤代、傅-克烷基化和傅-克酰基化。每种反应都需要催化剂生成强亲电试剂。
Nitration uses a mixture of concentrated nitric and sulfuric acids to generate the nitronium ion, NO₂⁺. Halogenation uses a halogen carrier such as FeBr₃ or AlCl₃ to polarise the halogen molecule.
硝化使用浓硝酸和浓硫酸的混合物生成硝鎓离子 NO₂⁺。卤代使用 FeBr₃ 或 AlCl₃ 等卤素载体使卤素分子极化。
Friedel-Crafts acylation uses an acyl chloride and AlCl₃ to generate an acylium ion, RCO⁺, which introduces an acyl group onto the ring. Alkylation uses a haloalkane and AlCl₃, but it can lead to polyalkylation.
傅-克酰基化使用酰氯和 AlCl₃ 生成酰基正离子 RCO⁺，将酰基引入苯环。烷基化使用卤代烷和 AlCl₃，但可能导致多取代。
8. Amines and Amides | 胺与酰胺
Amines are organic derivatives of ammonia. Primary amines can be prepared by reducing nitriles with LiAlH₄ or by reacting halogenoalkanes with excess ammonia. Amines act as bases because the lone pair on the nitrogen atom can accept a proton.
胺是氨的有机衍生物。伯胺可通过 LiAlH₄ 还原腈或卤代烷与过量氨反应制备。胺因氮原子上的孤对电子能接受质子而显碱性。
Amides are formed from acyl chlorides and ammonia or amines. They are much less basic than amines because the lone pair on nitrogen is delocalised onto the carbonyl oxygen.
酰胺由酰氯与氨或胺反应生成。酰胺的碱性远弱于胺，因为氮上的孤对电子离域到羰基氧上。
9. Amino Acids and Condensation Polymers | 氨基酸与缩合聚合物
Alpha-amino acids contain both a basic amino group and an acidic carboxylic acid group attached to the same carbon. In aqueous solution they exist as zwitterions, which carry both a positive and a negative charge.
α-氨基酸在同一个碳原子上同时含有碱性氨基和酸性羧基。在水溶液中，它们以同时带正电荷和负电荷的两性离子形式存在。
Proteins are condensation polymers of amino acids linked by peptide bonds. Condensation polymerisation also produces polyesters and polyamides. Common polyamides include nylon-6,6 and Kevlar.
蛋白质是由氨基酸通过肽键连接而成的缩合聚合物。缩聚反应也可制备聚酯和聚酰胺。常见的聚酰胺包括尼龙-6,6 和凯夫拉。
10. Planning Organic Synthesis Routes | 有机合成路线设计
A-Level synthesis questions often ask you to plan a sequence of reactions to convert one molecule into another. You need to know how to change functional groups, extend carbon chains, and state the conditions for each step.
A-Level 合成题常要求设计反应序列将一种分子转化为另一种分子。你需要掌握官能团转化、碳链增长以及每一步的反应条件。
For example, an alkene can be converted to an alcohol by acid-catalysed hydration or to a diol by reaction with acidified KMnO₄. A primary alcohol can be
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