A-Level Chemistry: Structure and Properties of Amines | A-Level 化学：胺类化合物的结构与性质

📚 A-Level Chemistry: Structure and Properties of Amines | A-Level 化学：胺类化合物的结构与性质

Amines are organic derivatives of ammonia, formed by replacing one or more hydrogen atoms of NH₃ with alkyl or aryl groups. Their chemistry is dominated by the lone pair of electrons on the nitrogen atom, which gives them basic, nucleophilic and hydrogen-bonding properties.

胺是氨的有机衍生物，由 NH₃ 中的一个或多个氢原子被烷基或芳基取代而形成。它们的化学性质主要由氮原子上的孤对电子决定，这使胺具有碱性、亲核性和形成氢键的能力。

1. What Are Amines? | 什么是胺？

An amine contains a nitrogen atom bonded to carbon-containing groups. The general formula for a simple aliphatic amine is RNH₂ for primary, R₂NH for secondary and R₃N for tertiary amines. The nitrogen atom also bears a lone pair of electrons in a roughly tetrahedral arrangement.

胺含有与含碳基团相连的氮原子。简单脂肪胺的通用式为：伯胺 RNH₂、仲胺 R₂NH、叔胺 R₃N。氮原子还带有一对孤对电子，呈近似四面体排列。

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2. Classification of Amines | 胺的分类

Amines are classified according to the number of alkyl or aryl groups attached to nitrogen. This classification is based on the number of hydrogen atoms in ammonia that have been replaced, not on the number of nitrogen atoms in the molecule.

胺根据与氮原子相连的烷基或芳基数目进行分类。这种分类依据的是氨中被取代的氢原子数目，而不是分子中氮原子的数目。

• Primary amine (1°) / 伯胺: one hydrogen replaced, RNH₂

• Secondary amine (2°) / 仲胺: two hydrogens replaced, R₂NH

• Tertiary amine (3°) / 叔胺: three hydrogens replaced, R₃N

• Quaternary ammonium salt (4°) / 季铵盐: four groups attached, R₄N⁺

Class / 类别	Structure / 结构	Example / 示例
Primary (1°) / 伯胺	RNH₂	CH₃NH₂ (methylamine / 甲胺)
Secondary (2°) / 仲胺	R₂NH	(CH₃)₂NH (dimethylamine / 二甲胺)
Tertiary (3°) / 叔胺	R₃N	(CH₃)₃N (trimethylamine / 三甲胺)
Quaternary (4°) / 季铵	R₄N⁺	(CH₃)₄N⁺ (tetramethylammonium / 四甲铵)

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3. Nomenclature | 命名

In common naming, simple amines are named by adding the suffix “-amine” to the alkyl group name. For example, CH₃NH₂ is methylamine and C₂H₅NH₂ is ethylamine. In IUPAC nomenclature, the longest carbon chain is taken as the parent alkane, and the -NH₂ group is indicated by the suffix “-amine” or the prefix “amino-“.

在普通命名法中，简单胺是在烷基名称后加上”胺”后缀。例如，CH₃NH₂ 称为甲胺，C₂H₅NH₂ 称为乙胺。在 IUPAC 命名法中，取最长碳链为母体烷烃，-NH₂ 基团用后缀”-胺”或前缀”氨基-“表示。

For secondary and tertiary amines, the largest alkyl group is used as the parent name and the remaining alkyl groups are listed as N-substituents. Quaternary ammonium salts use the name of the organic groups followed by “-ammonium”.

对于仲胺和叔胺，取最大烷基作为母体名称，其余烷基作为 N-取代基。季铵盐则以有机基团名称后加”-铵”命名。

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4. Physical Properties | 物理性质

Amines are polar compounds. Primary and secondary amines can form intermolecular N-H···N hydrogen bonds because they have N-H bonds; tertiary amines cannot donate hydrogen bonds but can accept them. Therefore, for similar molar masses, boiling points generally follow the order: primary > secondary > tertiary.

胺是极性化合物。伯胺和仲胺具有 N-H 键，因此可以形成分子间 N-H···N 氢键；叔胺不能提供氢键，但可以接受氢键。因此在相对分子质量相近时，沸点一般按伯胺 > 仲胺 > 叔胺的顺序排列。

Low molar mass amines are soluble in water because they form hydrogen bonds with water molecules. As the hydrocarbon chain length increases, solubility decreases because the non-polar alkyl chain begins to dominate.

低相对分子质量的胺能与水分子形成氢键，因此可溶于水。随着碳链增长，非极性烷基占主导，溶解度下降。

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5. Basicity of Amines | 胺的碱性

Amines are weak bases. In aqueous solution, they accept a proton from water forming alkylammonium ions and hydroxide ions. For example:

胺是弱碱。在水溶液中，它们从水中接受质子，生成烷基铵离子和氢氧根离子。例如：

CH₃NH₂ + H₂O ⇌ CH₃NH₃⁺ + OH⁻

The base dissociation constant Kb is defined as:

碱解离常数 Kb 定义为：

Kb = [RNH₃⁺][OH⁻] / [RNH₂]

A larger Kb means a stronger base. Simple alkyl amines are generally stronger bases than ammonia because the alkyl groups donate electron density to the nitrogen atom, stabilising the positive charge on the protonated ammonium ion.

Kb 越大，碱性越强。简单烷基胺通常比氨的碱性强，因为烷基向氮原子提供电子密度，稳定了质子化铵离子上的正电荷。

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6. Factors Affecting Basicity | 影响碱性的因素

Several factors influence the basicity of amines in aqueous solution.

多种因素影响胺在水溶液中的碱性。

• Inductive effect / 诱导效应: Alkyl groups are electron-donating. They increase the electron density on nitrogen, making the lone pair more available for protonation. In the gas phase, basicity increases with more alkyl groups: R₃N > R₂NH > RNH₂.

  烷基是给电子基。它们增加氮上的电子密度，使孤对电子更容易质子化。在气相中，碱性随烷基数增加而增强：R₃N > R₂NH > RNH₂。

• Solvation effect / 溶剂化效应: In water, the ammonium ion is stabilised by hydration. A primary ammonium ion RNH₃⁺ can form three N-H···O hydrogen bonds, while a tertiary ammonium ion can only form one. This makes tertiary amines less strongly stabilised in water, so their apparent basicity is reduced.

  在水中，铵离子通过水合作用稳定。伯铵离子 RNH₃⁺ 能形成三个 N-H···O 氢键，而叔铵离子只能形成一个。因此叔胺在水中稳定性较低，表观碱性减弱。

• Steric hindrance / 位阻效应: Bulky alkyl groups can hinder the approach of a proton to the nitrogen lone pair, reducing the rate and extent of protonation.

  体积大的烷基会阻碍质子接近氮的孤对电子，从而降低质子化的速率和程度。

As a result, in water the order for methylamines is approximately: (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃.

因此在水中，甲胺类的碱性顺序约为：(CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃。

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7. Aromatic Amines | 芳香胺

In aromatic amines such as phenylamine (aniline, C₆H₅NH₂), the lone pair on nitrogen is delocalised into the benzene ring. This conjugation makes the lone pair less available for protonation, so aniline is a much weaker base than ammonia.

在芳香胺如苯胺 (C₆H₅NH₂) 中，氮上的孤对电子离域到苯环上。这种共轭使孤对电子不易被质子化，因此苯胺的碱性强弱于氨。

Electron-withdrawing groups on the benzene ring, such as -NO₂, further decrease basicity, while electron-donating groups such as -CH₃ or -OCH₃ increase it. Aromatic amines are important intermediates in the synthesis of dyes and pharmaceuticals.

苯环上的吸电子基团如 -NO₂ 会进一步降低碱性，而给电子基团如 -CH₃ 或 -OCH₃ 则增加碱性。芳香胺是染料和药物合成中的重要中间体。

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8. Reaction with Acids: Salt Formation | 与酸反应：成盐

Amines react with acids to form ammonium salts. For example, methylamine reacts with hydrochloric acid to give methylammonium chloride:

胺与酸反应生成铵盐。例如，甲胺与盐酸反应生成氯化甲铵：

CH₃NH₂ + HCl → CH₃NH₃⁺Cl⁻

These salts are usually white, crystalline solids that are soluble in water and insoluble in non-polar solvents. The conversion of amines to their salts is used in purification and to improve water solubility of amine drugs.

这些盐通常为白色晶体，可溶于水，不溶于非极性溶剂。将胺转化为其盐可用于提纯，并提高胺类药物的水溶性。

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9. Acylation of Amines | 胺的酰化

Primary and secondary amines react with acyl chlorides or acid anhydrides to form amides. For example, methylamine reacts with ethanoyl chloride to give N-methylethanamide:

伯胺和仲胺与酰氯或酸酐反应生成酰胺。例如，甲胺与乙酰氯反应生成 N-甲基乙酰胺：

CH₃NH₂ + CH₃COCl → CH₃CONHCH₃ + HCl

Tertiary amines have no N-H bond and therefore cannot be acylated. Acylation is an important method for protecting amino groups in organic synthesis and is used in the manufacture of drugs such as paracetamol.

叔胺没有 N-H 键，因此不能发生酰化。酰化是有机合成中保护氨基的重要方法，也用于生产对乙酰氨基酚等药物。

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10. Reaction with Nitrous Acid | 与亚硝酸的反应

Nitrous acid (HNO₂) is unstable and is prepared in situ from sodium nitrite and dilute hydrochloric acid. Amines react with nitrous acid in different ways depending on whether they are primary, secondary or tertiary.

亚硝酸 (HNO₂) 不稳定，通常由亚硝酸钠和稀盐酸现场制备。胺与亚硝酸的反应方式取决于它们是伯胺、仲胺还是叔胺。

• Primary aliphatic amines / 脂肪伯胺: RNH₂ + HNO₂ → ROH + N₂ + H₂O. Nitrogen gas is evolved, which can be used to measure the amount of primary amine.

  RNH₂ + HNO₂ → ROH + N₂ + H₂O。放出氮气，可用于测定伯胺的量。

• Primary aromatic amines / 芳香伯胺: At 0-5 °C, aniline reacts with nitrous acid to form a diazonium salt: C₆H₅NH₂ + HNO₂ + HCl → C₆H₅N₂⁺Cl⁻ + 2H₂O. Diazonium salts are useful in dye synthesis.

  在 0-5 °C 下，苯胺与亚硝酸反应生成重氮盐：C₆H₅NH₂ + HNO₂ + HCl → C₆H₅N₂⁺Cl⁻ + 2H₂O。重氮盐在染料合成中非常有用。

• Secondary amines / 仲胺: React with nitrous acid to form N-nitrosamines, which are yellow oily liquids.

  仲胺与亚硝酸反应生成 N-亚硝胺，呈黄色油状液体。

• Tertiary amines / 叔胺: Do not react with nitrous acid in the same way; aliphatic tertiary amines simply form salts.

  叔胺与亚硝酸的反应方式不同，脂肪叔胺仅形成盐。

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11. Identification of Amines | 胺的鉴别

The Hinsberg test uses benzenesulfonyl chloride to distinguish between primary, secondary and tertiary amines. The amine is shaken with the reagent in the presence of aqueous alkali.

兴斯堡试验使用苯磺酰氯来区分伯胺、仲胺和叔胺。将胺与试剂在碱液中振荡混合。

• Primary amine: forms a sulfonamide that dissolves in alkali because the remaining N-H hydrogen is acidic.

  伯胺：生成的磺酰胺可溶于碱液，因为剩余的 N-H 氢具有酸性。

• Secondary amine: forms a sulfonamide with no N-H hydrogen, so it is insoluble in alkali.

  仲胺：生成的磺酰胺没有 N-H 氢，因此不溶于碱液。

• Tertiary amine: does not react with benzenesulfonyl chloride.

  叔胺：不与苯磺酰氯反应。

This test is a classic method for qualitative analysis of amines.

该试验是胺的定性分析经典方法。

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12. Importance in Biology and Medicine | 在生物和医药中的重要性

Amines are fundamental to life and medicine. Amino acids, the building blocks of proteins, contain an amino group. Many neurotransmitters and hormones, such as adrenaline and dopamine, are amines. Alkaloids isolated from plants, such as nicotine, quinine and morphine, also contain amine groups.

胺是生命和医药的基础。氨基酸是蛋白质的构建单元，含有氨基。许多神经递质和激素如肾上腺素、多巴胺都是胺类。从植物中分离出的生物碱如尼古丁、奎宁和吗啡也含有胺基。

Numerous synthetic drugs are amines, including local anaesthetics, antihistamines and antibiotics. Quaternary ammonium salts are used as cationic surfactants, fabric softeners and disinfectants because of their positive charge and antibacterial activity.

许多合成药物属于胺类，包括局部麻醉剂、抗组胺药和抗生素。季铵盐因其正电荷和抗菌活性，被用作阳离子表面活性剂、衣物柔顺剂和消毒剂。

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