📚 Amines | 胺
Amines are organic compounds derived from ammonia (NH₃) where one or more hydrogen atoms are replaced by alkyl or aryl groups. They are fundamental to many biological molecules (such as amino acids and alkaloids) and are widely used in pharmaceuticals, dyes and polymers. In this revision guide, we focus on the structure, properties, preparation and reactions of amines, aligned with the AQA A-level Chemistry specification.
胺是由氨（NH₃）衍生出的有机化合物，其中一个或多个氢原子被烷基或芳基取代。它们是许多生物分子（如氨基酸和生物碱）的基础，并广泛用于制药、染料和聚合物。在本复习指南中，我们围绕AQA A-level化学考纲，重点探讨胺的结构、性质、制备和反应。
1. Structure, Classification and Nomenclature | 结构、分类与命名
Amines are classified as primary (RNH₂), secondary (R₂NH) or tertiary (R₃N) according to the number of alkyl or aryl groups attached to the nitrogen atom. The nitrogen atom also carries a lone pair of electrons, which gives the molecule a trigonal pyramidal shape and makes the nitrogen atom nucleophilic and basic.
胺根据氮原子上连接的烷基或芳基数目分为伯胺（RNH₂）、仲胺（R₂NH）和叔胺（R₃N）。氮原子还含有一对孤对电子，使分子呈三角锥形，并使氮原子具有亲核性和碱性。
For simple amines, naming uses the suffix -amine. For example, CH₃NH₂ is methylamine, CH₃CH₂NH₂ is ethylamine, and (CH₃)₂NH is dimethylamine. In more complex cases, the larger alkyl group is the parent chain and the substituent on nitrogen is indicated by the prefix N-, e.g., N-methylethylamine.
对于简单胺，命名使用后缀“-amine”。例如，CH₃NH₂是甲胺，CH₃CH₂NH₂是乙胺，(CH₃)₂NH是二甲胺。在更复杂的情况下，较长的烷基作为主链，氮上的取代基用前缀“N-”表示，例如N-甲基乙胺。
Common examples include phenylamine (aniline, C₆H₅NH₂) and cyclohexylamine. You should also be able to identify primary, secondary and tertiary amines from their structural formulas or skeletal formulas.
常见实例包括苯胺（C₆H₅NH₂）和环己胺。你也应当能根据结构式或键线式识别伯、仲、叔胺。
2. Physical Properties | 物理性质
Low-molar-mass amines are gases or volatile liquids with a fishy, unpleasant odour. As the molar mass increases, they become less volatile. Primary and secondary amines can form hydrogen bonds between molecules because of the N–H bond, but these hydrogen bonds are weaker than those in alcohols because nitrogen is less electronegative than oxygen.
低摩尔质量的胺是气体或挥发性液体，具有鱼腥般难闻的气味。随着摩尔质量增加，挥发性降低。伯胺和仲胺由于存在N–H键，能在分子间形成氢键，但这些氢键比醇中的弱，因为氮的电负性低于氧。
Amines are soluble in water – especially those with low molar masses – because they can form hydrogen bonds with water molecules. Solubility decreases as the hydrocarbon chain gets longer. Tertiary amines do not have N–H bonds, so they cannot form hydrogen bonds with each other; consequently, they have lower boiling points than secondary amines of similar molar mass.
胺可溶于水，尤其是低摩尔质量的胺，因为它们能与水分子形成氢键。随着碳链增长，溶解度降低。叔胺没有N–H键，因此彼此之间不能形成氢键；因此它们的沸点低于类似摩尔质量的仲胺。
3. Basicity of Amines | 胺的碱性
All amines are weak bases. The lone pair on the nitrogen atom can accept a proton (H⁺) to form a substituted ammonium ion. For example, ethylamine reacts with water to form ethylammonium and hydroxide ions:
所有胺都是弱碱。氮原子上的孤对电子能够接受质子（H⁺）形成取代铵离子。例如，乙胺与水反应生成乙铵离子和氢氧根离子：
CH₃CH₂NH₂ + H₂O ⇌ CH₃CH₂NH₃⁺ + OH⁻
The strength of an amine as a base depends on how readily its lone pair is available to accept a proton. In aqueous solution, the equilibrium lies to the left for most amines, but the extent of protonation varies.
胺的碱性强弱取决于其孤对电子接受质子的容易程度。在水溶液中，大多数胺的平衡向左移动，但质子化程度不同。
Key comparison: Aliphatic amines are stronger bases than ammonia because alkyl groups are electron-releasing groups. They donate electron density toward the nitrogen, which makes the lone pair more available and also stabilises the positively charged ammonium ion formed after protonation. In contrast, phenylamine (aniline) is a much weaker base than ammonia, because the lone pair on nitrogen is delocalised into the benzene ring, making it less available to accept a proton.
关键比较：脂肪胺的碱性比氨强，因为烷基是推电子基团。它们向氮原子供电子密度，使孤对电子更容易提供，同时稳定质子化后形成的正电荷铵离子。相反，苯胺的碱性比氨弱得多，因为氮上的孤对电子离域到苯环中，使其不易接受质子。
4. Factors Affecting Basicity | 影响碱性的因素
Inductive effect of alkyl groups: Alkyl groups (e.g., methyl, ethyl) are electron-donating through the σ-bond framework. The more alkyl groups attached to nitrogen, the greater the electron density on nitrogen, usually enhancing basicity. However, in aqueous solution the trend is not straightforward due to solvent effects and steric hindrance.
烷基的诱导效应：烷基（如甲基、乙基）通过σ键框架供电。氮上连接的烷基越多，氮上的电子密度越大，通常会增强碱性。然而，在水溶液中，由于溶剂效应和空间位阻，趋势并不简单。
Delocalisation of the lone pair orbital overlap. Therefore phenylamine is far less basic than ammonia (pKb ≈ 9.4 vs 4.8).
孤对电子的离域：当氮原子直接连在芳环上（例如苯胺）时，孤对电子离域进入芳环的π体系。这使游离胺稳定，但使铵离子上的正电荷不稳定，因为质子化破坏了轨道重叠。因此苯胺的碱性强弱远低于氨（pKb约9.4 vs 4.8）。
Solvation effects in water: For primary and secondary amines, the resulting ammonium ions can hydrogen-bond with water, which increases their stability. Tertiary ammonium ions, though stabilised by inductive effects, lack these N–H hydrogen bonds, so in practice secondary amines often prove to be slightly more basic in water than tertiary amines.
水中的溶剂化效应：对于伯胺和仲胺，生成的铵离子能与水形成氢键，从而增加稳定性。叔铵离子虽然受诱导效应稳定，但缺少N–H氢键，因此实际上仲胺在水中碱性稍强于叔胺。
5. Amines as Nucleophiles | 胺作为亲核试剂
The nitrogen lone pair also makes amines good nucleophiles. Amines can attack electron-deficient carbon atoms in halogenoalkanes, acyl chlorides and other electrophilic species. The reaction with halogenoalkanes is a nucleophilic substitution and is a key route for preparing amines.
氮的孤对电子也使胺成为良好的亲核试剂。胺可以进攻卤代烷、酰氯和其他亲电物种中缺电子的碳原子。与卤代烷的反应是亲核取代，也是制备胺的重要途径。
For example, ethylamine can react with bromoethane to form diethylamine (a secondary amine) and, with further substitution, triethylamine (a tertiary amine) and tetraethylammonium bromide (a quaternary ammonium salt). The degree of substitution depends on the relative concentrations of reactants and the conditions used.
例如，乙胺可与溴乙烷反应生成二乙胺（仲胺），进一步取代得到三乙胺（叔胺）和四乙基溴化铵（季铵盐）。取代程度取决于反应物的相对浓度和条件。
6. Preparation of Amines: Halogenoalkanes with Ammonia | 胺的制备：卤代烷与氨
Primary amines can be prepared by heating a halogenoalkane with an excess of ammonia in a sealed tube or under pressure. An excess of ammonia is necessary to minimise further substitution. The initial product is an ammonium salt, which is then treated with a strong base to liberate the free amine.
伯胺可通过卤代烷与过量氨在密封管中加热（或在压力下）来制备。必须使用过量氨以减少进一步取代。初始产物是铵盐，然后用强碱处理释放游离胺。
Overall equation (using bromoethane):
总反应方程式（以溴乙烷为例）：
CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br
Mechanistically, ammonia acts as a nucleophile, displacing the bromide ion. The intermediate ammonium salt (CH₃CH₂NH₃⁺Br⁻) is then deprotonated by a second molecule of ammonia (or by added NaOH) to give the primary amine. Because the product (CH₃CH₂NH₂) is itself a nucleophile, it can react further with the halogenoalkane to form secondary and tertiary amines, hence the need for excess ammonia.
从机理上看，氨作为亲核试剂，取代了溴离子。中间体铵盐（CH₃CH₂NH₃⁺Br⁻）随后被第二个氨分子（或加入的NaOH）去质子化，得到伯胺。由于产物（CH₃CH₂NH₂）本身也是亲核试剂，它能进一步与卤代烷反应生成仲胺和叔胺，因此需要过量氨。
7. Preparation of Amines: Reduction of Nitriles | 胺的制备：腈的还原
Another important route to primary amines is the reduction of nitriles (R–C≡N) using lithium aluminium hydride (LiAlH₄) in dry ether, or by catalytic hydrogenation over a nickel or platinum catalyst. This method is particularly useful because it does not normally produce secondary or tertiary amine by-products.
制备伯胺的另一重要途径是将腈（R–C≡N）用氢化铝锂（LiAlH₄）在干燥乙醚中还原，或在镍或铂催化剂下催化氢化。该方法特别有用，因为通常不会产生仲胺或叔胺副产物。
The reduction converts the –C≡N group to a –CH₂NH₂ group:
还原将–C≡N基团转化为–CH₂NH₂基团：
R–C≡N + 4[H] → R–CH₂NH₂
For example, ethanenitrile (CH₃CN) reduces to ethylamine (CH₃CH₂NH₂). This route gives a good yield of the primary amine and is often preferred in a multi-step synthesis because the carbon chain is extended by one carbon atom when the nitrile is prepared from a halogenoalkane and cyanide ion.
例如，乙腈（CH₃CN）还原为乙胺（CH₃CH₂NH₂）。该方法伯胺产率高，在多步合成中常被优先选择，因为当腈由卤代烷和氰离子制备时，碳链会延长一个碳原子。
8. Reaction with Acids: Formation of Ammonium Salts | 胺与酸的反应：生成铵盐
Amines, being bases, react readily with acids to form ammonium salts. For example, ethylamine reacts with hydrochloric acid to give ethylammonium chloride:
胺作为碱，能与酸反应生成铵盐。例如，乙胺与盐酸反应生成氯化乙铵：
CH₃CH₂NH₂ + HCl → CH₃CH₂NH₃⁺Cl⁻
These salts are white crystalline solids, soluble in water, and have much higher melting points than the parent amines. The formation of a salt is often used to separate amines from organic mixtures or to make them water-soluble for extraction.
这些盐是白色晶体，溶于水，熔点远高于相应的胺。生成盐常用于将胺从有机混合物中分离，或使其水溶性提高以便萃取。
The reverse reaction is simply a treatment with a strong alkali such as aqueous sodium hydroxide, which regenerates the free amine:
反向反应是用强碱（如氢氧化钠水溶液）处理，重新生成游离胺：
CH₃CH₂NH₃⁺Cl⁻ + NaOH → CH₃CH₂NH₂ + NaCl + H₂O
This behaviour is similar to the reaction of ammonia with acids, but the strength of the base affects the position of equilibrium and the degree of salt formation.
这一行为与氨和酸的反应类似，但碱的强度影响平衡位置和成盐程度。
9. Acylation of Amines | 胺的酰化
Primary and secondary amines react with acyl chlorides (acid chlorides) to form amides. For example, ethanoyl chloride reacts with methylamine to produce N-methylethanamide and hydrogen chloride:
伯胺和仲胺与酰氯（酰氯）反应生成酰胺。例如，乙酰氯与甲胺反应生成N-甲基乙酰胺和氯化氢：
CH₃COCl + CH₃NH₂ → CH₃CONHCH₃ + HCl
In practice, the hydrogen chloride released reacts with another amine molecule to form an ammonium salt, so the balanced equation often uses two moles of amine or an additional base such as pyridine to absorb the HCl. Acylation is useful for converting an amine into a more stable, less basic derivative.
实际上，释放的氯化氢会与另一个胺分子反应生成铵盐，因此平衡方程式常使用两摩尔胺或额外加入吡啶等碱来吸收HCl。酰化反应可将胺转化为更稳定、碱性更弱的衍生物。
This reaction is important in the synthesis of paracetamol and other drugs. You may also encounter the reaction with ethanoic anhydride, which produces the same amide and ethanoic acid, avoiding the need to handle a corrosive acyl chloride.
该反应在扑热息痛等药物的合成中很重要。你也可能遇到与乙酸酐的反应，它生成相同的酰胺和乙酸，避免了处理腐蚀性酰氯的需要。
10. Phenylamine (Aniline) and Its Reactions | 苯胺及其反应
Phenylamine (C₆H₅NH₂) is an aromatic amine. It is prepared by the reduction of nitrobenzene using tin and concentrated hydrochloric acid (or catalytic hydrogenation). It is a weaker base than aliphatic amines because the nitrogen lone pair is delocalised into the benzene ring, as discussed earlier.
苯胺（C₆H₅NH₂）是一种芳香胺。它通过硝基苯的还原制备，可使用锡和浓盐酸（或催化氢化）。由于氮的孤对电子离域到苯环中，它的碱性比脂肪胺弱，如前所述。
The presence of the –NH₂ group activates the benzene ring toward electrophilic substitution. For instance, phenylamine reacts rapidly with bromine water to form 2,4,6-tribromophenylamine, a white precipitate. This reaction is so sensitive that it can be used as a test for phenylamine:
–NH₂基团的存在使苯环对亲电取代反应活化。例如，苯胺与溴水迅速反应生成2,4,6-三溴苯胺白色沉淀。该反应非常灵敏，可用于检验苯胺：
C₆H₅NH₂ + 3Br₂ → C₆H₂Br₃NH₂ + 3HBr
In addition, phenylamine can be diazotised by reacting with nitrous acid (HNO₂, generated in situ from NaNO₂ and dilute HCl) at temperatures below 5 °C to form a benzenediazonium salt. This salt can then undergo coupling reactions with phenols or amines to produce brightly coloured azo dyes, an important industrial use.
此外，苯胺可与亚硝酸（由NaNO₂和稀盐酸现场生成）在5 °C以下反应发生重氮化，生成重氮苯盐。该盐随后可与酚或胺发生偶联反应，生成颜色鲜艳的偶氮染料，这在工业上非常重要。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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