Edexcel A-Level Chemistry Topic 17.5 Amines, Amides, Amino Acids and Proteins | 爱德思A-Level化学主题17.5 胺、酰胺、氨基酸与蛋白质

📚 Edexcel A-Level Chemistry Topic 17.5 Amines, Amides, Amino Acids and Proteins | 爱德思A-Level化学主题17.5 胺、酰胺、氨基酸与蛋白质

This combined topic focuses on nitrogen-containing organic compounds: amines, amides, amino acids and proteins. It is a high-yield area of Edexcel A-Level Chemistry, linking organic synthesis, acid-base behaviour and biochemistry.

本综合主题重点讨论含氮有机化合物：胺、酰胺、氨基酸和蛋白质。这是爱德思A-Level化学中的高频考点，将有机合成、酸碱行为和生物化学联系在一起。

1. Classification and Nomenclature of Amines | 胺的分类与命名

An amine is derived from ammonia, NH₃, by replacing one or more hydrogen atoms with alkyl or aryl groups. Amines are classified as primary (one alkyl/aryl group), secondary (two) or tertiary (three) depending on the number of carbon-containing groups attached to the nitrogen atom.

胺是氨 NH₃ 中的一个或多个氢原子被烷基或芳基取代后形成的化合物。根据与氮原子相连的含碳基团数量，胺分为伯胺（一个）、仲胺（两个）和叔胺（三个）。

Primary amine: CH₃CH₂NH₂ (ethylamine)
Secondary amine: (CH₃)₂NH (dimethylamine)
Tertiary amine: (CH₃)₃N (trimethylamine)
Aromatic amine: C₆H₅NH₂ (phenylamine, also called aniline)

伯胺：CH₃CH₂NH₂（乙胺）
仲胺：(CH₃)₂NH（二甲胺）
叔胺：(CH₃)₃N（三甲胺）
芳香胺：C₆H₅NH₂（苯胺）

In exam answers, always name the longest carbon chain bonded to nitrogen and use the suffix -amine, for example CH₃CH₂CH₂NH₂ is propylamine. Do not confuse this with amides, which contain a carbonyl group next to nitrogen.

在考试作答时，始终命名与氮原子相连的最长碳链，并以 -amine 结尾，例如 CH₃CH₂CH₂NH₂ 为丙胺。不要与酰胺混淆，酰胺在氮原子旁边含有羰基。

2. Structure and Shape of the Amino Group | 氨基的结构与空间形状

The nitrogen atom in an amine has three sigma bonds and one lone pair of electrons. This gives a trigonal pyramidal shape around nitrogen, with bond angles of approximately 107°, similar to ammonia.

胺中的氮原子有三个 σ 键和一对孤对电子。这使得氮原子周围呈三角锥形，键角约为 107°，与氨类似。

N: three σ bonds + one lone pair → trigonal pyramidal, 107°

N：三个 σ 键 + 一对孤对电子 → 三角锥形，107°

The lone pair is not involved in bonding but occupies space. As a result, the H-N-H bond angle in ammonia is slightly less than the tetrahedral angle of 109.5° because lone pair-bond pair repulsion is greater than bond pair-bond pair repulsion.

孤对电子不参与成键但占据空间。因此，氨的 H-N-H 键角略小于正四面体角 109.5°，因为孤对电子-键对之间的排斥力大于键对-键对之间的排斥力。

3. Basicity of Amines: Lone Pair Protonation | 胺的碱性：孤对电子的质子化

Amines are weak bases because the nitrogen lone pair can accept a proton, H⁺, to form a substituted ammonium ion. For example, ethylamine reacts with water to produce ethylammonium ions and hydroxide ions.

胺是弱碱，因为氮原子上的孤对电子可以接受质子 H⁺，形成取代铵离子。例如，乙胺与水反应生成乙基铵离子和氢氧根离子。

CH₃CH₂NH₂ + H₂O ⇌ CH₃CH₂NH₃⁺ + OH⁻

Amines also react with acids to form salts. For instance, phenylamine reacts with hydrochloric acid to form phenylammonium chloride, an ionic salt that is soluble in water.

胺也可以与酸反应生成盐。例如，苯胺与盐酸反应生成苯基铵盐酸盐，这是一种可溶于水的离子盐。

C₆H₅NH₂ + HCl → C₆H₅NH₃⁺Cl⁻

4. Comparing Base Strength: Ammonia, Amines and Phenylamine | 碱性强弱比较：氨、脂肪胺和苯胺

Alkyl groups release electron density towards the nitrogen atom through the inductive effect. This makes the lone pair more available for protonation, so primary, secondary and tertiary aliphatic amines are usually stronger bases than ammonia.

烷基通过诱导效应向氮原子释放电子密度。这使孤对电子更容易接受质子，因此伯、仲、叔脂肪胺的碱性通常比氨强。

In contrast, phenylamine is a much weaker base than ammonia. The nitrogen lone pair is delocalised into the π system of the benzene ring, so it is less available to accept a proton.

相比之下，苯胺的碱性比氨弱得多。氮原子上的孤对电子离域进入苯环的 π 体系中，因此不易接受质子。

Substance	Relative basicity	Reason
Ammonia NH₃	Reference	Lone pair localised on N
Ethylamine CH₃CH₂NH₂	Stronger	Alkyl inductive effect increases electron density on N
Phenylamine C₆H₅NH₂	Weaker	Lone pair delocalised into benzene ring

物质	相对碱性	原因
氨 NH₃	参照	孤对电子定域在 N 上
乙胺 CH₃CH₂NH₂	更强	烷基诱导效应增加 N 上电子密度
苯胺 C₆H₅NH₂	更弱	孤对电子离域进入苯环

Do not simply state “more alkyl groups always make a stronger base” without explaining the inductive effect. Edexcel mark schemes reward explanation of electron density availability.

不要只写“烷基越多碱性越强”而不解释诱导效应。爱德思评分标准要求解释电子密度的可获得性。

5. Preparation of Aliphatic Amines | 脂肪胺的制备

Aliphatic amines can be prepared by nucleophilic substitution between a halogenoalkane and ammonia. Ammonia acts as a nucleophile because its lone pair attacks the partially positive carbon atom bonded to the halogen.

脂肪胺可以通过卤代烷与氨的亲核取代反应制备。氨作为亲核试剂，其孤对电子进攻与卤素相连的带部分正电荷的碳原子。

CH₃CH₂Br + NH₃ → CH₃CH₂NH₃⁺Br⁻ then CH₃CH₂NH₂ + NH₄Br

CH₃CH₂Br + NH₃ → CH₃CH₂NH₃⁺Br⁻ 然后 CH₃CH₂NH₂ + NH₄Br

A key practical detail is to use excess ammonia. This reduces the chance of the primary amine reacting further with halogenoalkane to form secondary, tertiary amines and quaternary ammonium salts.

一个关键的实验细节是使用过量的氨。这可以降低伯胺继续与卤代烷反应生成仲胺、叔胺和季铵盐的可能性。

Another method is the reduction of a nitrile, RCN, using lithium aluminium hydride, LiAlH₄, in dry ether. This gives a primary amine with one more carbon atom than the starting halogenoalkane.

另一种方法是用氢化铝锂 LiAlH₄ 在干燥乙醚中还原腈 RCN。这可以得到比起始卤代烷多一个碳原子的伯胺。

RCN + 4[H] → RCH₂NH₂ (using LiAlH₄)

RCN + 4[H] → RCH₂NH₂（使用 LiAlH₄）

6. Preparation of Phenylamine: Nitrobenzene Reduction | 苯胺的制备：硝基苯的还原

Phenylamine is made by reducing nitrobenzene, C₆H₅NO₂, using tin metal and concentrated hydrochloric acid, followed by addition of sodium hydroxide to liberate the free amine.

苯胺通过用锡和浓盐酸还原硝基苯 C₆H₅NO₂ 制得，然后加入氢氧化钠释放出游离胺。

C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O

Reagent: tin (Sn) and concentrated HCl, then NaOH
Conditions: reflux
Product: phenylamine

试剂：锡（Sn）和浓盐酸，然后加入 NaOH
条件：回流
产物：苯胺

This reaction is important because it shows a reduction of an aromatic nitro compound to an aromatic amine, a common pathway in dye and pharmaceutical synthesis.

该反应很重要，因为它展示了芳香族硝基化合物还原为芳香胺的路径，这在染料和药物合成中很常见。

7. Amides: Formation, Hydrolysis and Condensation | 酰胺：形成、水解与缩合

Amides contain the functional group RCONH₂. They can be formed by reacting an acyl chloride with ammonia or a primary amine. This is an addition-elimination reaction, not a simple nucleophilic substitution.

酰胺含有官能团 RCONH₂。它们可以通过酰氯与氨或伯胺反应制备。这是一个加成-消除反应，而不是简单的亲核取代。

CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl

Amides can be hydrolysed under acidic or basic conditions. Acidic hydrolysis produces a carboxylic acid and an ammonium salt; alkaline hydrolysis produces a carboxylate salt and ammonia or an amine.

酰胺可以在酸性或碱性条件下水解。酸性水解生成羧酸和铵盐；碱性水解生成羧酸盐和氨或胺。

CH₃CONH₂ + H₂O + HCl → CH₃COOH + NH₄Cl

CH₃CONH₂ + NaOH → CH₃COO⁻Na⁺ + NH₃

8. Amino Acids: Zwitterions and Isoelectric Point | 氨基酸：两性离子与等电点

Amino acids are bifunctional compounds containing both a basic amino group, NH₂, and an acidic carboxyl group, COOH. In aqueous solution, an internal acid-base reaction occurs to form a zwitterion.

氨基酸是双官能团化合物，同时含有碱性氨基 NH₂ 和酸性羧基 COOH。在水溶液中，分子内部发生酸碱反应形成两性离子。

H₂NCHRCOOH → H₃N⁺CHRCOO⁻

The zwitterion has both a positive and a negative charge but is overall neutral. At a specific pH called the isoelectric point, the zwitterion concentration is maximised and the amino acid has minimum solubility in water.

两性离子同时带正电荷和负电荷，但整体呈电中性。在特定 pH 值即等电点时，两性离子浓度最大，氨基酸在水中的溶解度最小。

In acid, the carboxylate group accepts a proton, giving a positive ion. In alkali, the ammonium group loses a proton, giving a negative ion. This makes amino acids amphoteric.

在酸性条件下，羧酸根接受质子形成正离子。在碱性条件下，铵基失去质子形成负离子。这使得氨基酸具有两性。

9. Peptide Bond Formation and Hydrolysis | 肽键的形成与水解

Two amino acids can join by a condensation reaction between the carboxyl group of one and the amino group of another. A molecule of water is eliminated and a peptide bond, -CONH-, is formed.

两个氨基酸可以通过一个氨基酸的羧基与另一个氨基酸的氨基之间的缩合反应连接。脱去一分子水，形成肽键 -CONH-。

H₂NCHRCOOH + H₂NCHR’COOH → H₂NCHRCONHCHR’COOH + H₂O

The product is a dipeptide. Longer chains are called polypeptides, and one or more polypeptide chains folded into a specific shape form a protein.

产物是二肽。更长的链称为多肽，一条或多条折叠成特定形状的多肽链形成蛋白质。

Peptide bonds can be broken by hydrolysis, either using hot aqueous acid or enzymes. This regenerates individual amino acids, which is important in digestion and protein analysis.

肽键可以通过水解断裂，使用热酸或酶均可。这可以再生单个氨基酸，在消化和蛋白质分析中十分重要。

10. Polyamides and Proteins | 聚酰胺与蛋白质

Polyamides are condensation polymers formed when diamine and dicarboxylic acid monomers react together. A familiar example is nylon-6,6, made from hexane-1,6-diamine and hexanedioic acid.

聚酰胺是由二胺和二羧酸单体通过缩聚反应形成的缩合聚合物。一个熟悉的例子是尼龙-6,6，由己二胺和己二酸制成。

n H₂N(CH₂)₆NH₂ + n HOOC(CH₂)₄COOH → [-NH(CH₂)₆NHCO(CH₂)₄CO-]ₙ + 2n H₂O

Proteins are natural polyamides. Hydrogen bonding between N-H and C=O groups in the peptide backbone holds the secondary structure, such as alpha-helices and beta-pleated sheets, in place.

蛋白质是天然聚酰胺。肽链骨架中 N-H 与 C=O 之间的氢键维持二级结构，如 α-螺旋和 β-折叠片层。

Disulfide bridges, ionic bonds and hydrophobic interactions also stabilise the tertiary structure. Disrupting these forces, for example by heating or changing pH, denatures the protein.

二硫键、离子键和疏水相互作用也稳定三级结构。加热或改变 pH 会破坏这些作用力，使蛋白质变性。

11. Exam Technique and Common Errors | 考试技巧与常见错误

When describing basicity, always mention the availability of the lone pair on nitrogen. Simply saying “alkyl groups push electrons” is not enough; link it to increased electron density on nitrogen and easier protonation.

在描述碱性时，一定要提到氮原子上孤对电子的可获得性。只写“烷基推电子”是不够的；要联系到氮上电子密度增加，更容易接受质子。

For phenylamine, avoid saying the lone pair is “removed” from the nitrogen. The accepted wording is that it is delocalised into the π system of the benzene ring, making it less available.

对于苯胺，不要说孤对电子从氮上“被拿走”。正确表述是它离域进入苯环的 π 体系中，使其不易参与反应。

When balancing equations for amino acid condensation or polyamide formation, check that water molecules are shown in the correct molar ratio. For nylon-6,6, 2n molecules of water are released per n repeating units.

在配平氨基酸缩合或聚酰胺生成方程式时，检查水分子的摩尔比是否正确。对于尼龙-6,6，每 n 个重复单元释放 2n 分子水。

Finally, remember that amide hydrolysis under alkaline conditions does not produce a free carboxylic acid. It produces a carboxylate salt, because the acid is immediately neutralised by the alkali.

最后，记住酰胺在碱性条件下水解不会

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Edexcel A-Level Chemistry Topic 17.5 Amines, Amides, Amino Acids and Proteins | 爱德思A-Level化学主题17.5 胺、酰胺、氨基酸与蛋白质