A-Level Chemistry | Typical Reactions of Amides | A-Level 化学：酰胺的典型反应总结

📚 A-Level Chemistry | Typical Reactions of Amides | A-Level 化学：酰胺的典型反应总结

Amides are derivatives of carboxylic acids in which the hydroxyl group of the acid is replaced by an amino group or a substituted amino group. They are an important class of nitrogen-containing organic compounds frequently examined in CIE A-Level Chemistry, particularly in Paper 4 structured questions. This article summarises the key reactions of amides, including hydrolysis, reduction, the Hofmann degradation, nitrous acid reactions, and dehydration, with a focus on conditions, reagents, and reaction mechanisms.

酰胺是羧酸的衍生物，其结构特点是羧酸中的羟基被氨基或取代氨基替代。酰胺是含氮有机化合物中的重要一类，在 CIE A-Level 化学考试中，尤其 Paper 4 的问答题中经常出现。本文总结了酰胺的主要反应，包括水解、还原、霍夫曼降解、与亚硝酸的反应以及脱水反应，重点关注试剂、条件和反应机理。

1. Structure and Basicity of Amides | 酰胺的结构与碱性

The amide functional group consists of a carbonyl group (C=O) directly bonded to a nitrogen atom. The general formula of a simple amide is RCONH₂. The nitrogen atom in an amide is less basic than that in an amine because the lone pair on nitrogen is delocalised into the carbonyl group via resonance, forming a partial C-N double bond character.

酰胺官能团包含一个与氮原子直接相连的羰基（C=O），简单酰胺的通式为 RCONH₂。酰胺中的氮原子碱性弱于胺中的氮原子，这是因为氮原子上的孤对电子通过共振离域到羰基上，形成了部分 C=N 双键特征。

RCONH₂ ⇌ RC(OH)=NH

This delocalisation makes the nitrogen lone pair less available for protonation, so amides are essentially neutral or very weakly basic. Consequently, amides do not turn red litmus blue and do not form stable salts with dilute acids.

这种离域效应使氮原子上的孤对电子不易参与质子化，因此酰胺几乎呈中性或极弱的碱性。因此，酰胺不能使红色石蕊变蓝，也不能与稀酸形成稳定的盐。

2. Preparation of Amides | 酰胺的制备

A common laboratory method for preparing amides is the reaction between an acyl chloride (or acid anhydride) and ammonia or an amine. Acyl chlorides are highly reactive and react vigorously with ammonia or amines at room temperature.

实验室制备酰胺的常用方法是用酰氯（或酸酐）与氨或胺反应。酰氯反应活性高，在室温下即可与氨或胺剧烈反应。

CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl

In this reaction, one mole of ammonia reacts as a nucleophile to form the amide, while a second mole of ammonia removes the hydrogen chloride produced, forming ammonium chloride. When a primary amine is used instead of ammonia, an N-substituted amide is obtained.

在此反应中，一分子氨作为亲核试剂进攻酰氯生成酰胺，另一分子氨则除去反应中生成的氯化氢，形成氯化铵。若使用伯胺代替氨，则得到 N-取代酰胺。

CH₃COCl + CH₃NH₂ → CH₃CONHCH₃ + HCl

Amides can also be prepared by heating the ammonium salt of a carboxylic acid. For example, heating ammonium ethanoate produces ethanamide and water.

酰胺还可以通过加热羧酸的铵盐来制备。例如，加热乙酸铵可生成乙酰胺和水。

CH₃COONH₄ → CH₃CONH₂ + H₂O

3. Acid-Catalysed Hydrolysis | 酸催化水解

Amides undergo hydrolysis in the presence of hot dilute acid, typically dilute hydrochloric acid or dilute sulfuric acid. Under acidic conditions, the amide is converted back to the parent carboxylic acid and an ammonium salt.

在热的稀酸（通常为稀盐酸或稀硫酸）存在下，酰胺发生水解。在酸性条件下，酰胺被转化回原来的羧酸和铵盐。

RCONH₂ + H₂O + H⁺ → RCOOH + NH₄⁺

For example, ethanamide heated with dilute hydrochloric acid produces ethanoic acid and ammonium chloride. This reaction is the reverse of amide formation and requires heating under reflux for a considerable time because the amide group is relatively stable.

例如，乙酰胺与稀盐酸共热时生成乙酸和氯化铵。该反应是酰胺生成的逆反应，需要在回流条件下加热较长时间，因为酰胺基团相对稳定。

CH₃CONH₂ + HCl + H₂O → CH₃COOH + NH₄Cl

The carboxylic acid product can be recovered by cooling the reaction mixture and separating the organic layer, or by distillation as appropriate.

羧酸产物可以通过冷却反应混合物并分离有机层来回收，或在适当时通过蒸馏分离。

4. Base-Catalysed Hydrolysis | 碱催化水解

When an amide is heated with hot aqueous sodium hydroxide or potassium hydroxide, hydrolysis occurs to produce a carboxylate salt and ammonia gas. The ammonia can be detected by its characteristic smell and by its ability to turn moist red litmus paper blue.

当酰胺与热氢氧化钠或氢氧化钾水溶液共热时，发生水解生成羧酸盐和氨气。氨气可通过其特征气味以及使湿润的红色石蕊试纸变蓝来检验。

RCONH₂ + NaOH → RCOONa + NH₃

For example, ethanamide heated with sodium hydroxide solution gives sodium ethanoate and ammonia. The carboxylate salt can then be acidified to liberate the free carboxylic acid.

例如，乙酰胺与氢氧化钠溶液共热生成乙酸钠和氨。随后可向羧酸盐中加入酸以释放出游离的羧酸。

CH₃CONH₂ + NaOH → CH₃COONa + NH₃

This is a useful method for distinguishing an amide from an amine, as amines do not produce ammonia gas when treated with hot alkali unless they are ammonium salts.

这是区分酰胺与胺的有用方法，因为胺（除非是铵盐）在热碱处理下不会产生氨气。

5. Reduction of Amides | 酰胺的还原

Amides can be reduced to primary amines using lithium aluminium hydride (LiAlH₄) in anhydrous ether, followed by hydrolysis of the intermediate complex. This is a powerful reducing agent that completely reduces the carbonyl group of the amide to a methylene group.

酰胺可用氢化铝锂（LiAlH₄）在无水乙醚中还原为伯胺，随后水解中间络合物。LiAlH₄ 是一种强还原剂，可将酰胺中的羰基完全还原为亚甲基。

RCONH₂ + 4[H] → RCH₂NH₂ + H₂O

For example, ethanamide is reduced to ethylamine. Note that the product has one more carbon-nitrogen bond but the carbon chain length remains the same, with the carbonyl carbon becoming a CH₂ group.

例如，乙酰胺被还原为乙胺。注意产物中碳链长度不变，但羰基碳变为 CH₂ 基团。

CH₃CONH₂ → CH₃CH₂NH₂

Although LiAlH₄ is a standard reagent for this conversion, catalytic hydrogenation is generally not effective for reducing amides under mild conditions. The reduction of amides is an important synthetic route to amines, especially when primary amines of specific chain length are required.

虽然 LiAlH₄ 是实现该转化的标准试剂，但催化氢化在温和条件下通常不能有效还原酰胺。酰胺的还原是合成胺的重要途径，尤其适合制备特定链长的伯胺。

6. Hofmann Degradation | 霍夫曼降解反应

The Hofmann degradation, also known as the Hofmann rearrangement, is a reaction in which a primary amide is treated with bromine or chlorine in the presence of aqueous sodium hydroxide to form a primary amine with one fewer carbon atom. The carbon chain is shortened by one carbon because the carbonyl carbon is lost as carbon dioxide.

霍夫曼降解（又称霍夫曼重排）是指伯酰胺在氢氧化钠水溶液存在下与溴或氯反应，生成少一个碳原子的伯胺的反应。由于羰基碳以二氧化碳形式失去，所以碳链缩短了一个碳原子。

RCONH₂ + Br₂ + 4NaOH → RNH₂ + Na₂CO₃ + 2NaBr + 2H₂O

For example, ethanamide undergoes Hofmann degradation to produce methylamine:

例如，乙酰胺经霍夫曼降解生成甲胺：

CH₃CONH₂ + Br₂ + 4NaOH → CH₃NH₂ + Na₂CO₃ + 2NaBr + 2H₂O

The mechanism involves formation of an N-bromoamide, which rearranges to an isocyanate intermediate; the isocyanate then undergoes hydrolysis to give the primary amine and carbon dioxide. This reaction is particularly important because it enables the conversion of a carboxylic acid derivative into a shorter-chain amine, a transformation not easily achieved by other methods.

该反应机理涉及 N-溴代酰胺的生成，然后重排为异氰酸酯中间体；异氰酸酯随后水解生成伯胺和二氧化碳。该反应之所以重要，是因为它可以将羧酸衍生物转化为少一个碳的胺，实现其他方法难以完成的转化。

7. Reaction with Nitrous Acid | 酰胺与亚硝酸的反应

Primary amides react with nitrous acid (HNO₂) at room temperature to produce the corresponding carboxylic acid, nitrogen gas, and water. Nitrous acid is usually generated in situ by reacting sodium nitrite with a dilute mineral acid such as hydrochloric acid.

伯酰胺在室温下与亚硝酸（HNO₂）反应生成相应的羧酸、氮气和水。亚硝酸通常通过亚硝酸钠与稀酸（如盐酸）反应原位生成。

RCONH₂ + HNO₂ → RCOOH + N₂ + H₂O

For example, ethanamide reacts with nitrous acid to give ethanoic acid, nitrogen, and water:

例如，乙酰胺与亚硝酸反应生成乙酸、氮气和水：

CH₃CONH₂ + HNO₂ → CH₃COOH + N₂ + H₂O

The effervescence of nitrogen gas is observable, and this reaction provides a qualitative test for the presence of a primary amide group. It is analogous to the reaction of primary amines with nitrous acid, where nitrogen gas is also evolved, but the organic product differs: amines give alcohols, whereas amides give carboxylic acids.

观察到氮气气泡即可定性确认伯酰胺基团的存在。这一反应与伯胺和亚硝酸的反应类似，两者均产生氮气，但有机产物不同：胺生成醇，而酰胺生成羧酸。

8. Dehydration of Amides | 酰胺的脱水反应

Primary amides can be dehydrated to form nitriles (alkanenitriles) using a strong dehydrating agent such as phosphorus(V) oxide (P₂O₅) or thionyl chloride (SOCl₂). The reaction removes a molecule of water from the amide group, leaving a C≡N triple bond.

伯酰胺可用强脱水剂如五氧化二磷（P₂O₅）或氯化亚砜（SOCl₂）脱水生成腈。反应从酰胺基团中脱去一分子水，留下 C≡N 三键。

RCONH₂ → RCN + H₂O

For example, ethanamide heated with phosphorus(V) oxide yields ethanenitrile:

例如，乙酰胺与五氧化二磷共热生成乙腈：

CH₃CONH₂ → CH₃CN + H₂O

This reaction is the reverse of nitrile hydrolysis. Nitriles are useful intermediates because they can be hydrolysed back to carboxylic acids or reduced to primary amines, providing versatile synthetic routes.

该反应是腈水解的逆反应。腈是有用的中间体，因为它们可以被水解回羧酸或还原为伯胺，提供多种合成路径。

9. Relative Reactivity of Amide Derivatives | 酰胺衍生物的相对反应活性

Amides are the least reactive of the carboxylic acid derivatives. The order of reactivity for nucleophilic acyl substitution is: acyl chlorides > acid anhydrides > esters > amides. This is due to the electron-donating effect of the nitrogen atom, which reduces the positive character of the carbonyl carbon and stabilises the amide through resonance.

酰胺是羧酸衍生物中反应活性最低的。亲核酰基取代反应活性顺序为：酰氯 > 酸酐 > 酯 > 酰胺。这是因为氮原子的给电子效应降低了羰基碳的正电性，并通过共振稳定了酰胺分子。

As a result, amides do not react with water, alcohols, or ammonia under mild conditions. Harsh conditions, such as prolonged heating with strong acids or bases, are required for hydrolysis. This low reactivity explains why amide bonds are stable in proteins and synthetic polymers such as nylon.

因此，酰胺在温和条件下不与水、醇或氨反应。水解需要较强烈的条件，如长时间与强酸或强碱加热。这种低反应活性解释了为何酰胺键在蛋白质和尼龙等合成聚合物中是稳定的。

10. Summary of Amide Reactions | 酰胺反应总结

The table below summarizes the key reactions of amides, including reagents, conditions, and major products. This provides a quick revision reference for examinations.

下表总结了酰胺的主要反应，包括试剂、条件和主要产物，便于考试前快速复习。

Reaction	Reagent / Condition	Major Product
Acidic hydrolysis	Dilute HCl, heat under reflux	Carboxylic acid + NH₄⁺
Basic hydrolysis	NaOH (aq), heat	Carboxylate salt + NH₃
Reduction	LiAlH₄ in dry ether, then H₂O	Primary amine
Hofmann degradation	Br₂ + NaOH (aq)	Primary amine (one C less)
Nitrous acid	HNO₂ (from NaNO₂ + HCl), room temp	Carboxylic acid + N₂ + H₂O
Dehydration	P₂O₅, heat	Nitrile

Amides can also be converted to N-substituted amides through reaction with acyl chlorides or acid anhydrides, which is an important route for building larger molecules in organic synthesis.

酰胺还可以通过与酰氯或酸酐反应转化为 N-取代酰胺，这是在有机合成中构建更大分子的重要途径。

Understanding the distinct reactivity of amides enables students to predict products, plan multi-step syntheses, and explain the stability of biological and synthetic amide bonds. In CIE A-Level examinations, questions on amides often appear in the context of organic synthesis pathways, so it is essential to memorise the reagents and conditions for each reaction.

理解酰胺的反应特性有助于学生预测产物、设计多步合成路线，并解释生物及合成酰胺键的稳定性。在 CIE A-Level 考试中，有关酰胺的题目常出现在有机合成路线题中，因此务必牢记每个反应所需的试剂和条件。

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A-Level Chemistry | Typical Reactions of Amides | A-Level 化学：酰胺的典型反应总结