📚 Reactions of the Amides | 酰胺的反应
Amides are an important class of organic compounds derived from carboxylic acids. They contain the functional group –CONH₂, –CONHR, or –CONR₂. Understanding the characteristic reactions of amides is essential for A‑Level Chemistry, as these reactions illustrate key principles of nucleophilic acyl substitution, reduction, and molecular rearrangement. This article provides a detailed, bilingual guide to the principal reactions of amides relevant to the Cambridge International A‑Level syllabus.
酰胺是一类由羧酸衍生的重要有机化合物，其官能团为–CONH₂、–CONHR 或 –CONR₂。掌握酰胺的特征反应对 A‑Level 化学至关重要，因为这些反应展示了亲核酰基取代、还原和分子重排的核心原理。本文为 Cambridge International A‑Level 考试提供一份详尽的双语指南，系统讲解酰胺的主要反应。
1. Introduction to Amides | 酰胺简介
Amides are carboxylic acid derivatives in which the hydroxyl (–OH) group is replaced by an amino group (–NH₂, –NHR, or –NR₂). Depending on the degree of substitution at the nitrogen atom, they are classified as primary, secondary, or tertiary amides. Primary amides (RCONH₂) are the most frequently encountered in A‑Level reactions and serve as the focus of this article. The amide functional group is planar due to resonance, which imparts considerable stability and partial double‑bond character to the C–N bond.
酰胺是羧酸衍生物，其中的羟基 (–OH) 被氨基 (–NH₂、–NHR 或 –NR₂) 取代。根据氮原子上的取代程度，可分为伯酰胺、仲酰胺和叔酰胺。伯酰胺 (RCONH₂) 在 A‑Level 反应中最常见，也是本文的重点。由于共振作用，酰胺官能团呈平面结构，这使得 C–N 键具有相当大的稳定性和部分双键性质。
Because of this resonance stabilisation, amides are less reactive than acid chlorides or acid anhydrides towards nucleophilic attack. Their reactions typically require more vigorous conditions, such as prolonged heating with concentrated acids or strong bases, or the use of powerful reducing agents.
由于这种共振稳定作用，酰胺对亲核进攻的反应活性低于酰氯或酸酐。它们发生反应通常需要更剧烈的条件，例如与浓酸或强碱长时间加热，或者使用强还原剂。
2. Acidic Hydrolysis of Amides | 酰胺的酸性水解
When a primary amide is heated under reflux with a dilute aqueous acid, typically hydrochloric acid, it undergoes hydrolysis. The reaction cleaves the amide bond and produces the corresponding carboxylic acid and an ammonium salt.
当伯酰胺与稀酸（通常是盐酸）一起加热回流时，会发生水解反应。该反应断裂酰胺键，生成相应的羧酸和铵盐。
The general equation for the acidic hydrolysis of a primary amide is:
RCONH₂ + H₂O + H⁺ → RCOOH + NH₄⁺
伯酰胺酸性水解的通用方程式为：
RCONH₂ + H₂O + H⁺ → RCOOH + NH₄⁺
For example, ethanamide reacts with hydrochloric acid to form ethanoic acid and ammonium chloride:
例如，乙酰胺与盐酸反应生成乙酸和氯化铵：
CH₃CONH₂ + H₂O + HCl → CH₃COOH + NH₄Cl
The mechanism of acidic hydrolysis involves protonation of the carbonyl oxygen, which makes the carbonyl carbon more electrophilic. Water then acts as a nucleophile, attacking the carbonyl carbon. After a series of proton transfers, the tetrahedral intermediate collapses, expelling ammonia (which is immediately protonated to NH₄⁺) and forming the carboxylic acid.
酸性水解的机理包括羰基氧的质子化，这使羰基碳更具亲电性。然后水作为亲核试剂进攻羰基碳。经过一系列质子转移后，四面体中间体瓦解，释放出氨（随即被质子化形成 NH₄⁺）并生成羧酸。
3. Alkaline Hydrolysis of Amides | 酰胺的碱性水解
Primary amides are also hydrolysed when heated with an aqueous alkali, such as sodium hydroxide solution. This reaction produces the sodium salt of the carboxylic acid and liberates ammonia gas.
伯酰胺与碱的水溶液（如氢氧化钠溶液）一起加热时也会发生水解。该反应生成羧酸钠盐并释放出氨气。
The general equation is:
RCONH₂ + NaOH → RCOONa + NH₃
通用方程式为：
RCONH₂ + NaOH → RCOONa + NH₃
As a specific example, propanamide yields sodium propanoate and ammonia on heating with sodium hydroxide:
具体例子如丙酰胺与氢氧化钠加热生成丙酸钠和氨：
CH₃CH₂CONH₂ + NaOH → CH₃CH₂COONa + NH₃
In the alkaline hydrolysis mechanism, the hydroxide ion acts as a nucleophile and attacks the carbonyl carbon directly. The resulting tetrahedral intermediate loses an amide ion (–NH₂), which then deprotonates the carboxylic acid to form the carboxylate anion and ammonia. The evolution of ammonia gas on warming is a useful test for the presence of a primary amide.
在碱性水解机理中，氢氧根离子作为亲核试剂直接进攻羰基碳。生成的四面体中间体失去氨基负离子 (–NH₂)，接着氨基负离子夺取羧酸的质子形成羧酸根负离子和氨。加热时释放氨气是检验伯酰胺存在的一个有用方法。
4. Dehydration of Amides to Nitriles | 酰胺脱水生成腈
Primary amides can be dehydrated to form nitriles (R–CN) by treatment with a strong dehydrating agent. The reagent commonly used is phosphorus(V) oxide, P₄O₁₀, and the reaction is carried out by heating the solid amide with the oxide.
伯酰胺可以通过强脱水剂作用脱水生成腈 (R–CN)。通常使用的试剂是五氧化二磷 (P₄O₁₀)，反应是将固态酰胺与该氧化物一起加热。
The general conversion is:
RCONH₂ → RCN + H₂O
一般转化过程为：
RCONH₂ → RCN + H₂O
For instance, ethanamide is dehydrated to ethanenitrile (acetonitrile) when heated with P₄O₁₀:
例如，乙酰胺与 P₄O₁₀ 一起加热时脱水生成乙腈：
CH₃CONH₂ → CH₃CN + H₂O
This reaction is important in organic synthesis because the nitrile group can be further transformed into carboxylic acids (by hydrolysis) or amines (by reduction). The strong dehydrating agent removes the elements of water from the –CONH₂ group, leaving the highly polar cyano group.
该反应在有机合成中很重要，因为氰基可以进一步通过水解转化为羧酸，或通过还原转化为胺。强脱水剂从 –CONH₂ 基团中除去水的成分，留下高极性的氰基。
5. Reduction of Amides with Lithium Aluminium Hydride | 氢化铝锂还原酰胺
Amides are reduced to primary amines by the powerful reducing agent lithium aluminium hydride, LiAlH₄, in dry ether. This reaction converts the carbonyl group of the amide into a –CH₂– unit, giving a primary amine with the same number of carbon atoms as the original amide.
酰胺可以被强还原剂氢化铝锂 (LiAlH₄) 在无水乙醚中还原为伯胺。该反应将酰胺的羰基转化为 –CH₂– 单元，生成碳原子数与原来酰胺相同的伯胺。
The general reduction can be represented as:
一般还原过程可表示为：
RCONH₂ + 4[H] → RCH₂NH₂ + H₂O
For example, reduction of propanamide gives propylamine:
例如，丙酰胺还原得到丙胺：
CH₃CH₂CONH₂ + 4[H] → CH₃CH₂CH₂NH₂ + H₂O
The mechanism involves nucleophilic attack by hydride ions (H⁻) from LiAlH₄ at the carbonyl carbon, followed by elimination of the oxygen as an aluminate complex. The reaction is typically followed by an aqueous work‑up to destroy excess LiAlH₄ and liberate the amine. This reduction is particularly valuable because it produces a pure primary amine without the risk of further alkylation that occurs in other synthetic routes.
机理包括来自 LiAlH₄ 的氢负离子 (H⁻) 对羰基碳的亲核进攻，随后氧以铝酸盐配合物形式离去。反应通常在水溶液后处理以破坏过量的 LiAlH₄ 并释放出胺。这种还原特别有价值，因为它生成纯净的伯胺，而不会出现其他合成路线中进一步烷基化的风险。
6. Reaction of Amides with Nitrous Acid | 酰胺与亚硝酸的反应
Primary amides react with nitrous acid (HNO₂, generated in situ from sodium nitrite and a mineral acid) to give the corresponding carboxylic acid and nitrogen gas. This reaction is analogous to the reaction of primary aliphatic amines with nitrous acid, but the amide yields an acid instead of an unstable diazonium salt.
伯酰胺与亚硝酸 (由亚硝酸钠和无机酸原位生成的 HNO₂) 反应，生成相应的羧酸和氮气。该反应类似于脂肪族伯胺与亚硝酸的反应，但酰胺生成的是羧酸，而不是不稳定的重氮盐。
The overall transformation for a primary amide is:
伯酰胺的整体转化过程为：
RCONH₂ + HNO₂ → RCOOH + N₂ + H₂O
Thus, ethanamide treated with nitrous acid yields ethanoic acid with evolution of nitrogen gas:
因此，乙酰胺用亚硝酸处理时生成乙酸并释放氮气：
CH₃CONH₂ + HNO₂ → CH₃COOH + N₂ + H₂O
The mechanism proceeds via the formation of an N‑nitrosoamide intermediate, which rapidly decomposes to the carboxylic acid and dinitrogen. The vigorous evolution of nitrogen gas can be used to confirm the presence of a –CONH₂ group. Secondary and tertiary amides react differently with nitrous acid, often producing N‑nitroso compounds, but these are not typically required at A‑Level.
机理是通过形成 N‑亚硝基酰胺中间体进行的，该中间体迅速分解为羧酸和氮气。剧烈冒出氮气可用于确认 –CONH₂ 基团的存在。仲酰胺和叔酰胺与亚硝酸的反应方式不同，通常生成 N‑亚硝基化合物，但 A‑Level 一般不作要求。
7. Hofmann Rearrangement | 霍夫曼重排
The Hofmann rearrangement is a highly characteristic reaction of primary amides. When a primary amide is treated with bromine and an excess of strong base (usually aqueous sodium hydroxide), the carbonyl carbon is lost as carbonate, and a primary amine containing one carbon atom fewer than the original amide is produced.
霍夫曼重排是伯酰胺的一个极具特征的反应。当伯酰胺与溴和过量的强碱（通常为氢氧化钠水溶液）作用时，羰基碳以碳酸盐形式离去，生成的伯胺比原来的酰胺少一个碳原子。
The general equation for the Hofmann rearrangement is:
霍夫曼重排的通用方程式为：
RCONH₂ + Br₂ + 4NaOH → RNH₂ + Na₂CO₃ + 2NaBr + 2H₂O
For a specific example, ethanamide is converted into methylamine:
具体例子中，乙酰胺转变为甲胺：
CH₃CONH₂ + Br₂ + 4NaOH → CH₃NH₂ + Na₂CO₃ + 2NaBr + 2H₂O
The mechanism involves the formation of an N‑ ion to give a bromoamide anion. This anion undergoes an alkyl migration from the carbonyl carbon to the nitrogen atom, with simultaneous loss of bromide ion, forming an isocyanate intermediate (R‑ lysed by the aqueous alkali to the primary amine and carbonate. The Hofmann rearrangement is an important method for preparing pure primary amines in the laboratory.
该机理包括形成 N‑溴代酰胺，随即去质子化得到溴代酰胺负离子。该负离子发生烷基从羰基碳向氮原子的迁移，同时失去溴离子，形成异氰酸酯中间体 (R‑N=C=O)。然后异氰酸酯被碱水溶液水解成伯胺和碳酸盐。霍夫曼重排是实验室制备纯净伯胺的重要方法。
8. Comparison of Amide Reactivity with Other Carboxylic Acid Derivatives | 酰胺与其他羧酸衍生物的反应活性比较
Amides are the least reactive among the common carboxylic acid derivatives. The order of reactivity towards nucleophilic acyl substitution is: acid chloride > acid anhydride > ester > amide. This low reactivity is primarily due to the strong resonance donation of the nitrogen lone pair into the carbonyl π system, which reduces the electrophilicity of the carbonyl carbon.
在常见的羧酸衍生物中，酰胺的反应活性最低。亲核酰基取代的反应活性顺序为：酰氯 > 酸酐 > 酯 > 酰胺。这种低活性主要是因为氮的孤对电子通过共振强烈地供给羰基 π 系统，降低了羰基碳的亲电性。
Consequently, the hydrolysis of amides requires concentrated acids or alkalis and prolonged heating, whereas acid chlorides react violently with water at room temperature. Furthermore, amides cannot be converted directly to esters by simple alcoholysis under mild conditions; more forcing conditions or activation is required.
因此，酰胺的水解需要浓酸或浓碱并长时间加热，而酰氯在室温下就会与水剧烈反应。此外，酰胺在温和条件下无法通过简单的醇解直接转化为酯，需要更剧烈的条件或活化。
The table below summarises the relative reactivities of common carboxylic acid derivatives under typical hydrolysis conditions:
下表总结了常见羧酸衍生物在典型水解条件下的相对反应活性：
| Derivative | Typical hydrolysis conditions | Products |
|---|---|---|
| Acid chloride, RCOCl | Water, room temp. | RCOOH + HCl |
| Acid anhydride, (RCO)₂O | Warm water | 2RCOOH |
| Ester, RCOOR´ | Dil. acid or alkali, heat | RCOOH + R´OH |
| Amide, RCONH₂ | Conc. acid or alkali, prolonged heat | RCOOH + NH₄⁺ / NH₃ |
9. Reaction Conditions and Key Observations | 反应条件与关键现象
Identifying the products of amide reactions often relies on observing characteristic changes. During acidic hydrolysis, the reaction mixture becomes a homogeneous solution of the carboxylic acid and ammonium salt. Alkaline hydrolysis produces the unmistakable odour of ammonia gas, which turns red litmus paper blue.
鉴别酰胺反应的产物通常依赖于观察特征变化。在酸性水解过程中，反应混合物变为羧酸和铵盐的均一溶液。碱性水解会产生明显的氨气气味，使红色石蕊试纸变蓝。
In the Hofmann rearrangement, the gradual disappearance of the orange‑brown bromine colour signals the consumption of Br₂ and the formation of the amine. When amides are dehydrated with P₄O₁₀, the nitrile formed can be identified by its characteristic sweet, ethereal odour and by boiling point determination.
在霍夫曼重排中，橙棕色溴颜色逐渐消失，标志着 Br₂ 被消耗和胺的形成。当酰胺用 P₄O₁₀ 脱水时，生成的腈可通过其特有的甜味、醚样气味及沸点测定来鉴定。
It is also worth noting that the reaction with nitrous acid can be used as a qualitative test for primary amides: the evolution of a colourless, odourless gas (N₂) that does not support combustion is a positive indication.
还值得注意的是，与亚硝酸的反应可用作伯酰胺的定性检验：放出无色、无味且不支持燃烧的气体 (N₂) 即表明有伯酰胺存在。
10. Summary of Key Amide Reactions | 酰胺关键反应总结
The following table provides a concise summary of the principal reactions of primary amides covered in the Cambridge International A‑Level syllabus. It is designed as a quick reference for revision.
下表简洁地总结了 Cambridge International A‑Level 大纲中涉及的伯酰胺主要反应，可作为复习时的快速参考。
| Reaction | Reagents & Conditions | Organic Product |
|---|---|---|
| Acidic hydrolysis | Dil. HCl (aq.), heat under reflux | Carboxylic acid (RCOOH) |
| Alkaline hydrolysis | NaOH (aq.), heat | Carboxylate salt (RCOONa) |
| Dehydration | P₄O₁₀, heat | Nitrile (RCN) |
| Reduction | LiAlH₄ in dry ether, then H₂O | Primary amine (RCH₂NH₂) |
| Reaction with HNO₂ | NaNO₂ + dil. HCl, cold | Carboxylic acid (RCOOH) + N₂ |
| Hofmann rearrangement | Br₂ + excess NaOH, heat | Primary amine (RNH₂, one C fewer) |
By mastering these transformations, students can confidently tackle examination questions on amide chemistry, including prediction of products, reaction conditions, and mechanistic understanding. Remember that the relatively low reactivity of amides means that most reactions require heating and strong reagents.
通过掌握这些转化，学生可以自信地应对涉及酰胺化学的考试题目，包括产物预测、反应条件和机理理解。请记住，酰胺相对较低的反应活性意味着大多数反应需要加热和强试剂。
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