Acyl Chlorides | 酰基氯

📚 Acyl Chlorides | 酰基氯

Acyl chlorides (also known as acid chlorides) are a highly reactive family of organic compounds derived from carboxylic acids by replacing the –OH group with a chlorine atom. They contain the functional group –COCl. The presence of the electronegative chlorine atom makes the carbonyl carbon extremely susceptible to attack by nucleophiles. In the Cambridge A-Level Chemistry syllabus, you are expected to understand their preparation, physical properties, and the characteristic nucleophilic addition–elimination mechanism that governs their reactions with water, alcohols, ammonia, amines, and other nucleophiles. Their vigorous reactivity and ability to form useful derivatives make them important intermediates in organic synthesis.

酰基氯（又称酰氯）是一族反应活性极高的有机化合物，由羧酸中的 –OH 被氯原子取代而衍生得到，其官能团为 –COCl。电负性极强的氯原子使羰基碳极易受到亲核试剂的进攻。在剑桥 A-Level 化学大纲中，学生需要掌握酰基氯的制备、物理性质，以及支配其与水、醇、氨、胺等亲核试剂反应的特征性亲核加成–消除机理。它们反应活泼，能生成多种重要的衍生物，因此是有机合成中不可或缺的中间体。

1. Introduction to Acyl Chlorides | 酰基氯简介

Acyl chlorides are carboxylic acid derivatives where the hydroxyl group is replaced by a chlorine atom. The general formula for an aliphatic acyl chloride is R–COCl, where R is an alkyl or aryl group. The simplest member is ethanoyl chloride, CH₃COCl, widely used in the laboratory. The –COCl group is planar, with the carbonyl carbon being sp² hybridised and bonded to a highly electronegative chlorine atom. This structural arrangement withdraws electron density from the carbonyl carbon, enhancing its electrophilicity and making it reactive towards nucleophiles such as water, alcohols, and amines.

酰基氯是羧酸衍生物，分子中的羟基被氯原子替代。脂肪族酰基氯的通式为 R–COCl，R 代表烷基或芳基。最简单的成员是乙酰氯 CH₃COCl，在实验室中广泛使用。–COCl 基团呈平面形，羰基碳为 sp² 杂化，并与电负性极强的氯原子相连。这一结构从羰基碳上拉走电子云，增强了它的亲电性，使其对水、醇、胺等亲核试剂表现出高反应活性。

2. Nomenclature of Acyl Chlorides | 酰基氯的命名

The systematic naming of acyl chlorides follows IUPAC conventions: the name is derived from the corresponding carboxylic acid by replacing '-oic acid' with '-oyl chloride'. For example, ethanoic acid becomes ethanoyl chloride, propanoic acid becomes propanoyl chloride. If the –COCl group is attached to a ring, the suffix '-carbonyl chloride' is used, as in benzenecarbonyl chloride (commonly known as benzoyl chloride). When numbering the carbon chain, the carbonyl carbon is always designated as carbon number 1.

酰基氯的系统命名遵循 IUPAC 规则：名称由相应的羧酸衍生而来，将 “-oic acid” 替换为 “-oyl chloride”。例如，ethanoic acid 对应 ethanoyl chloride，propanoic acid 对应 propanoyl chloride。若 –COCl 基团连接到环上，则使用后缀 “-carbonyl chloride”，如苯甲酰氯（benzenecarbonyl chloride，俗称 benzoyl chloride）。碳链编号时，羰基碳始终定为 1 号碳。

3. Physical Properties | 物理性质

tly lower than those of the parent carboxylic acids of comparable molecular mass, because they cannot form intermolecular hydrogen bonds. For instance, ethanoyl chloride (CH₃COCl, Mᵣ = 78.5) boils at 51 °C, whereas ethanoic acid (CH₃COOH, Mᵣ = 60) boils at 118 °C. Acyl chlorides are immiscible with water but react violently with it, so they are not simply 'insoluble'—they undergo chemical decomposition upon contact with water.

低级酰基氯（如乙酰氯）是无色发烟液体，具有刺激性辛辣气味。它们的沸点略低于相对分子质量相近的母体羧酸，这是因为酰基氯分子间无法形成氢键。例如，乙酰氯（CH₃COCl，相对分子质量 78.5）沸点为 51 °C，而乙酸（CH₃COOH，相对分子质量 60）沸点为 118 °C。酰基氯与水不混溶，但遇水剧烈反应，因此它们不仅仅是“不溶于水”——一旦接触水便发生化学分解。

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4. Preparation of Acyl Chlorides | 酰基氯的制备

Acyl chlorides are typically prepared by treating a carboxylic acid with a chlorinating agent such as thionyl chloride (SOCl₂), phosphorus pentachloride (PCl₅), or phosphorus trichloride (PCl₃). The reaction with thionyl chloride is preferred in the laboratory because gaseous by-products (SO₂ and HCl) escape, leaving the crude acyl chloride that can be purified by distillation:

RCOOH + SOCl₂ → RCOCl + SO₂ + HCl

Using phosphorus pentachloride, solid products and phosphoryl chloride (POCl₃) are formed, which may complicate isolation. In all cases, the hydroxyl –OH group of the carboxylic acid is replaced by a chlorine atom. The reaction is not reversible, and yields are generally high.

制备酰基氯通常使用氯化试剂与羧酸反应，常用的试剂有氯化亚砜（SOCl₂）、五氯化磷（PCl₅）或三氯化磷（PCl₃）。在实验室中优先使用氯化亚砜，因为副产物（SO₂ 和 HCl）均为气体逸出，得到粗酰基氯，再通过蒸馏提纯：

RCOOH + SOCl₂ → RCOCl + SO₂ + HCl

使用五氯化磷时，会生成固体产物和三氯氧磷（POCl₃），使分离变得复杂。无论哪种方法，羧酸中的 –OH 都被氯原子取代。该反应不可逆，产率通常很高。

5. General Reactivity: Nucleophilic Addition–Elimination | 一般反应性：亲核加成-消除机理

The high reactivity of acyl chlorides is explained by the nucleophilic addition–elimination mechanism. The carbonyl carbon bears a significant partial positive charge due to the electron-withdrawing effect of both the oxygen and chlorine atoms. A nucleophile, Nu⁻, attacks this electrophilic carbon, forming a tetrahedral intermediate. In the next step, the chloride ion acts as a leaving group, and the carbonyl group is reformed. The overall outcome is substitution of the –Cl by the nucleophile, accompanied by elimination of HCl. This two-step process is general for all reactions involving water, alcohols, ammonia, and amines.

酰基氯的高反应活性可通过亲核加成–消除机理解释。由于氧和氯原子的吸电子效应，羰基碳带有显著的正电性。亲核试剂 Nu⁻ 进攻这个亲电碳，形成四面体中间体。接着，氯离子作为离去基团离去，羰基得以重建。总的结果是 –Cl 被亲核试剂取代，同时消除一分子 HCl。这一两步过程普遍适用于水、醇、氨和胺等亲核试剂的反应。

The tetrahedral intermediate is short-lived; the expulsion of the chloride ion is facilitated by the fact that Cl⁻ is a weak base and a good leaving group. The reactivity order of carboxylic acid derivatives is: acyl chloride > acid anhydride > ester > amide. This can be rationalised by the electron-withdrawing ability and leaving-group aptitude of the substituent attached to the carbonyl carbon.

四面体中间体存在时间极短；Cl⁻ 是弱碱且是良好的离去基团，从而使氯离子的离去较为容易。羧酸衍生物的反应活性顺序为：酰基氯 > 酸酐 > 酯 > 酰胺。这可以用与羰基碳相连的取代基的吸电子能力和离去基团倾向加以解释。

6. Hydrolysis: Reaction with Water | 水解反应：与水的反应

Acyl chlorides react vigorously with water at room temperature to produce the corresponding carboxylic acid and hydrogen chloride. The reaction is exothermic and, for lower acyl chlorides, appears violent with visible fumes of HCl gas. The general equation is:

RCOCl + H₂O → RCOOH + HCl

For example, ethanoyl chloride reacts with water to give ethanoic acid and HCl. This is why acyl chlorides must be stored in sealed containers away from moisture. The mechanism involves water acting as the nucleophile: one lone pair on the oxygen of water attacks the carbonyl carbon, chloride leaves, and a proton is lost from the intermediate to form the carboxylic acid.

酰基氯在室温下与水剧烈反应，生成相应的羧酸和氯化氢。该反应放热，对于低级酰基氯，可见明显的 HCl 烟雾，反应十分剧烈。通式如下：

RCOCl + H₂O → RCOOH + HCl

例如，乙酰氯与水反应生成乙酸和 HCl。这就是为什么酰基氯必须密封保存、避免接触潮气。反应机理中，水作为亲核试剂：水分子氧上的一对孤对电子进攻羰基碳，氯离子离去，中间体失去一个质子便得到羧酸。

7. Reaction with Alcohols and Phenols | 与醇和酚的反应

Acyl chlorides react with alcohols to produce esters and hydrogen chloride. This reaction occurs rapidly at room temperature and does not require an acid catalyst, unlike Fischer esterification of carboxylic acids. The general equation is:

RCOCl + R'OH → RCOOR' + HCl

For example, ethanoyl chloride reacts with ethanol to form ethyl ethanoate. The reaction with phenols proceeds similarly, yielding phenyl esters. In the mechanism, the alcohol oxygen attacks the carbonyl carbon, the chloride leaves, and deprotonation gives the ester. This method is particularly useful for preparing esters that are difficult to obtain via direct acid–alcohol equilibrium, such as those from tertiary alcohols or phenols.

酰基氯与醇反应生成酯和氯化氢。此反应在室温下即可迅速发生，且不需要酸催化剂，这与羧酸的费歇尔酯化反应不同。反应通式为：

RCOCl + R'OH → RCOOR' + HCl

例如，乙酰氯与乙醇反应生成乙酸乙酯。与酚的反应类似，得到苯基酯。在机理中，醇的氧原子进攻羰基碳，氯离子离去，脱质子后形成酯。该方法特别适用于那些难以通过酸-醇平衡直接得到的酯，例如由叔醇或酚制备的酯。

8. Reaction with Ammonia and Amines | 与氨和胺的反应

Acyl chlorides react with ammonia and amines to form amides. With ammonia, an unsubstituted amide is produced together with ammonium chloride. The reaction is rapid and can be represented as:

RCOCl + 2NH₃ → RCONH₂ + NH₄Cl

Two equivalents of ammonia are required: one acts as the nucleophile, attacking the carbonyl carbon, and the second neutralises the HCl produced. With primary amines, N-substituted amides are obtained. The mechanism is nucleophilic addition–elimination, with the amine acting as the nucleophile. The nitrogen atom donates its lone pair to the carbonyl carbon, the tetrahedral intermediate forms, chloride leaves, and deprotonation yields the amide. This reaction is an excellent method for making amides under mild conditions.

酰基氯与氨和胺反应生成酰胺。与氨反应时，生成未取代的酰胺和氯化铵。该反应迅速，可表示为：

RCOCl + 2NH₃ → RCONH₂ + NH₄Cl

需要两当量的氨：一分子作为亲核试剂进攻羰基碳，另一分子用于中和生成的 HCl。与伯胺反应则得到 N-取代酰胺。机理为亲核加成–消除，胺充当亲核试剂。氮原子提供孤对电子进攻羰基碳，形成四面体中间体，氯离子离去，脱质子后得到酰胺。该反应是在温和条件下制备酰胺的优良方法。

9. Formation of Acid Anhydrides | 酸酐的生成

Acyl chlorides react with carboxylate salts (e.g., sodium ethanoate) to produce acid anhydrides. The carboxylate ion acts as a nucleophile, displacing the chloride ion. The general reaction is:

RCOCl + R'COO⁻Na⁺ → RCO-O-COR' + NaCl

This is a useful route to symmetric or mixed anhydrides. For instance, ethanoyl chloride reacting with sodium ethanoate gives ethanoic anhydride, (CH₃CO)₂O. In the laboratory, this reaction can be performed under anhydrous conditions. The nucleophilic addition–elimination mechanism again applies, with the carboxylate oxygen attacking the carbonyl carbon of the acyl chloride.

酰基氯与羧酸盐（如乙酸钠）反应生成酸酐。羧酸根离子作为亲核试剂，取代氯离子。一般反应为：

RCOCl + R'COO⁻Na⁺ → RCO-O-COR' + NaCl

这是制备对称或混合酸酐的有效途径。例如，乙酰氯与醋酸钠反应得到乙酸酐 (CH₃CO)₂O。在实验室中，此反应可在无水条件下进行。机理同样为亲核加成–消除，由羧酸根的氧原子进攻酰基氯的羰基碳。

10. Reduction of Acyl Chlorides | 酰基氯的还原

Acyl chlorides can be reduced to primary alcohols or aldehydes, depending on the reducing agent used. Lithium aluminium hydride (LiAlH₄) in dry ether reduces acyl chlorides all the way to primary alcohols. The overall transformation replaces the –COCl group with –CH₂OH, with breaking of the C–Cl bond and reduction of the carbonyl group. The reaction pathway is aggressive and not easily stopped at the aldehyde stage because the intermediate aldehyde is also reduced under the conditions. A milder reducing agent such as lithium tri-tert-butoxyaluminium hydride can be used to obtain aldehydes from acyl chlorides, but LiAlH₄ reduction is the commonly cited example in A-Level syllabuses.

酰基氯可以被还原为伯醇或醛，取决于所使用的还原剂。在干燥乙醚中使用氢化铝锂 (LiAlH₄)，可将酰基氯完全还原为伯醇。总过程是将 –COCl 基团转化为 –CH₂OH，包括 C–Cl 键的断裂和羰基的还原。此反应路径非常剧烈，很难停止在醛的阶段，因为生成的醛在反应条件下也会被还原。若使用位阻更大的还原剂，如三叔丁氧基氢化铝锂，可以从酰基氯得到醛，但在 A-Level 大纲中通常只提及 LiAlH₄ 还原得到伯醇。

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11. Comparison with Carboxylic Acids | 与羧酸的比较

Acyl chlorides are much more reactive than the parent carboxylic acids. This is because the –OH group in carboxylic acids is a poor leaving group, whereas the –Cl in acyl chlorides is an excellent leaving group. In nucleophilic substitution reactions, the rate-determining step for acyl chlorides is the initial nucleophilic attack; the chloride leaves readily. For carboxylic acids, the –OH group must be activated (e.g. by protonation) before it can leave as water, which is why esterification with alcohols requires an acid catalyst and heating. Acyl chlorides achieve the same transformations under mild conditions, often at room temperature, with faster rates and higher yields. However, acyl chlorides are more hazardous—they are corrosive, lachrymatory, and hydrolyse exothermically, whereas carboxylic acids are generally safer to handle.

酰基氯的反应活性远高于母体羧酸。这是因为羧酸中的 –OH 是较差的离去基团，而酰基氯中的 –Cl 是极好的离去基团。在亲核取代反应中，酰基氯的决速步骤是最初的亲核进攻；氯离子轻易离去。而羧酸的 –OH 必须先被活化（如质子化）才能以水的形式离去，因此与醇的酯化反应需要酸催化剂和加热。酰基氯在温和条件下（常为室温）就能实现相同的转化，速率更快、产率更高。不过，酰基氯更危险——具有腐蚀性和催泪性，且水解放热，而羧酸通常更安全。

Furthermore, acyl chlorides are more easily converted into amides, esters, and anhydrides without the need for coupling reagents or forcing conditions. In synthetic organic chemistry, this reactivity is exploited to efficiently install acyl groups onto molecules, making acyl chlorides a valuable tool for synthesis.

此外，酰基氯更容易转化为酰胺、酯和酸酐，无需偶联试剂或苛刻条件。在合成有机化学中，人们利用这一反应活性，将酰基高效地接入目标分子，使酰基氯成为宝贵的合成工具。

12. Summary and Key Points | 总结与要点

Acyl chlorides possess the functional group –COCl. They are named by replacing '-oic acid' with '-oyl chloride'. They are prepared by reacting carboxylic acids with SOCl₂, PCl₅, or PCl₃, and their key chemical feature is high reactivity towards nucleophiles due to the electron-withdrawing chlorine and the good leaving ability of chloride. All characteristic reactions—hydrolysis, esterification, amidation, anhydride formation—proceed by the nucleophilic addition–elimination mechanism. Acyl chlorides are more reactive than carboxylic acids but require careful handling. Remember: reaction with water gives carboxylic acid + HCl; with alcohol gives ester + HCl; with ammonia/amine gives amide + ammonium chloride; with carboxylate salt gives acid anhydride + NaCl. These reactions are fast, exothermic, and generally quantitative.

酰基氯含 –COCl 官能团，命名时将 “-oic acid” 替换为 “-oyl chloride”。它们由羧酸与 SOCl₂、PCl₅ 或 PCl₃ 反应制得，其关键化学特征是由于氯原子的吸电性和 Cl⁻ 良好的离去能力，对亲核试剂表现出高反应活性。所有特征反应——水解、酯化、酰胺化、酸酐生成——均遵循亲核加成–消除机理。酰基氯比羧酸更活泼，但操作需谨慎。请牢记：与水反应生成羧酸和 HCl；与醇反应生成酯和 HCl；与氨/胺反应生成酰胺和氯化铵；与羧酸盐反应生成酸酐和 NaCl。这些反应速率快、放热，且近乎定量进行。

For the Cambridge A-Level examination, it is essential to be able to write balanced equations, identify the products, outline the addition–elimination mechanism using curly arrows showing electron pair movements, and appreciate the relative reactivity of acyl chlorides compared to other carboxylic acid derivatives.

对于剑桥 A-Level 考试，必须能够写出配平的方程式，识别产物，用弯箭头勾勒出加成–消除机理以表示电子对的移动，并理解酰基氯相对其他羧酸衍生物的反应活性顺序。

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