Acyl Chlorides: Structure, Reactivity and Key Reactions | 酰氯：结构、反应性与核心反应

📚 Acyl Chlorides: Structure, Reactivity and Key Reactions | 酰氯：结构、反应性与核心反应

Acyl chlorides, also known as acid chlorides, are highly reactive derivatives of carboxylic acids. They contain the functional group -COCl and play a central role in A-Level organic synthesis because they convert readily into esters, amides, carboxylic acids and acid anhydrides.

酰氯，又称酸性氯化物，是羧酸的高活性衍生物。它们含有 -COCl 官能团，在 A-Level 有机合成中占有核心地位，因为它们容易转化为酯、酰胺、羧酸和酸酐。

1. Introduction to Acyl Chlorides | 酰氯简介

Acyl chlorides have the general formula RCOCl, where R can be an alkyl or aryl group. The carbonyl carbon is bonded to both a chlorine atom and a hydrocarbon group, making the molecule highly polar and very susceptible to nucleophilic attack.

酰氯的通式为 RCOCl，其中 R 可以是烷基或芳基。羰基碳同时与氯原子和烃基相连，使分子极性很强，极易受到亲核试剂的进攻。

In the Cambridge International A-Level syllabus, ethanoyl chloride (CH₃COCl) is the most commonly studied example. You must be able to write equations for its reactions with water, alcohols, ammonia and amines, and explain why acyl chlorides are more reactive than carboxylic acids.

在剑桥国际 A-Level 考纲中，乙酰氯（CH₃COCl）是最常考查的例子。你必须能够书写它与水、醇、氨和胺反应的方程式，并解释为什么酰氯比羧酸更活泼。

2. Nomenclature and Structure | 命名与结构

Acyl chlorides are named by replacing the -oic acid ending of the parent carboxylic acid with -oyl chloride. For example, ethanoic acid becomes ethanoyl chloride, and propanoic acid becomes propanoyl chloride.

酰氯的命名是将母体羧酸名称中的 -oic acid 结尾替换为 -oyl chloride。例如，ethanoic acid 变为 ethanoyl chloride，propanoic acid 变为 propanoyl chloride。

The carbonyl group in an acyl chloride is planar, with bond angles of approximately 120° around the carbonyl carbon. The C=O bond is strongly polarised, and the C–Cl bond is also polar, making the carbonyl carbon strongly electrophilic.

酰氯中的羰基是平面结构，羰基碳周围的键角约为 120°。C=O 键强烈极化，C–Cl 键也具有极性，使羰基碳具有很强的亲电性。

CH₃COCl ethanoyl chloride

CH₃COCl 乙酰氯

3. Physical Properties | 物理性质

Lower acyl chlorides such as ethanoyl chloride are colourless, fuming liquids with a sharp, pungent smell. They are volatile and have boiling points lower than the corresponding carboxylic acids.

低级酰氯如乙酰氯是无色、发烟的液体，具有刺激性气味。它们易挥发，沸点低于相应的羧酸。

Although acyl chlorides are polar molecules, they cannot form hydrogen bonds with themselves because they lack a hydrogen atom bonded to oxygen or nitrogen. Their intermolecular forces are mainly permanent dipole-dipole interactions, so their boiling points are lower than those of carboxylic acids of similar molar mass.

尽管酰氯是极性分子，但由于缺少与氧或氮相连的氢原子，它们不能在分子间形成氢键。它们的分子间作用力主要是永久偶极-偶极相互作用，因此沸点低于摩尔质量相近的羧酸。

Acyl chlorides fume in moist air because they react rapidly with water vapour to produce hydrogen chloride gas. They are usually stored under anhydrous conditions and handled in a fume cupboard.

酰氯在潮湿空气中会发烟，因为它们与水蒸气迅速反应生成氯化氢气体。它们通常在无水条件下储存，并在通风橱中操作。

4. Preparation of Acyl Chlorides | 酰氯的制备

Acyl chlorides are prepared from carboxylic acids by replacing the -OH group with a chlorine atom. Common reagents include thionyl chloride (SOCl₂), phosphorus pentachloride (PCl₅) and phosphorus trichloride (PCl₃).

酰氯由羧酸制备，即用氯原子取代 -OH 基团。常用试剂包括亚硫酰氯（SOCl₂）、五氯化磷（PCl₅）和三氯化磷（PCl₃）。

The reaction with thionyl chloride is particularly convenient because the by-products are gases: sulfur dioxide and hydrogen chloride. This allows a clean preparation.

与亚硫酰氯的反应特别方便，因为副产物是气体：二氧化硫和氯化氢。这样可以进行干净的制备。

RCOOH + SOCl₂ → RCOCl + SO₂ + HCl

With PCl₅, the reaction produces phosphoryl chloride (POCl₃) and HCl. This method is also effective but requires careful handling of the solid reagent.

与 PCl₅ 反应时，产物为三氯氧磷（POCl₃）和 HCl。该方法同样有效，但需要小心处理固体试剂。

RCOOH + PCl₅ → RCOCl + POCl₃ + HCl

5. General Nucleophilic Addition-Elimination Mechanism | 亲核加成-消除反应通式

Most reactions of acyl chlorides follow the same two-step mechanism: nucleophilic addition followed by elimination. This is often called nucleophilic addition-elimination or acyl substitution.

酰氯的大多数反应遵循相同的两步机理：先亲核加成，再消除。这通常称为亲核加成-消除或酰基取代。

In the first step, a nucleophile attacks the electron-deficient carbonyl carbon. The C=O π bond breaks, and a tetrahedral intermediate with a negatively charged oxygen is formed.

第一步，亲核试剂进攻缺电子的羰基碳。C=O π 键断裂，形成一个带有负电荷氧的四面体中间体。

In the second step, the tetrahedral intermediate collapses. The chloride ion is expelled as a leaving group, and the carbonyl group is reformed. Chloride is a weak base, so it is a good leaving group.

第二步，四面体中间体解体。氯离子作为离去基团被排出，羰基重新形成。氯离子是弱碱，因此是良好的离去基团。

The general mechanism can be summarised by the sequence: addition of Nu⁻ to C=O, formation of a tetrahedral intermediate, then loss of Cl⁻ and reformation of C=O.

该通式机理可概括为：Nu⁻ 进攻 C=O，形成四面体中间体，然后失去 Cl⁻ 并重新生成 C=O。

6. Reaction with Water: Hydrolysis | 与水反应：水解

Acyl chlorides react vigorously with water to form the corresponding carboxylic acid and hydrogen chloride. Ethanoyl chloride reacts violently with cold water, producing a mist of HCl gas.

酰氯与水剧烈反应，生成相应的羧酸和氯化氢。乙酰氯与冷水剧烈反应，产生 HCl 气体雾。

CH₃COCl + H₂O → CH₃COOH + HCl

This is a hydrolysis reaction. It is much faster than the hydrolysis of a carboxylic acid derivative such as an ester because the Cl⁻ leaving group is much better than an alkoxide ion.

这是一个水解反应。它比酯等羧酸衍生物的水解快得多，因为 Cl⁻ 离去基团比烷氧基离子好得多。

In the laboratory, acyl chlorides must be kept away from moisture. If you accidentally add water to ethanoyl chloride, the mixture can spit and release corrosive fumes, so the reaction must be carried out with cooling and controlled addition.

在实验室中，酰氯必须远离水分。如果不慎将水加入乙酰氯，混合物可能飞溅并释放腐蚀性烟雾，因此反应必须在冷却和受控加入的条件下进行。

7. Reaction with Alcohols: Ester Formation | 与醇反应：酯的生成

Acyl chlorides react with alcohols to produce esters and hydrogen chloride. This is an esterification reaction that is much faster than the acid-catalysed reaction between a carboxylic acid and an alcohol.

酰氯与醇反应生成酯和氯化氢。这是酯化反应，比羧酸与醇在酸催化下的反应快得多。

CH₃COCl + CH₃OH → CH₃COOCH₃ + HCl

The reaction is usually carried out in the presence of a base such as pyridine or sodium hydroxide, which reacts with the HCl produced. This prevents the acid from protonating the alcohol and slowing the reaction.

该反应通常在吡啶或氢氧化钠等碱存在下进行，碱会与生成的 HCl 反应。这可以防止酸质子化醇而减慢反应。

Phenols also react with acyl chlorides to form esters, although the reaction is slower than with alcohols because phenols are less nucleophilic.

酚也能与酰氯反应生成酯，但由于酚的亲核性较弱，反应比醇慢。

8. Reaction with Ammonia and Amines: Amide Formation | 与氨和胺反应：酰胺的生成

Acyl chlorides react with ammonia to form primary amides. Because the HCl produced would protonate ammonia, two moles of ammonia are needed per mole of acyl chloride.

酰氯与氨反应生成伯酰胺。由于生成的 HCl 会使氨质子化，每摩尔酰氯需要两摩尔氨。

CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl

With primary amines, N-substituted amides are formed. Again, two moles of amine are required: one acts as the nucleophile and one neutralises the HCl.

与伯胺反应生成 N-取代酰胺。同样需要两摩尔胺：一摩尔作为亲核试剂，一摩尔用于中和 HCl。

CH₃COCl + 2CH₃NH₂ → CH₃CONHCH₃ + CH₃NH₃⁺Cl⁻

Secondary amines react similarly to form tertiary amides. In all amide-forming reactions, the nucleophile is the nitrogen atom, and the product contains a C–N bond.

仲胺发生类似反应生成叔酰胺。在所有形成酰胺的反应中，亲核试剂是氮原子，产物含有 C–N 键。

9. Reaction with Carboxylic Acids and Carboxylates | 与羧酸及羧酸盐反应

Acyl chlorides react with carboxylic acids to form acid anhydrides. This is a useful way to prepare anhydrides that may be difficult to obtain otherwise.

酰氯与羧酸反应生成酸酐。这是制备酸酐的有效方法，尤其是那些难以用其他方法获得的酸酐。

CH₃COCl + CH₃COOH → (CH₃CO)₂O + HCl

Acyl chlorides also react with sodium carboxylate salts to produce acid anhydrides and sodium chloride. This reaction is sometimes used in organic synthesis because the salt is less acidic and the reaction can be more controlled.

酰氯也能与羧酸钠盐反应生成酸酐和氯化钠。该反应有时用于有机合成，因为盐的酸性较弱，反应更易控制。

CH₃COCl + CH₃COONa → (CH₃CO)₂O + NaCl

Acid anhydrides are themselves useful acylating agents, although they are less reactive than acyl chlorides. This allows chemists to tune reactivity by choosing between an acyl chloride and an anhydride.

酸酐本身也是有用的酰化试剂，尽管其反应活性低于酰氯。这使化学家可以通过选择酰氯或酸酐来调节反应活性。

10. Reduction and Organometallic Reactions | 还原与有机金属反应

Acyl chlorides can be reduced to primary alcohols using lithium aluminium hydride (LiAlH₄). This is a strong reducing agent that also reduces carboxylic acids and esters, so the acyl chloride must be converted under controlled conditions.

酰氯可以被氢化铝锂（LiAlH₄）还原为伯醇。这是一种强还原剂，也能还原羧酸和酯，因此酰氯的还原需要在受控条件下进行。

Partial reduction to an aldehyde can be achieved using the Rosenmund reduction, in which hydrogen gas reacts over a palladium catalyst poisoned with barium sulfate. This stops the reduction at the aldehyde stage.

使用 Rosenmund 还原法可将酰氯部分还原为醛，即氢气在经硫酸钡毒化的钯催化剂上反应。这使还原停止在醛阶段。

Acyl chlorides also react with Grignard reagents. However, the product after the first addition is a ketone, which itself reacts further with the Grignard reagent, so the final product is usually a tertiary alcohol after two equivalents of RMgX.

酰氯也能与格氏试剂反应。然而，第一次加成后的产物是酮，酮会进一步与格氏试剂反应，因此使用两当量 RMgX 时最终产物通常是叔醇。

11. Relative Reactivity and Comparison with Carboxylic Acids | 相对反应活性及与羧酸比较

Acyl chlorides are far more reactive than carboxylic acids, esters and amides. This is because the chlorine atom is both electron-withdrawing and a good leaving group, so the carbonyl carbon is more electrophilic and the substitution step is faster.

酰氯比羧酸、酯和酰胺活泼得多。这是因为氯原子既具有吸电子性，又是良好的离去基团，因此羰基碳更具亲电性，取代步骤更快。

Carboxylic acids react with alcohols only slowly and require an acid catalyst, while acyl chlorides react rapidly at room temperature. This is why acyl chlorides are preferred in synthesis when a fast, high-yielding esterification or amidation is needed.

羧酸与醇的反应缓慢且需要酸催化，而酰氯在室温下即可快速反应。因此，当需要快速、高产率的酯化或酰胺化时，合成中优先使用酰氯。

The reactivity order for carboxylic acid derivatives is generally: acyl chlorides > acid anhydrides > esters > amides. Amides are the least reactive because the nitrogen lone pair donates electron density into the carbonyl group, reducing its electrophilicity.

羧酸衍生物的反应活性顺序通常为：酰氯 > 酸酐 > 酯 > 酰胺。酰胺的反应活性最低，因为氮的孤对电子向羰基提供电子密度，降低了羰基的亲电性。

12. Summary and Exam Tips | 总结与考试提示

Acyl chlorides contain the -COCl group and react by nucleophilic addition-elimination. You should be able to write balanced equations for their reactions with water, alcohols, ammonia, primary amines and carboxylate salts.

酰氯含有 -COCl 基团，通过亲核加成-消除机理反应。你必须能够书写它们与水、醇、氨、伯胺和羧酸盐反应的配平方程式。

In exam answers, always show the formation of HCl or NH₄Cl where appropriate. For reactions with ammonia or amines, remember that two equivalents are needed because one neutralises the HCl formed.

在考试答案中，务必在适当处写出 HCl 或 NH₄Cl 的生成。对于与氨或胺的反应，记住需要两当量，因为其中一当量用于中和生成的 HCl。

When asked to compare reactivity, link your answer to two factors: the electron-withdrawing effect of Cl, which increases the positive charge on the carbonyl carbon, and the ability of Cl⁻ to act as a good leaving group.

当要求比较反应活性时，请将答案与两个因素联系起来：Cl 的吸电子效应使羰基碳正电荷增加，以及 Cl⁻ 作为良好离去基团的能力。

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Acyl Chlorides: Structure, Reactivity and Key Reactions | 酰氯：结构、反应性与核心反应