Making Esters | 酯的制备

📚 Making Esters | 酯的制备

Esters are sweet-smelling organic compounds that are responsible for many natural flavours and fragrances, such as banana, pineapple, and vanilla. In the IGCSE chemistry practical course, the preparation of an ester is a classic experiment that tests your understanding of organic reactions, reflux techniques, and purification methods. This article explains the chemistry behind making esters and guides you through the laboratory procedure step by step.

酯类是带有甜味的有机化合物，许多天然香料和水果风味（如香蕉、菠萝和香草）都源自它们。在 IGCSE 化学实验课程中，制备酯是经典实验之一，它综合考查你对有机反应、回流操作和纯化方法的理解。本文将解释酯制备背后的化学原理，并逐步指导你完成实验流程。

1. What Are Esters? | 什么是酯？

An ester is an organic compound formed by the reaction between a carboxylic acid and an alcohol. The functional group of an ester is –COO–, which is often written as –COOR. The R group comes from the alcohol, and the other part comes from the acid. For example, ethyl ethanoate (CH₃COOC₂H₅) is an ester made from ethanoic acid and ethanol.

酯是由羧酸和醇反应生成的有机化合物。酯的官能团是 –COO–，通常也写成 –COOR。其中 R 基来自醇，而其余部分来自酸。例如，乙酸乙酯（CH₃COOC₂H₅）就是由乙酸和乙醇制成的酯。

Esters are less polar than alcohols and acids, so they are usually insoluble in water but soluble in organic solvents. They have lower boiling points than the carboxylic acids of similar molecular mass, which makes them easy to purify by distillation.

酯的极性比醇和酸小，因此通常不溶于水，但可溶于有机溶剂。它们的沸点低于相近相对分子质量的羧酸，因此可以通过蒸馏方便地进行纯化。

2. Structure and Nomenclature | 结构与命名

To name an ester, take the alcohol part first (from the alcohol) as the alkyl group, then the acid part as the “-anoate” ending. The acid loses its “-ic acid” ending and becomes “-ate”. For example, ethanol + ethanoic acid → ethyl ethanoate.

酯的命名规则：先取来自醇的烷基，再用来自酸的“-酸”改为“-酸酯”。例如，乙醇 + 乙酸 → 乙酸乙酯。

Common examples include: methyl butanoate (pineapple smell), pentyl ethanoate (banana smell), and octyl ethanoate (orange smell). The table below shows some familiar esters and their odours.

常见的例子有：丁酸甲酯（菠萝味）、乙酸戊酯（香蕉味）、乙酸辛酯（橙子味）。下表列出了一些常见酯及其气味。

Ester	Odour
Ethyl ethanoate	Nail polish remover
Pentyl ethanoate	Banana
Octyl ethanoate	Orange
Methyl butanoate	Pineapple

3. The General Esterification Reaction | 酯化反应的一般方程式

Esterification is the reaction between a carboxylic acid and an alcohol in the presence of concentrated sulfuric acid. The general equation is:

RCOOH + R’OH ⇌ RCOOR’ + H₂O

This is a reversible reaction, indicated by the ⇌ symbol. The forward reaction produces an ester and water; the reverse reaction breaks the ester back into acid and alcohol (hydrolysis).

酯化是羧酸与醇在浓硫酸存在下发生的反应。通式为：

RCOOH + R’OH ⇌ RCOOR’ + H₂O

这是一个可逆反应，因此使用 ⇌ 符号。正向反应生成酯和水；逆向反应（水解）将酯分解回酸和醇。

For the specific reaction between ethanol and ethanoic acid:

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

乙醇和乙酸的具体反应为：

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

4. The Role of Concentrated Sulfuric Acid | 浓硫酸的作用

Concentrated sulfuric acid plays two important roles in esterification. First, it acts as a catalyst, speeding up the reaction by protonating the carboxylic acid, making it more reactive towards the alcohol. Second, it acts as a dehydrating agent, removing water from the reaction mixture. Since esterification is reversible, removing water shifts the equilibrium to the right, increasing the yield of ester.

浓硫酸在酯化中起两个重要作用。首先，它是催化剂，通过质子化羧酸使其更易与醇反应，从而加快反应速率。其次，它是脱水剂，会移除反应体系中的水。由于酯化是可逆反应，除去水可以使平衡向右移动，提高酯的产率。

Only a small amount of concentrated sulfuric acid is needed as a catalyst, but a larger excess is often used to absorb the water produced. Always add concentrated sulfuric acid slowly to the alcohol–acid mixture with cooling, because the reaction is exothermic.

作为催化剂，只需要少量浓硫酸；但为了吸收生成的水，实验中常使用过量浓硫酸。将浓硫酸缓慢加入醇和酸的混合物中并冷却，因为该反应放热。

5. Reflux: The Safe Heating Technique | 回流：安全的加热方法

Esterification is slow at low temperatures and requires heating. However, both ethanol and ethyl ethanoate are volatile and flammable. To heat the mixture without losing volatile reactants or products, a reflux apparatus is used. A reflux condenser is placed vertically above the reaction flask. Cold water circulates through the outer jacket, condensing any vapour back into the flask.

酯化反应在低温下进行缓慢，需要加热。但乙醇和乙酸乙酯都易挥发且易燃。为防止挥发性反应物和产物损失，需使用回流装置。将回流冷凝管垂直安装在反应瓶上方，冷凝管外层通入冷水，使蒸气冷凝回流至烧瓶中。

Reflux allows the reaction to continue at the boiling temperature of the solvent without losing material. In the IGCSE practical exam, you should be able to draw and label this apparatus, including the anti-bumping granules and the water inlet/outlet.

回流使反应能在溶剂沸点温度下持续进行而不损失物料。在 IGCSE 实验考试中，你需要能画出并标出该装置，包括防暴沸石以及冷凝水的进水口和出水口。

6. Step-by-Step Preparation of Ethyl Ethanoate | 制备乙酸乙酯的逐步操作

Here is a standard method used in IGCSE laboratories to prepare ethyl ethanoate:

以下是 IGCSE 实验室制备乙酸乙酯的标准方法：

Step 1: Place 5 cm³ of ethanol and 4 cm³ of glacial ethanoic acid in a round-bottomed flask. Add a few drops of concentrated sulfuric acid slowly with swirling.

步骤 1：在圆底烧瓶中加入 5 cm³ 乙醇和 4 cm³ 冰乙酸，然后缓慢滴入少量浓硫酸，边加边旋摇。

Step 2: Add anti-bumping granules and attach a reflux condenser. Heat the mixture gently in a water bath (or electric heater) for about 10–15 minutes.

步骤 2：加入防暴沸石，安装回流冷凝管。在热水浴（或电加热器）中温和加热约 10–15 分钟。

Step 3: Stop heating and allow the mixture to cool. Then pour it into a separating funnel containing a saturated sodium carbonate solution. The sodium carbonate neutralises the excess acid and increases the separation of layers.

步骤 3：停止加热，冷却混合物。然后将其倒入盛有饱和碳酸钠溶液的分液漏斗中。碳酸钠会中和多余的酸，并有助于分层。

Step 4: Shake the funnel gently and release the pressure frequently. Let the layers separate. The upper layer is the ester layer; the lower aqueous layer is discarded.

步骤 4：轻轻振荡漏斗并不断放气，静置分层。上层为酯层，下层为水层，可将水层弃去。

Step 5: Wash the ester layer with water, then add anhydrous calcium chloride to dry it. Finally, decant or filter the liquid and, if required, distil it to collect pure ethyl ethanoate at its boiling point (77 °C).

步骤 5：用水洗涤酯层，然后加入无水氯化钙干燥固体，干燥后过滤或倾析。如有需要，可在 77 °C 下蒸馏收集纯乙酸乙酯。

7. Purification and Separation | 纯化与分离

Crude ester mixture contains unreacted alcohol, acid, water, and sulfuric acid. Purification involves several stages:

粗酯混合物中含有未反应的醇、酸、水和硫酸。纯化需经过多个阶段：

Neutralisation: Sodium carbonate solution removes any acidic impurities and converts carboxylic acid into its water-soluble sodium salt.

中和：碳酸钠溶液可除去酸性杂质，将羧酸转化为可溶于水的羧酸钠盐。
Separation: Using a separating funnel, the ester layer can be separated from the aqueous layer because esters are less dense and immiscible with water.

分离：由于酯的密度比水小且不溶于水，可直接用分液漏斗将酯层与水层分离。
Washing: Shaking the ester with water removes any remaining alcohol or water-soluble impurities.

洗涤：用水振荡酯层可除去残留的醇及其他水溶性杂质。
Drying: Anhydrous calcium chloride (CaCl₂) absorbs water from the ester. Other drying agents include anhydrous sodium sulfate and magnesium sulfate.

干燥：无水氯化钙（CaCl₂）可以吸收酯中的水。其他干燥剂还有无水硫酸钠和硫酸镁。
Distillation: Final purification by distillation collects the ester at its characteristic boiling point, leaving behind any higher-boiling impurities.

蒸馏：最后通过蒸馏可在酯的特征沸点处收集产物，留下沸点较高的杂质。

8. Testing the Ester Product | 检验酯产物

You can confirm that an ester has been formed by checking its physical properties and smell. Ethyl ethanoate has a sweet, fruity smell (like pear drops or nail polish remover). It is insoluble in water and forms a separate layer. It is not acidic — it does not turn blue litmus red.

你可以通过物理性质和气味来确认酯是否生成。乙酸乙酯具有甜美的水果香味（类似梨糖或指甲油去光水）。它不溶于水，会形成分层。它不显酸性——不会使蓝色石蕊变红。

In an exam, you may be asked to describe how you could distinguish an ester from a carboxylic acid. A simple test is to add aqueous sodium carbonate: a carboxylic acid produces carbon dioxide gas (effervescence), while an ester does not.

考试中可能要求你区分酯和羧酸。一个简单的方法是加入碳酸钠溶液：羧酸会放出二氧化碳气体（有气泡），而酯则无气泡。

Another useful property is the boiling point. Pure ethyl ethanoate boils at 77 °C. Measuring the boiling point can confirm the identity and purity of your product.

另一个有用性质是沸点。纯乙酸乙酯的沸点为 77 °C。测定沸点可以确认产物及其纯度。

9. Uses of Esters in Daily Life and Industry | 酯在日常生活中的应用

Esters are widely used in the food, cosmetic and chemical industries. Their pleasant smells make them perfect for perfumes and artificial flavourings. For example, isoamyl acetate is used as banana flavouring, and benzyl acetate gives jasmine-like scents.

酯广泛用于食品、化妆品和化学工业。它们宜人的气味使其非常适合制作香水和人工香精。例如，乙酸异戊酯用作香蕉香精，乙酸苄酯具有茉莉香气。

Esters are also excellent solvents for paints, varnishes and adhesives because they dissolve many non-polar substances and evaporate quickly. In addition, natural fats and oils are esters of glycerol with fatty acids; they form the basis of our diet and are used to make soaps through hydrolysis.

酯还是油漆、清漆和胶粘剂的优良溶剂，因为它们可以溶解许多非极性物质且挥发快。此外，天然油脂是甘油与高级脂肪酸形成的酯，它们是人类饮食的基础，并可通过水解制造肥皂。

Some esters are used as plasticisers to make plastics flexible, while others are found in medicines and cosmetics. Biodiesel is another important application: it is a mixture of methyl or ethyl esters made from vegetable oils.

一些酯被用作增塑剂使塑料柔韧，另一些酯用于药物和化妆品中。生物柴油也是一个重要应用：它是由植物油制成的甲酯或乙酯混合物。

10. Common Mistakes and Exam Tips | 易错点与考试技巧

Students often make the following mistakes in esterification experiments and questions:

学生在酯化实验和题目中常犯以下错误：

Using the wrong arrow: Remember esterification is reversible, so you must write ⇌, not →.

混淆箭头：酯化是可逆反应，必须写 ⇌，而不是 →。
Naming errors: Always name the alkyl group from the alcohol first, then the ethanoate/propanoate etc. from the acid. Ethyl ethanoate, not ethanoic ethyl.

命名错误：先写来自醇的烷基，再写来自酸的“某酸酯”。是乙酸乙酯（ethyl ethanoate），而不是乙基乙酸。
Missing the catalyst: Concentrated sulfuric acid must be mentioned as the catalyst. Avoid saying dilute sulfuric acid — it will not work well.

漏写催化剂：必须指明浓硫酸作为催化剂。不能写稀硫酸——稀硫酸效果不好。
Forgetting water: The other product is water. In the equation, do not forget H₂O.

漏写水：另一个产物是水，方程式中不要忘记 H₂O。
Not using reflux: Heating an open flask will lose volatile alcohol and ester. In the practical, use a reflux condenser.

不用回流：敞口加热会导致易挥发的醇和酯损失。实验中必须使用回流冷凝管。
Inhaling or tasting chemicals: Do not smell esters directly by putting your nose over the flask; waft the vapour towards your nose. Never taste them, as many are toxic.

直接吸入或品尝：不要将鼻子凑到瓶口闻酯；应用手扇动蒸气闻其气味。切勿品尝，因为许多酯有毒。

Exam questions often ask you to explain why sodium carbonate is used, why the mixture is heated under reflux, or why the product is dried with anhydrous calcium chloride. Practice writing clear, concise explanations using the concepts in this article.

考试常考的问题包括：为什么使用碳酸钠溶液？为什么需要回流加热？为什么用无水氯化钙干燥？请用本文中的概念练习写出清晰、简洁的解释。

Published by TutorHao | Chemistry Revision Series | aleveler.com

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Making Esters | 酯的制备