Chemistry of Ethanol: Reactions and Chemical Properties | 乙醇的化学性质与反应

📚 Chemistry of Ethanol: Reactions and Chemical Properties | 乙醇的化学性质与反应

Ethanol (C₂H₅OH) is arguably the most important organic compound in the A-Level chemistry syllabus. It serves as the gateway to understanding alcohols, nucleophilic substitution, elimination, oxidation, and esterification. This article systematically covers every examinable reaction of ethanol, with mechanisms, conditions, and practical observations.

乙醇（C₂H₅OH）可以说是 A-Level 化学大纲中最重要的有机化合物。它是理解醇类、亲核取代、消除、氧化和酯化反应的入门钥匙。本文系统性地覆盖乙醇所有可考的反应，包括机理、条件和实验现象。

1. Structure and Physical Properties | 结构与物理性质

Ethanol has the molecular formula C₂H₅OH and a structural formula CH₃CH₂OH. The molecule consists of an ethyl group (CH₃CH₂–) attached to a hydroxyl group (–OH). The oxygen atom is sp³ hybridised, with a bent geometry around the C–O–H bond, giving a bond angle of approximately 104.5°.

乙醇的分子式为 C₂H₅OH，结构式为 CH₃CH₂OH。分子由一个乙基（CH₃CH₂–）连接一个羟基（–OH）组成。氧原子为 sp³ 杂化，C–O–H 键角约为 104.5°，呈弯曲几何构型。

Key physical properties you must recall:

你必须记住的关键物理性质：

Boiling point: 78 °C — much higher than ethane (–89 °C) due to hydrogen bonding between –OH groups.
沸点：78 °C——远高于乙烷（–89 °C），原因是 –OH 基团之间的氢键作用。
Solubility: completely miscible with water because the hydroxyl group forms hydrogen bonds with water molecules.
溶解性：与水完全混溶，因为羟基能与水分子形成氢键。
Polarity: the –OH group makes ethanol a polar molecule, though the ethyl part is non-polar.
极性：–OH 基团使乙醇成为极性分子，但乙基部分是非极性的。

2. Combustion of Ethanol | 乙醇的燃烧反应

Ethanol is highly flammable and undergoes complete combustion in excess oxygen to form carbon dioxide and water:

乙醇高度易燃，在过量氧气中完全燃烧生成二氧化碳和水：

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O ΔH = –1371 kJ mol⁻¹

Incomplete combustion occurs when the oxygen supply is limited, producing carbon monoxide (CO) and/or carbon (soot). This is a key point for energy chemistry questions comparing ethanol with fossil fuels.

当氧气供应不足时发生不完全燃烧，生成一氧化碳（CO）和/或碳（烟灰）。这是能量化学中比较乙醇与化石燃料时的关键点。

Exam tip: The enthalpy of combustion of ethanol is often measured using a spirit burner and a calorimeter. Sources of error include heat loss to the surroundings and incomplete combustion — you should be able to discuss these systematically.

考试提示：乙醇的燃烧焓常用酒精灯和量热计测量。误差来源包括向周围环境散热和不完全燃烧——你应当能够系统性地讨论这些因素。

3. Reaction with Sodium | 与钠的反应

Ethanol reacts with sodium metal, replacing the acidic hydrogen of the hydroxyl group. This is a redox reaction in which sodium is oxidised to Na⁺ and hydrogen is reduced:

乙醇与金属钠反应，置换出羟基上的酸性氢。这是一个氧化还原反应，钠被氧化为 Na⁺，氢被还原：

2C₂H₅OH + 2Na → 2C₂H₅O⁻Na⁺ + H₂↑

The product is sodium ethoxide (C₂H₅ONa), also called sodium ethanolate. The reaction is slower than the corresponding reaction of water with sodium because the ethyl group donates electron density to the oxygen atom, making the O–H bond less polar.

产物是乙醇钠（C₂H₅ONa），又称乙氧基钠。该反应比水与钠的反应慢，因为乙基向氧原子推电子，使 O–H 键的极性减弱。

Practical observations to quote in exams:

考试中要引用的实验现象：

Sodium sinks and floats intermittently (less dense than ethanol but denser than the solution formed).
钠先沉后漂浮，间歇性上下（密度介于乙醇与生成的溶液之间）。
Effervescence (bubbles of hydrogen gas) is observed.
观察到气泡（氢气）产生。
When the gas is tested with a lighted splint, it burns with a pop sound (hydrogen test).
用燃着的火柴检验气体时，发出爆鸣声（氢气的检验）。

4. Substitution with Hydrogen Halides | 与氢卤酸的取代反应

Ethanol reacts with hydrogen halides (HX) to form haloalkanes and water. This is a nucleophilic substitution in reverse — the –OH group, a poor leaving group, is converted into water in acidic conditions, making it a good leaving group.

乙醇与卤化氢（HX）反应生成卤代烷和水。这是一个反向的亲核取代——–OH 是较差的离去基团，在酸性条件下转化为水后成为良好的离去基团。

C₂H₅OH + HX → C₂H₅X + H₂O

The most examinable version uses hydrogen bromide, generated in situ from sodium bromide and concentrated sulfuric acid:

最常考的是溴化氢版本，由溴化钠与浓硫酸原位生成 HBr：

NaBr + H₂SO₄ → HBr + NaHSO₄
C₂H₅OH + HBr → C₂H₅Br + H₂O

In the lab, a mixture of ethanol, sodium bromide, and concentrated sulfuric acid is heated under reflux. The bromoethane is distilled off and collected, often with anti-bumping granules added to ensure even boiling.

在实验室中，将乙醇、溴化钠和浓硫酸的混合物加热回流。溴乙烷被蒸馏收集，通常加入沸石以保证平稳沸腾。

Alternatively, phosphorus tribromide (PBr₃) can be used: 3C₂H₅OH + PBr₃ → 3C₂H₅Br + H₃PO₃. This method avoids the formation of ethene as a side product.

另一种方法是用三溴化磷（PBr₃）：3C₂H₅OH + PBr₃ → 3C₂H₅Br + H₃PO₃。此方法避免副产物乙烯的生成。

5. Dehydration to Ethene | 脱水生成乙烯

Ethanol can be dehydrated (elimination of water) to form ethene. This is an elimination reaction, and it requires an acidic catalyst:

乙醇可以脱水（消除一分子水）生成乙烯。这是一个消除反应，需要酸性催化剂：

C₂H₅OH → C₂H₄ + H₂O

Conditions for the reaction:

反应条件：

Catalyst: concentrated H₂SO₄ or concentrated H₃PO₄
催化剂：浓 H₂SO₄ 或浓 H₃PO₄
Temperature: 170–180 °C (with H₂SO₄); for H₃PO₄, around 200 °C
温度：170–180 °C（用 H₂SO₄ 时）；用 H₃PO₄ 时约 200 °C

Alternatively, aluminium oxide (Al₂O₃) can serve as a solid catalyst at 350 °C. The mechanism proceeds via protonation of the –OH group, loss of water to form a carbocation (CH₃CH₂⁺), and deprotonation to give the alkene.

另一种方法是使用氧化铝（Al₂O₃）作为固体催化剂，温度 350 °C。机理为：–OH 基团质子化，脱去水生成碳正离子（CH₃CH₂⁺），再失去质子得到烯烃。

The dehydration of ethanol is reversible. At lower temperatures (around 140 °C), two ethanol molecules condense to form diethyl ether (C₂H₅OC₂H₅), a side reaction you must know:

乙醇脱水是可逆反应。在较低温度（约 140 °C）下，两分子乙醇缩合生成乙醚（C₂H₅OC₂H₅），这是一个你必须要知道的副反应：

2C₂H₅OH → C₂H₅OC₂H₅ + H₂O

6. Oxidation of Ethanol | 乙醇的氧化反应

Oxidation is arguably the most important reaction type for ethanol in the A-Level exam. Ethanol is a primary alcohol; therefore, it can be oxidised twice.

氧化反应可以说是乙醇在 A-Level 考试中最重要的反应类型。乙醇为伯醇，因此可以被氧化两次。

First oxidation (to ethanal):

第一步氧化（生成乙醛）：

CH₃CH₂OH + [O] → CH₃CHO + H₂O

The oxidising agent is often acidified potassium dichromate(VI) (K₂Cr₂O₇ / H⁺) or acidified potassium manganate(VII) (KMnO₄ / H⁺). In the dichromate test, the orange dichromate ion (Cr₂O₇²⁻) is reduced to green Cr³⁺(aq) — this colour change is the basis for the breathalyser test.

氧化剂常用酸性重铬酸钾（K₂Cr₂O₇ / H⁺）或酸性高锰酸钾（KMnO₄ / H⁺）。在重铬酸盐检验中，橙色的重铬酸根离子（Cr₂O₇²⁻）被还原为绿色的 Cr³⁺(aq)——这个颜色变化是呼气测醉仪的原理。

Second oxidation (to ethanoic acid):

第二步氧化（生成乙酸）：

CH₃CHO + [O] → CH₃COOH

The overall oxidation of ethanol to ethanoic acid can be written as:

乙醇氧化为乙酸的总反应式可写为：

CH₃CH₂OH + 2[O] → CH₃COOH + H₂O

To obtain ethanal as the product, the aldehyde must be distilled off immediately, as it boils at 21 °C and can escape before further oxidation. To obtain ethanoic acid, reflux the mixture to ensure complete oxidation.

要获得乙醛产物，必须立即将乙醛蒸出，因为乙醛的沸点为 21 °C，可在进一步氧化前逸出。要获得乙酸，则需加热回流使氧化完全。

Exam tip: You must be able to write the ionic half-equation for the dichromate reduction:

考试提示：你必须能写出重铬酸盐还原的离子半反应式：

Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O

And for the oxidation of ethanol: C₂H₅OH + H₂O → CH₃COOH + 4H⁺ + 4e⁻. Multiplying to balance electrons is a common calculation skill tested.

乙醇氧化的半反应：C₂H₅OH + H₂O → CH₃COOH + 4H⁺ + 4e⁻。乘以系数使电子守恒是常见的计算考查技能。

7. Esterification with Carboxylic Acids | 与羧酸的酯化反应

Ethanol reacts with carboxylic acids in the presence of a strong acid catalyst to form an ester and water. This is a condensation reaction (two molecules join with the loss of a small molecule).

乙醇在强酸催化下与羧酸反应生成酯和水。这是一个缩合反应（两分子结合并脱去一个小分子）。

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

The product is ethyl ethanoate, a sweet-smelling ester. The catalyst is concentrated sulfuric acid, which also acts as a dehydrating agent to shift the equilibrium to the right.

产物是乙酸乙酯，具有水果香味。催化剂为浓硫酸，同时也可作为脱水剂使平衡右移。

In the lab, a mixture of ethanol, glacial ethanoic acid, and concentrated sulfuric acid is heated under reflux for about 20 minutes. The ester is then separated using a separating funnel with sodium carbonate solution to neutralise excess acid.

在实验室中，将乙醇、冰乙酸和浓硫酸的混合物回流加热约 20 分钟。然后用分液漏斗加入碳酸钠溶液分离酯层，以中和过量的酸。

Key facts for memorisation:

需要记忆的关键事实：

Esterification is reversible and slow without a catalyst.
酯化反应可逆，无催化剂时速率慢。
The –OH of the acid is removed with the H of the alcohol’s –OH to form water, not the other way around. This can be proven using ¹⁸O isotopic labelling.
酸提供 –OH，醇的 –OH 提供 H 结合成水，而不是相反。这可以用 ¹⁸O 同位素标记法验证。
Ester names follow the pattern “alkyl alkanoate”: ethyl ethanoate.
酯的名称遵循”烷基烷酸酯”的规律：乙酸乙酯。

8. Reaction with Phosphorus Halides | 与磷卤化物的反应

Ethanol reacts with phosphorus trichloride (PCl₃), phosphorus pentachloride (PCl₅), or thionyl chloride (SOCl₂) to produce chloroethane.

乙醇与三氯化磷（PCl₃）、五氯化磷（PCl₅）或氯化亚砜（SOCl₂）反应生成氯乙烷。

C₂H₅OH + PCl₅ → C₂H₅Cl + POCl₃ + HCl

The reaction with PCl₅ is particularly useful as a chemical test for the presence of the hydroxyl group. At room temperature, white fumes of hydrogen chloride are given off, which turn blue litmus paper red. This is a quick qualitative test for alcohols in an organic lab.

与 PCl₅ 的反应特别有用，可作为羟基存在的化学检验方法。室温下，产生白色烟雾状的氯化氢气体，能使蓝色石蕊试纸变红。这是有机实验室中快速检验醇的定性方法。

Thionyl chloride is the cleanest method because the by-products (SO₂ and HCl) are both gaseous, leaving pure chloroethane:

氯化亚砜法最干净，因为副产物（SO₂ 和 HCl）均为气体，留下纯净的氯乙烷：

C₂H₅OH + SOCl₂ → C₂H₅Cl + SO₂ + HCl

9. Reaction with Acyl Chlorides | 与酰氯的反应

Ethanol reacts readily with acyl chlorides (also called acid chlorides) to form esters. This is a more reactive alternative to direct esterification with carboxylic acids, and it does not require a catalyst.

乙醇可迅速与酰氯（也称氯化酰）反应生成酯。这是比直接与羧酸酯化更活泼的替代方法，且不需要催化剂。

CH₃COCl + C₂H₅OH → CH₃COOC₂H₅ + HCl

This reaction is violently exothermic. The hydrogen chloride gas evolved can be observed as steamy white fumes. Ammonia or a base is sometimes added to neutralise the HCl and drive the reaction to completion.

该反应剧烈放热。产生的氯化氢气体呈白色酸雾。有时加入氨或碱来中和 HCl，并促使反应完全。

Exam point: Acyl chlorides are derived from carboxylic acids by replacing the –OH group with a chlorine atom. They are much more reactive because the chlorine atom is a better leaving group than hydroxide under nucleophilic attack.

考点：酰氯由羧酸中的 –OH 被氯原子取代衍生而来。它们的反应活性高得多，因为氯原子比氢氧根是更好的离去基团，更容易受到亲核进攻。

10. Acid-Base Behaviour: Acidity of Ethanol | 乙醇的酸碱行为：乙醇的酸性

Ethanol is a very weak acid. Its pKₐ value is approximately 16, while water has pKₐ ≈ 14 and ethanoic acid has pKₐ ≈ 4.76. This means ethanol is a weaker acid than water.

乙醇是一种极弱的酸。其 pKₐ 约为 16，水的 pKₐ ≈ 14，乙酸的 pKₐ ≈ 4.76。这意味着乙醇的酸性比水还弱。

The acidity arises from the ability of the oxygen atom to stabilise the negative charge of the ethoxide ion (C₂H₅O⁻). However, the ethyl group is electron-donating (via the inductive effect), which destabilises the ethoxide ion compared to the hydroxide ion, making ethanol less acidic than water.

其酸性源于氧原子稳定乙氧基离子（C₂H₅O⁻）负电荷的能力。然而，乙基通过诱导效应推电子，使乙氧基离子相对于氢氧根离子更不稳定，因此乙醇的酸性比水弱。

Trend to remember for exam comparisons:

考试比较记忆规律：

Compound	pKₐ	Acidity trend
Ethanoic acid	4.76	most acidic
Water	14	↓
Ethanol	16	least acidic

Ethanol does not react with bases such as NaOH or Na₂CO₃ — this distinguishes it from carboxylic acids, which react with carbonates to release CO₂.

乙醇不与 NaOH 或 Na₂CO₃ 等碱反应——这使其区别于羧酸，羧酸能与碳酸盐反应放出 CO₂。

11. Fermentation: The Biological Production of Ethanol | 发酵法：乙醇的生物制备

While not strictly a chemical reaction of ethanol itself, fermentation is the most important industrial route to ethanol and is frequently examined alongside ethanol’s chemistry.

虽然发酵严格来说不是乙醇本身的化学反应，但它是乙醇最重要的工业制备途径，经常与乙醇化学一起考查。

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Conditions required:

所需条件：

Enzyme: zymase, found in yeast
酶：酵母菌中的酒化酶
Temperature: 30–40 °C (optimum around 37 °C, as enzymes denature at high temperatures)
温度：30–40 °C（最适约 37 °C，温度过高会使酶失活）
Anaerobic conditions: no oxygen, otherwise ethanol is oxidised to ethanoic acid
厌氧条件：无氧气，否则乙醇会被氧化为乙酸
Neutral pH: to maintain enzyme activity
中性 pH：维持酶活性

Fermentation produces only dilute ethanol (about 12–15% concentration) because the yeast dies when the alcohol concentration exceeds this level. Concentration is achieved by fractional distillation.

发酵法只能产生稀乙醇（约 12–15% 浓度），因为当酒精浓度超过这一水平时酵母会死亡。乙醇的浓缩通过分馏实现。

12. Summary Table of Ethanol Reactions | 乙醇反应总结表

Use this summary table for rapid revision before the exam. It covers all the reagents, conditions, and key products in one place.

请在考前使用此总结表快速复习。它涵盖所有试剂、条件和关键产物。

Reaction type	Reagent(s)	Conditions	Product(s)
Combustion	O₂	Ignition	CO₂ + H₂O
With sodium	Na	Room temp.	C₂H₅ONa + H₂
Substitution	HBr / NaBr + H₂SO₄	Heat under reflux	C₂H₅Br + H₂O
Dehydration	Concentrated H₂SO₄	170–180 °C	C₂H₄ + H₂O
Oxidation (partial)	K₂Cr₂O₇ / H⁺	Distil off product	CH₃CHO
Oxidation (full)	K₂Cr₂O₇ / H⁺ excess	Reflux	CH₃COOH
Esterification	CH₃COOH + conc. H₂SO₄	Heat under reflux	CH₃COOC₂H₅ + H₂O
With PCl₅	PCl₅	Room temp.	C₂H₅Cl + POCl₃ + HCl
With CH₃COCl	CH₃COCl	Room temp.	CH₃COOC₂H₅ + HCl

Mastering this table gives you a complete command of ethanol chemistry. The single most common exam error is confusing the oxidation products — remember: primary alcohol → aldehyde → carboxylic acid, with each step requiring one “oxygen atom” from the oxidising agent.

掌握此表将使你完全掌握乙醇化学。最常见的考试错误是混淆氧化产物——请记住：伯醇 → 醛 → 羧酸，每一步需要氧化剂提供一个”氧原子”。

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Chemistry of Ethanol: Reactions and Chemical Properties | 乙醇的化学性质与反应