📚 Reactions of the Alcohols | 醇的反应
Alcohols are an important homologous series in A-Level Chemistry. Their hydroxyl group makes them reactive towards oxidation, dehydration, substitution and esterification, and the type of alcohol – primary, secondary or tertiary – controls the products formed.
醇是 A-Level 化学中重要的同系物。羟基使醇容易发生氧化、脱水、取代和酯化反应，而醇的类型——伯醇、仲醇或叔醇——决定了反应产物。
1. Alcohol Functional Group and Classification | 醇的官能团与分类
Alcohols contain the hydroxyl group –OH bonded to a saturated carbon atom. They are classified as primary, secondary or tertiary according to the number of carbon atoms directly attached to the carbon bearing the –OH group.
醇含有连接在饱和碳原子上的羟基 –OH。根据与带 –OH 的碳原子直接相连的碳原子数，醇可分为伯醇、仲醇和叔醇。
Primary alcohols have the general structure RCH₂OH, for example ethanol CH₃CH₂OH. Secondary alcohols have R₂CHOH, for example propan-2-ol CH₃CH(OH)CH₃. Tertiary alcohols have R₃COH, for example 2-methylpropan-2-ol (CH₃)₃COH.
伯醇的通式为 RCH₂OH，例如乙醇 CH₃CH₂OH。仲醇的通式为 R₂CHOH，例如丙-2-醇 CH₃CH(OH)CH₃。叔醇的通式为 R₃COH，例如 2-甲基丙-2-醇 (CH₃)₃COH。
2. Combustion of Alcohols | 醇的燃烧
Alcohols burn in excess oxygen to form carbon dioxide and water. Combustion is highly exothermic, and alcohols such as ethanol are used as biofuels because they release large amounts of energy per mole.
醇在过量氧气中燃烧生成二氧化碳和水。燃烧反应强烈放热，乙醇等醇类因每摩尔释放大量能量而被用作生物燃料。
CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O
Complete combustion requires a plentiful supply of oxygen. In a limited supply of air, incomplete combustion can produce carbon monoxide or carbon.
完全燃烧需要充足的氧气供应。在空气供应不足时，不完全燃烧会产生一氧化碳或碳。
3. Reaction with Sodium | 与钠的反应
Alcohols react with sodium metal to form sodium alkoxides and hydrogen gas. The reaction is a redox process in which the O–H bond breaks and sodium is oxidised.
醇与金属钠反应生成醇钠和氢气。该反应是氧化还原反应，O–H 键断裂，钠被氧化。
2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂
This reaction is less vigorous than the reaction of sodium with water. The observation is steady effervescence as hydrogen gas is released, and the sodium dissolves slowly.
该反应不如钠与水的反应剧烈。实验现象是稳定地产生气泡，即放出氢气，钠缓慢溶解。
4. Oxidation: Oxidising Agent and General Pattern | 氧化反应：氧化剂与一般规律
The common oxidising agent for alcohols is acidified potassium dichromate(VI), K₂Cr₂O₇ dissolved in dilute sulfuric acid. During oxidation, the orange dichromate(VI) ion Cr₂O₇²⁻ is reduced to green chromium(III) ion Cr³⁺.
醇的常用氧化剂是酸化重铬酸钾(VI)，即溶于稀硫酸的 K₂Cr₂O₇。氧化过程中，橙色的重铬酸根离子 Cr₂O₇²⁻ 被还原为绿色的铬(III)离子 Cr³⁺。
Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
In organic equations, the oxidising agent is often represented by [O]. Whether an alcohol can be oxidised, and which product is formed, depends on how many hydrogen atoms are attached to the carbon bearing the –OH group.
在有机方程式中，氧化剂常表示为 [O]。醇能否被氧化以及生成何种产物，取决于带 –OH 的碳原子上连接了多少个氢原子。
5. Oxidation of Primary Alcohols: Aldehydes and Carboxylic Acids | 伯醇的氧化：醛与羧酸
Primary alcohols are oxidised first to aldehydes and then to carboxylic acids. To obtain the aldehyde, the alcohol is warmed with a controlled amount of acidified dichromate(VI) and the aldehyde is distilled off as it forms, because aldehydes have lower boiling points than the corresponding alcohols and carboxylic acids.
伯醇首先被氧化为醛，进一步氧化为羧酸。为得到醛，醇与适量酸化重铬酸钾(VI)一起加热，并在醛生成时立即蒸馏出来，因为醛的沸点低于相应的醇和羧酸。
CH₃CH₂OH + [O] → CH₃CHO + H₂O
Heating under reflux with excess acidified dichromate(VI) brings about full oxidation to the carboxylic acid. The orange dichromate solution turns green in both stages.
用过量酸化重铬酸钾(VI)加热回流可完全氧化生成羧酸。两个阶段中橙色重铬酸根溶液都会变为绿色。
CH₃CHO + [O] → CH₃COOH
6. Oxidation of Secondary Alcohols: Ketones | 仲醇的氧化：酮
Secondary alcohols are oxidised to ketones. Reflux with acidified potassium dichromate(VI) is typically used, and the orange solution turns green as chromium(III) ions are formed.
仲醇被氧化为酮。通常使用酸化重铬酸钾(VI)回流，溶液由橙色变为绿色，因为生成了铬(III)离子。
CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O
The ketone product has no hydrogen atom directly attached to the carbonyl carbon, so it is resistant to further oxidation under these conditions. This is why secondary alcohols give only ketones, not carboxylic acids.
酮产物羰基碳上没有直接相连的氢原子，因此在这些条件下不易被进一步氧化。这就是仲醇只生成酮而不生成羧酸的原因。
7. Resistance of Tertiary Alcohols to Oxidation | 叔醇对氧化的抵抗
Tertiary alcohols are not oxidised by acidified potassium dichromate(VI) under normal conditions. The carbon bearing the –OH group has no hydrogen atom attached, so oxidation would require breaking a C–C bond, which does not occur with this mild oxidising agent.
叔醇在通常条件下不被酸化重铬酸钾(VI)氧化。带 –OH 的碳原子上没有连接氢原子，因此氧化需要断裂 C–C 键，而这种温和氧化剂无法实现。
With a tertiary alcohol, the orange dichromate solution remains orange. This is a useful test to distinguish tertiary alcohols from primary and secondary alcohols, although other tests such as Lucas reagent may also be used.
对于叔醇，橙色重铬酸根溶液保持橙色。这是区分叔醇与伯醇、仲醇的有效方法，尽管也可使用卢卡斯试剂等其他方法。
8. Dehydration to Alkenes | 脱水生成烯烃
Alcohols can be dehydrated to alkenes by heating with concentrated sulfuric acid or by passing the alcohol vapour over hot aluminium oxide, Al₂O₃. The reaction is an elimination, and the elements of water are removed from adjacent carbon atoms.
醇可以通过与浓硫酸共热或使醇蒸气通过热的氧化铝 Al₂O₃ 脱水生成烯烃。该反应是消除反应，水分子从相邻的两个碳原子上脱去。
CH₃CH₂OH → CH₂=CH₂ + H₂O
For ethanol, concentrated H₂SO₄ at about 170 °C produces ethene. In the mechanism, the –OH group is protonated to form –OH₂⁺, water leaves to generate a carbocation, and loss of H⁺ from an adjacent carbon forms the C=C double bond.
对于乙醇，约 170 °C 下浓硫酸可生成乙烯。反应机理为：–OH 基团先被质子化形成 –OH₂⁺，水离去生成碳正离子，相邻碳原子失去 H⁺ 形成 C=C 双键。
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