📚 Reactions of Alcohols | 醇的反应
Alcohols are versatile organic compounds whose chemistry is dominated by the hydroxyl group. For A-Level Cambridge, a firm grasp of the reactions of alcohols — including oxidation, elimination, substitution, and esterification — is essential. This article systematically explores each key reaction, linking mechanism, reagents, conditions and observations to real examination expectations.
醇是用途广泛的有机化合物，其化学性质主要由羟基决定。在剑桥A-Level考试中，牢固掌握醇的各类反应——包括氧化、消除、取代和酯化——至关重要。本文系统地梳理了每一种核心反应，将机理、试剂、条件与实验现象紧密结合，紧扣考试要求。
1. Combustion | 燃烧
Alcohols burn readily in plenty of air to form carbon dioxide and water. Ethanol, for example, is used as a biofuel and in spirit burners.
醇在充足空气中很容易燃烧，生成二氧化碳和水。例如，乙醇被用作生物燃料和酒精灯燃料。
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
The enthalpy change of combustion is highly exothermic, making short-chain alcohols excellent clean fuels. In limited oxygen, incomplete combustion can produce carbon monoxide and soot.
燃烧反应放热显著，因此短链醇是优良的清洁燃料。在氧气不足时，不完全燃烧会产生一氧化碳和碳烟。
2. Reaction with Sodium | 与钠反应
Alcohols react with reactive metals such as sodium, though less vigorously than water, forming an alkoxide and hydrogen gas.
醇能与钠等活泼金属反应，但反应比水缓慢，生成醇钠和氢气。
2C₂H₅OH + 2Na → 2C₂H₅O⁻Na⁺ + H₂
The observation is steady effervescence and the sodium gradually dissolves. This reaction demonstrates the weakly acidic nature of the –OH proton (pKa ~16). The resulting ethoxide ion is a strong base and a useful nucleophile.
实验现象为稳定冒泡，钠逐渐溶解。这个反应说明醇中–OH 质子的弱酸性(pKa约16)。生成的乙醇根离子是强碱和有用的亲核试剂。
3. Reaction with Hydrogen Halides | 与氢卤酸反应
Alcohols undergo nucleophilic substitution with hydrogen halides (HX) to yield haloalkanes. The reactivity order is tertiary > secondary > primary, reflecting carbocation stability.
醇与氢卤酸（HX）发生亲核取代反应生成卤代烷。反应速率顺序为叔醇 > 仲醇 > 伯醇，这与碳正离子稳定性一致。
ROH + HX → RX + H₂O
For primary alcohols, concentrated HCl requires a catalyst such as anhydrous ZnCl₂ (Lucas reagent); tertiary alcohols react almost instantly at room temperature, forming a cloudy layer of insoluble haloalkane. Rearrangements may occur with unstable carbocations.
对于伯醇，浓盐酸需用无水氯化锌催化（卢卡斯试剂）；叔醇在室温几乎立即反应，生成不溶于水的卤代烷浑浊液层。不稳定的碳正离子可能发生重排。
4. Reaction with Phosphorus Halides and SOCl₂ | 与卤化磷及亚硫酰氯反应
Several halogenating agents convert the –OH group into a good leaving group and facilitate halogen substitution without rearrangement.
多种卤化剂可将 –OH 转变为良好的离去基团，并促进卤代反应而不发生重排。
Common reagents and their overall equations:
常用试剂及其总反应方程式：
| Reagent | Equation | Notes |
|---|---|---|
| PCl₅ | ROH + PCl₅ → RCl + POCl₃ + HCl | Room temperature; violent reaction |
| PCl₃ | 3ROH + PCl₃ → 3RCl + H₃PO₃ | Heating required |
| PBr₃ | 3ROH + PBr₃ → 3RBr + H₃PO₃ | Used for bromoalkanes |
| SOCl₂ | ROH + SOCl₂ → RCl + SO₂ + HCl | Gaseous by-products leave pure chloroalkane |
Thionyl chloride (SOCl₂) is particularly useful because the SO₂ and HCl gases escape, leaving the chloroalkane in high purity. Phosphorus halides often produce side equilibria, but they are reliable for synthesising bromo- and iodoalkanes via PBr₃ and PI₃ generated in situ.
亚硫酰氯(SOCl₂)尤其有用，因为生成的SO₂和HCl气体逸出，留下的氯代烷纯度很高。卤化磷反应常伴随一些副反应，但通过原位生成的PBr₃和PI₃可可靠地合成溴代烷和碘代烷。
5. Oxidation of Alcohols – Overview | 醇的氧化总述
Oxidation is one of the most examined topics. Strong oxidising agents such as acidified potassium dichromate(VI) (K₂Cr₂O₇/H₂SO₄) or acidified potassium manganate(VII) (KMnO₄/H₂SO₄) are used, and the outcome depends on whether the alcohol is primary, secondary or tertiary.
氧化是考查频率最高的考点之一。使用酸化重铬酸钾(K₂Cr₂O₇/H₂SO₄)或酸化高锰酸钾(KMnO₄/H₂SO₄)等强氧化剂，产物取决于醇是伯醇、仲醇还是叔醇。
| Type of alcohol | Example | Observation with K₂Cr₂O₇ | Product |
|---|---|---|---|
| Primary (1°) | CH₃CH₂OH | Orange → Green | Aldehyde then carboxylic acid |
| Secondary (2°) | CH₃CH(OH)CH₃ | Orange → Green | Ketone |
| Tertiary (3°) | (CH₃)₃COH | Remains orange | No reaction |
The colour change from orange (Cr₂O₇²⁻) to green (Cr³⁺) is the key observation when identifying oxidisable alcohols.
颜色由橙(Cr₂O₇²⁻)变为绿(Cr³⁺)是鉴定可氧化醇的关键现象。
6. Oxidation of Primary Alcohols | 伯醇的氧化
Primary alcohols are oxidised first to aldehydes and then to carboxylic acids. Control of conditions determines the major product.
伯醇首先被氧化成醛，然后进一步氧化成羧酸。控制反应条件可决定主要产物。
RCH₂OH + [O] → RCHO + H₂O
RCHO + [O] → RCOOH
To obtain the aldehyde, the reaction mixture is heated gently and the aldehyde is distilled off as it forms, preventing further oxidation. To obtain the carboxylic acid, the alcohol is heated under reflux with excess oxidising agent, ensuring complete oxidation.
要得到醛，需轻微加热并随生成随蒸出，避免进一步氧化。要得到羧酸，则需要用过量的氧化剂在回流条件下充分氧化。
For example, ethanol yields ethanal (CH₃CHO) by distillation and then ethanoic acid (CH₃COOH) under reflux. Acidified potassium dichromate(VI) is the standard reagent in the laboratory.
例如，乙醇通过蒸馏可生成乙醛(CH₃CHO)，回流下则得到乙酸(CH₃COOH)。实验室标准试剂为酸化重铬酸钾。
7. Oxidation of Secondary Alcohols | 仲醇的氧化
Secondary alcohols are oxidised to ketones, which resist further oxidation under normal conditions because breaking a C–C bond would be required.
仲醇被氧化为酮，在常规条件下酮不易进一步氧化，因为进一步反应需要断裂C–C键。
R₂CHOH + [O] → R₂C=O + H₂O
Common examples include propan-2-ol giving propanone (acetone) and butan-2-ol giving butanone. The orange-to-green colour change is observed, and the ketone can be isolated by distillation.
常见例子如2-丙醇生成丙酮，2-丁醇生成丁酮。同样可观察到橙变绿的现象，酮可通过蒸馏分离。
Ketones do not react with mild oxidising agents; thus the dichromate test can distinguish secondary from primary alcohols when combined with additional tests (e.g. testing the product with Tollens' reagent).
酮不与温和氧化剂反应；因此重铬酸盐测试结合其它检验（例如用Tollens试剂检验产物）可区分仲醇和伯醇。
8. Resistance of Tertiary Alcohols to Oxidation | 叔醇的抗氧化性
Tertiary alcohols lack a hydrogen atom on the carbon bearing the –OH group, so they cannot be oxidised without breaking carbon–carbon bonds. With acidified dichromate(VI), the solution remains orange.
叔醇在连接羟基的碳原子上没有氢原子，因此在不破坏C–C键的情况下不能被氧化。遇酸化重铬酸盐溶液保持橙色。
This chemical inertness towards mild oxidising agents provides a simple qualitative test: a few drops of unknown alcohol added to warm acidified K₂Cr₂O₇; if the colour persists orange, the alcohol is likely tertiary. Primary and secondary alcohols will turn the solution green.
这种对温和氧化剂的稳定性提供了简易的定性鉴别：向温热酸化重铬酸钾中加入几滴未知醇；若颜色仍为橙色，则很可能为叔醇。伯醇和仲醇会使其变绿。
9. Dehydration to Alkenes | 脱水生成烯烃
Alcohols undergo elimination upon heating with concentrated sulfuric acid or by passing vapour over hot aluminium oxide (Al₂O₃) catalyst, forming alkenes.
醇在浓硫酸加热下或蒸气通过热的氧化铝(Al₂O₃)催化剂时发生消除反应，生成烯烃。
C₂H₅OH → CH₂=CH₂ + H₂O
With concentrated H₂SO₄, temperatures around 170 °C are required for complete dehydration. The reaction follows Zaitsev's rule where more substituted alkenes are favoured. For example, butan-2-ol yields mainly but-2-ene rather than but-1-ene.
使用浓硫酸完全脱水需要约170°C。反应遵循扎伊采夫规则，优先生成取代较多的烯烃。例如，2-丁醇主要生成2-丁烯而非1-丁烯。
can be reused and produces fewer side products. The mechanism involves protonation of the –OH group, water loss, and deprotonation to form the C=C double bond.
氧化铝催化法在工业上更有吸引力，因为催化剂可重复使用且副产物少。其机理包括–OH质子化、脱水、去质子化生成C=C双键。
10. Esterification | 酯化反应
Alcohols react with carboxylic acids in the presence of an acid catalyst (commonly concentrated H₂SO₄) to form esters and water. This is a reversible condensation reaction.
醇与羧酸在酸催化（常用浓H₂SO₄）下反应生成酯和水，这是一个可逆的缩合反应。
RCOOH + R'OH ⇌ RCOOR' + H₂O
For esterification to go to completion, the equilibrium can be driven by using an excess of one reactant or by removing water. Esters are identified by their fruity smells and are often used as solvents and flavourings.
为使酯化反应完成，可使用过量某反应物或不断移除水以拉动平衡。酯具有水果香味，常用于溶剂和调味剂。
ith alcohols at room temperature to give esters without the need for a catalyst: RCOCl + R'OH → RCOOR' ible and gives high yields, but may be less relevant for core A-Level synthesis questions.
酰氯也可在室温下与醇剧烈反应直接生成酯，无需催化剂：RCOCl + R'OH → RCOOR' + HCl。此法不可逆且产率高，但在A-Level核心合成题中较少直接涉及。
11. Triiodomethane (Iodoform) Test | 三碘甲烷（碘仿）反应
The iodoform test detects the presence of a methyl group adjacent to a carbonyl or a CH₃CH(OH)– group. When an alcohol containing the CH₃CH(OH)– moiety is warmed with iodine
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