📚 IB Chemistry: Alcohols – Key Concepts & Exam Tips | IB 化学：醇 考点精讲
Alcohols are one of the most versatile and commonly examined functional groups in the IB Chemistry syllabus. Understanding their structure, classification, physical properties, and characteristic reactions is essential for success in both Paper 1 and Paper 2, as well as the Internal Assessment. This article provides a comprehensive and exam-focused breakdown of everything you need to know about alcohols, from nomenclature to reaction mechanisms and identification tests.
醇是 IB 化学大纲中最常见、用途最广的官能团之一。理解其结构、分类、物理性质以及特征反应，对于在卷一、卷二乃至内部评估中取得高分至关重要。本文将从命名、反应机理到鉴别试验，全面、紧扣考点地梳理你所需掌握的醇类知识。
1. Introduction to Alcohols | 醇简介
An alcohol is an organic compound containing at least one hydroxyl (–OH) functional group attached to a saturated carbon atom. Alcohols can be considered as derivatives of water in which one hydrogen atom is replaced by an alkyl group. The general formula for a saturated monohydric alcohol is CₙH₂ₙ₊₁OH, or more simply R–OH, where R is an alkyl group.
醇是一类至少含有一个羟基（–OH）官能团的有机化合物，该羟基连接在饱和碳原子上。醇可视为水分子中的一个氢原子被烷基取代后的衍生物。饱和一元醇的通式为 CₙH₂ₙ₊₁OH，或简写作 R–OH，其中 R 为烷基。
The presence of the polar –OH group dominates both the physical properties and the chemical reactivity of alcohols. The oxygen atom is more electronegative than both carbon and hydrogen, creating a polar bond and allowing alcohols to form hydrogen bonds. This leads to significantly higher boiling points compared to alkanes of similar molar mass and explains their solubility in water, especially for shorter-chain alcohols.
极性的 –OH 基团决定了醇的物理性质和化学反应活性。氧原子的电负性高于碳和氢，形成极性键，使醇分子间能够形成氢键。因此，与相对分子质量相近的烷烃相比，醇的沸点明显更高；这也解释了为何短链醇易溶于水。
2. Classification of Alcohols | 醇的分类
Alcohols are classified as primary (1°), secondary (2°), or tertiary (3°) based on the number of carbon atoms directly bonded to the carbon atom that carries the –OH group. This classification is crucial because it determines the outcome of oxidation reactions and some substitution mechanisms.
根据连接羟基的碳原子所直接相连的碳原子数目，醇可分为伯醇（1°）、仲醇（2°）和叔醇（3°）。这一分类至关重要，因为它决定了氧化反应及某些取代反应的产物。
- A primary (1°) alcohol has the –OH group attached to a carbon that is bonded to only one other carbon atom (or none in methanol). Example: ethanol CH₃CH₂OH.
- 伯醇（1°）：羟基所在的碳原子只与一个其他碳原子相连（甲醇除外）。例如乙醇 CH₃CH₂OH。
- A secondary (2°) alcohol has the –OH group attached to a carbon bonded to two other carbon atoms. Example: propan-2-ol (CH₃)₂CHOH.
- 仲醇（2°）：羟基所在的碳原子与另外两个碳原子相连。例如 2-丙醇 (CH₃)₂CHOH。
- A tertiary (3°) alcohol has the –OH group attached to a carbon bonded to three other carbon atoms. Example: 2-methylpropan-2-ol (CH₃)₃COH.
- 叔醇（3°）：羟基所在的碳原子与另外三个碳原子相连。例如 2-甲基-2-丙醇 (CH₃)₃COH。
3. Nomenclature of Alcohols | 醇的命名
The IUPAC system names alcohols by identifying the longest continuous carbon chain containing the –OH group. The suffix “-e” of the corresponding alkane is replaced with “-ol”, and the position of the hydroxyl group is indicated by a number assigned to the carbon to which it is attached, using the lowest possible number. Substituents are named and numbered as usual.
IUPAC 命名法选取含有 –OH 的最长碳链为主链，将相应烷烃词尾的“-e”改为“-ol”，并用尽可能小的数字标明羟基所在的碳原子位置。取代基的命名与编号规则不变。
For example, CH₃CH₂CH₂OH is propan-1-ol, while CH₃CHOHCH₃ is propan-2-ol. When a molecule contains two –OH groups, it is called a diol, e.g., ethane-1,2-diol. If higher priority functional groups are present, the –OH group may be named as a “hydroxy” substituent.
例如，CH₃CH₂CH₂OH 是 1-丙醇，CH₃CHOHCH₃ 是 2-丙醇。分子中含有两个 –OH 时称为二醇，如 1,2-乙二醇。若存在优先级别更高的官能团，–OH 可作为“羟基”取代基命名。
4. Physical Properties | 物理性质
Alcohols exhibit higher boiling points than analogous alkanes and haloalkanes due to the presence of intermolecular hydrogen bonds. Each –OH group can both donate and accept hydrogen bonds, creating a strong network of intermolecular forces that requires more energy to overcome during boiling.
由于分子间存在氢键，醇的沸点比相应烷烃和卤代烷高。每个 –OH 基团既可以提供氢原子也可以接受氢原子，形成较强的分子间力网络，沸腾时需要更多能量来克服。
Solubility in water decreases as the length of the non-polar hydrocarbon chain increases. Short-chain alcohols such as methanol, ethanol, and propan-1-ol are fully miscible with water because the hydrogen bonding between alcohol and water molecules dominates. As the alkyl group grows larger, the hydrophobic effect outweighs hydrogen bonding, and solubility drops, with long-chain alcohols being virtually insoluble.
水溶性随非极性碳氢链的增长而降低。短链醇如甲醇、乙醇和 1-丙醇能与水互溶，因为醇与水分子间的氢键作用占主导。随着烷基增大，疏水效应超过氢键作用，溶解度下降，长链醇几乎不溶于水。
5. Preparation of Alcohols | 醇的制备
There are several key methods for synthesising alcohols covered in the IB syllabus. The hydration of alkenes is a common industrial method. Ethene reacts with steam at high temperature (≈300 °C) and pressure (60–70 atm) in the presence of a phosphoric acid catalyst to yield ethanol:
CH₂=CH₂ + H₂O → CH₃CH₂OH
IB 大纲涵盖几种关键的醇合成方法。烯烃水合是常见的工业制法。乙烯在磷酸催化、高温（约 300 °C）高压（60–70 atm）条件下与水蒸气反应生成乙醇：
CH₂=CH₂ + H₂O → CH₃CH₂OH
Another important route is the nucleophilic substitution of haloalkanes using aqueous sodium hydroxide. For instance, bromoethane reacts with warm aqueous NaOH to give ethanol via an Sₙ2 mechanism:
CH₃CH₂Br + NaOH → CH₃CH₂OH + NaBr
另一重要途径是卤代烷的亲核取代，使用氢氧化钠水溶液。例如，溴乙烷与温热的 NaOH 水溶液经 Sₙ2 机理反应生成乙醇：
CH₃CH₂Br + NaOH → CH₃CH₂OH + NaBr
Fermentation of glucose by yeast is a biological route to ethanol, operating under anaerobic conditions at around 37 °C:
C₆H₁₂O₆ → 2 CH₃CH₂OH + 2 CO₂
发酵法利用酵母在无氧条件下将葡萄糖转化为乙醇，温度约 37 °C：
C₆H₁₂O₆ → 2 CH₃CH₂OH + 2 CO₂
6. Combustion of Alcohols | 醇的燃烧
Alcohols are flammable and undergo complete combustion in a plentiful supply of oxygen to produce carbon dioxide and water. Methanol and ethanol are often used as fuels or fuel additives. The general equation for the complete combustion of a saturated monohydric alcohol is:
CₙH₂ₙ₊₁OH + (3n/2) O₂ → n CO₂ + (n+1) H₂O
醇易燃，在氧气充足时可完全燃烧生成二氧化碳和水。甲醇和乙醇常用作燃料或燃料添加剂。饱和一元醇完全燃烧的通式为：
CₙH₂ₙ₊₁OH + (3n/2) O₂ → n CO₂ + (n+1) H₂O
In the lab, the enthalpy change of combustion (ΔH_c) of alcohols can be determined using a spirit burner and calorimeter. This experiment is a classic IB practical, where students often compare experimental values with literature values and discuss heat loss and incomplete combustion as sources of error. Trends in ΔH_c show increasing exothermicity with chain length due to more carbon–carbon and carbon–hydrogen bonds being broken and formed.
实验室中可使用酒精灯和量热计测定醇的燃烧焓变（ΔH_c）。这是 IB 经典的实验，学生常需比较实验值与文献值，并讨论散热及不完全燃烧等误差来源。随着碳链增长，断裂和形成的 C–C 与 C–H 键增多，燃烧放出的热量呈上升趋势。
7. Oxidation of Alcohols | 醇的氧化
The oxidation behaviour of alcohols is directly linked to their classification. Oxidising agents such as acidified potassium dichromate(VI) (K₂Cr₂O₇/H₂SO₄) are commonly used, with the colour change from orange to green indicating the reduction of Cr(VI) to Cr(III).
醇的氧化行为与其分类直接相关。常用的氧化剂为酸化重铬酸钾（K₂Cr₂O₇/H₂SO₄），实验中橙色变为绿色表明 Cr(VI) 被还原为 Cr(III)。
A primary alcohol is first oxidised to an aldehyde, which can be further oxidised to a carboxylic acid. To isolate the aldehyde, distillation must be used because the aldehyde has a lower boiling point and can be removed before further oxidation. Refluxing with excess oxidising agent yields the carboxylic acid.
RCH₂OH + [O] → RCHO + H₂O
RCHO + [O] → RCOOH
伯醇先被氧化为醛，醛可进一步氧化为羧酸。若要获取醛，需采用蒸馏法，因为醛沸点较低，可先被蒸出以避免继续氧化。在过量氧化剂存在下回流加热则直接生成羧酸。
RCH₂OH + [O] → RCHO + H₂O
RCHO + [O] → RCOOH
A secondary alcohol is oxidised to a ketone. No further oxidation occurs under normal conditions because breaking a carbon–carbon bond would be required.
R₂CHOH + [O] → R₂C=O + H₂O
仲醇被氧化为酮。通常条件下不再继续氧化，因为需要断裂碳–碳键。
R₂CHOH + [O] → R₂C=O + H₂O
Tertiary alcohols resist oxidation because there is no hydrogen atom attached to the carbon bearing the –OH group. Acidified dichromate remains orange with tertiary alcohols, providing a simple test of classification.
叔醇难以被氧化，因为羟基所在碳上没有氢原子。叔醇与酸化重铬酸钾混合后溶液保持橙色，可作为分类的简易检验。
8. Reaction with Sodium | 与钠的反应
Alcohols react with reactive metals such as sodium to produce alkoxide ions and hydrogen gas. This is analogous to the reaction of sodium with water, but less vigorous. For ethanol:
2 CH₃CH₂OH + 2 Na → 2 CH₃CH₂O⁻Na⁺ + H₂
醇与活泼金属（如钠）反应生成醇钠（醇盐）和氢气，与钠和水的反应类似但较为缓和。以乙醇为例：
2 CH₃CH₂OH + 2 Na → 2 CH₃CH₂O⁻Na⁺ + H₂
This reaction is a useful test for the –OH group in a liquid organic compound, provided the compound is dry and no water is present to give a false positive. Observing effervescence of hydrogen gas, which can be tested with a lit splint producing a “pop”, confirms the presence of an alcohol group (though water, carboxylic acids, etc., also react).
该反应可用于检验液态有机物中的 –OH 基团，但必须确保样品干燥无水，以免假阳性。观察到气泡（氢气），用燃着的木条检验可听到爆鸣声，则可确认含 –OH（但水、羧酸等也有类似现象）。
9. Esterification | 酯化反应
Alcohols react with carboxylic acids in the presence of a strong acid catalyst (usually concentrated H₂SO₄) to form esters and water. This is a condensation reaction and an equilibrium process. For example, ethanol and ethanoic acid produce ethyl ethanoate:
CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O
醇与羧酸在强酸催化（通常为浓硫酸）下反应生成酯和水。这是一个缩合反应，也是可逆平衡过程。例如，乙醇与乙酸反应生成乙酸乙酯：
CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O
Esters are known for their pleasant, fruity odours and are widely used in flavourings and perfumes. In the lab, esterification is often carried out by heating the mixture under reflux, and the ester can be separated and identified by its smell. The reaction mechanism involves nucleophilic addition–elimination, with the alcohol acting as the nucleophile.
酯类以其宜人的果香闻名，广泛用于调味剂和香料。实验室中常通过回流加热进行酯化反应，通过气味可分离并识别酯。反应机理为亲核加成–消除，醇作为亲核试剂。
10. Dehydration of Alcohols | 醇的脱水
Alcohols can undergo elimination to form alkenes when heated with a concentrated acid catalyst such as H₂SO₄ or Al₂O₃. This is the reverse of alkene hydration. For ethanol:
CH₃CH₂OH → CH₂=CH₂ + H₂O
醇在浓酸（如浓硫酸）或氧化铝催化下加热，可发生消除反应生成烯烃，这是烯烃水合的逆反应。以乙醇为例：
CH₃CH₂OH → CH₂=CH₂ + H₂O
For unsymmetrical secondary and tertiary alcohols, dehydration can yield a mixture of alkene isomers, following Zaitsev’s rule: the major product is the more substituted (more stable) alkene. For example, dehydration of butan-2-ol gives but-2-ene as the major product and but-1-ene as the minor product.
对于不对称的仲醇和叔醇，脱水可能产生烯烃异构体混合物，遵循查依采夫规则：主要产物为取代较多（更稳定）的烯烃。例如，2-丁醇脱水主要得 2-丁烯，副产 1-丁烯。
11. Identification Tests for Alcohols | 醇的鉴别
Several qualitative tests can distinguish between primary, secondary, and tertiary alcohols. The Lucas test uses a mixture of concentrated hydrochloric acid and anhydrous zinc chloride (Lucas reagent). Tertiary alcohols react almost immediately to form a cloudy layer of alkyl chloride; secondary alcohols react within 5–10 minutes upon warming; primary alcohols show no visible reaction at room temperature.
若干定性试验可区分伯、仲、叔醇。卢卡斯试剂（浓盐酸与无水氯化锌混合液）试验中：叔醇立即反应生成氯代烷浑浊层；仲醇加热后 5–10 分钟内反应；伯醇室温下无明显变化。
The oxidation test with acidified potassium dichromate is also widely used: primary and secondary alcohols turn the solution from orange to green, while tertiary alcohols do not react. The iodoform (triiodomethane) test is specific for alcohols containing a methyl group adjacent to the –OH-bearing carbon (e.g., ethanol, propan-2-ol). A positive result yields a pale yellow precipitate of CHI₃ with a distinctive antiseptic smell.
酸化重铬酸钾氧化试验也很常用：伯醇和仲醇使溶液由橙色变为绿色，叔醇不反应。碘仿（三碘甲烷）试验专一性地检测含有 –CH(OH)CH₃ 结构的醇（如乙醇、2-丙醇），阳性结果产生淡黄色 CHI₃ 沉淀，有特殊消毒水味。
12. Summary & Exam Tips | 总结与应试技巧
When studying alcohols for IB Chemistry, focus on the relationship between structure, classification, and reactivity. Be able to write balanced equations for all key reactions, including combustion, oxidation, esterification, and elimination. Practice identifying conditions (distillation vs reflux, temperature, catalysts) and predicting products.
备考 IB 化学醇类章节时，需重点关注结构、分类与反应活性的关联。要能够书写所有关键反应的配平方程式，包括燃烧、氧化、酯化和消除。熟悉不同反应条件（蒸馏与回流、温度、催化剂），并能预测产物。
Predicting the outcome of oxidation reactions based on classification is a very common exam question. Remember that primary alcohols can over-oxidise to carboxylic acids under reflux, while aldehydes are obtained only by distillation. Be prepared to describe experimental results, such as the colour change of dichromate or the smells of products like esters and triiodomethane.
根据醇的分类预测氧化产物是高频考点。记住伯醇在回流下会被过度氧化为羧酸，只有通过蒸馏才能获得醛。做好准备描述实验现象，如重铬酸盐的颜色变化，或酯和碘仿等产物的特征气味。
Also, make links across topics: compare boiling points of alcohols with alkanes and water, understand the role of hydrogen bonding, and connect esterification to equilibria and organic synthesis pathways. Mastery of these concepts will give you confidence in data analysis questions, mechanism questions, and practical assessments.
同时注意跨专题联系：比较醇、烷烃和水的沸点，理解氢键的作用，将酯化反应与化学平衡和有机合成路线相结合。掌握了这些概念，你将能自信应对数据分析题、机理题和实践评估。
Published by TutorHao | Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导