A-Level OCR Chemistry: Alcohols Revision Essentials | A-Level OCR 化学：醇考点精讲

📚 A-Level OCR Chemistry: Alcohols Revision Essentials | A-Level OCR 化学：醇考点精讲

Alcohols are one of the most versatile functional groups in organic chemistry. For OCR A-Level Chemistry, a thorough understanding of their structure, nomenclature, physical properties, preparation methods, and characteristic reactions is essential. This article breaks down the key knowledge points, reaction mechanisms, and practical aspects you need to master, linking them directly to the OCR specification.

醇是有机化学中用途最广的官能团之一。在 OCR A-Level 化学中，透彻理解醇的结构、命名、物理性质、制备方法及其特征反应至关重要。本文拆解你需要掌握的关键知识点、反应机理和实验要点，并直接对接 OCR 考纲。

1. Classification of Alcohols | 醇的分类

The reactivity of an alcohol is largely determined by the number of carbon atoms directly attached to the carbon bearing the –OH group. Primary (1°) alcohols have the –OH group on a carbon that is bonded to only one other carbon atom. Secondary (2°) alcohols have the –OH group on a carbon bonded to two other carbons. Tertiary (3°) alcohols have the –OH group on a carbon bonded to three other carbons.

醇的反应活性很大程度上取决于与 –OH 所在碳直接相连的碳原子数目。一级（伯）醇的 –OH 连在只与一个碳相连的碳上；二级（仲）醇的 –OH 连在与两个碳相连的碳上；三级（叔）醇的 –OH 连在与三个碳相连的碳上。

Primary example: ethanol, CH₃CH₂OH
中文：一级醇示例：乙醇，CH₃CH₂OH
Secondary example: propan-2-ol, CH₃CH(OH)CH₃
中文：二级醇示例：丙-2-醇，CH₃CH(OH)CH₃
Tertiary example: 2-methylpropan-2-ol, (CH₃)₃COH
中文：三级醇示例：2-甲基丙-2-醇，(CH₃)₃COH

— 注意：此处列表混合语言，但符合“每个要点先英文后中文”，所以用了交替的

。这可以。每个li里英文，下个li中文。也可以分开。这里用

？其实用

更合适。规范说“每个要点先写英文

，紧接写中文

配对”，对于列表可以用

但也要配对。更保险的方法是用

英文，然后

中文。然后用

英文
中文

看起来两个条目。最好在每个要点内用段落而不是列表。我改成段落形式：

Primary (1°) alcohols … Example: ethanol, CH₃CH₂OH.

一级（伯）醇 … 示例：乙醇，CH₃CH₂OH。

然后同样处理二级和三级。这样可以。重新处理。

1. Classification of Alcohols | 醇的分类

The reactivity of an alcohol is largely determined by the number of carbon atoms directly attached to the carbon bearing the –OH group. Primary (1°) alcohols have the –OH group on a carbon that is bonded to only one other carbon atom, for example ethanol (CH₃CH₂OH). Secondary (2°) alcohols have the –OH group on a carbon bonded to two other carbons, e.g. propan-2-ol (CH₃CH(OH)CH₃). Tertiary (3°) alcohols have the –OH group on a carbon bonded to three other carbons, e.g. 2-methylpropan-2-ol ((CH₃)₃COH).

醇的反应活性很大程度上取决于与 –OH 所在碳直接相连的碳原子数目。一级（伯）醇的 –OH 连在只与一个碳相连的碳上，例如乙醇 (CH₃CH₂OH)；二级（仲）醇的 –OH 连在与两个碳相连的碳上，例如丙-2-醇 (CH₃CH(OH)CH₃)；三级（叔）醇的 –OH 连在与三个碳相连的碳上，例如 2-甲基丙-2-醇 ((CH₃)₃COH)。

2. IUPAC Nomenclature | IUPAC 命名法

Alcohols are named by identifying the longest continuous carbon chain containing the –OH group, replacing the terminal ‘-e’ of the corresponding alkane with ‘-ol’. The chain is numbered to give the –OH group the lowest possible locant. When multiple –OH groups are present, suffixes like ‘-diol’, ‘-triol’ are used, and the ‘e’ of the alkane name is retained.

命名醇时，先找出含有 –OH 的最长碳链，将相应烷烃名称末尾的‘-e’替换为‘-ol’。链的编号应使 –OH 的位置号尽可能小。当存在多个 –OH 时，使用‘-diol’、‘-triol’等后缀，并保留烷烃名称中的‘e’。

Examples: CH₃CH₂CH₂OH is propan-1-ol; CH₃CH(OH)CH₃ is propan-2-ol; CH₂OHCH₂OH is ethane-1,2-diol. The –OH group takes priority over halogens and alkyl groups when numbering.

示例：CH₃CH₂CH₂OH 为丙-1-醇；CH₃CH(OH)CH₃ 为丙-2-醇；CH₂OHCH₂OH 为乙-1,2-二醇。编号时 –OH 优先于卤素和烷基。

3. Physical Properties and Intermolecular Forces | 物理性质与分子间作用力

Alcohols exhibit significantly higher boiling points than analogous alkanes due to hydrogen bonding between –OH groups. Short-chain alcohols are miscible with water because they can form hydrogen bonds with water molecules. As the non-polar hydrocarbon chain lengthens, the influence of the hydrophobic alkyl group increases, reducing water solubility. Volatility decreases with increasing molar mass and with more extensive hydrogen bonding, as seen in polyols like glycerol.

由于 –OH 基团间的氢键作用，醇的沸点显著高于相应的烷烃。短链醇能与水混溶，因为它们可以和水分子形成氢键。随着非极性烃链增长，疏水烷基的影响增大，水溶性降低。随着摩尔质量增加以及氢键作用增强（如甘油等多元醇），挥发性下降。

For example, ethanol (b.p. 78 °C) is much higher than ethane (b.p. –89 °C), and butan-1-ol is less soluble in water than ethanol due to its larger hydrocarbon chain.

例如，乙醇沸点 78 °C，远高于乙烷的 –89 °C；丁-1-醇的水溶性低于乙醇，因为其烃链更长。

4. Preparation of Alcohols | 醇的制备方法

The main synthetic routes to alcohols covered in OCR include hydration of alkenes, fermentation of sugars, and nucleophilic substitution of halogenoalkanes. Hydration of alkenes uses steam and an acid catalyst (H₃PO₄) at high temperature and pressure, following Markovnikov’s rule to yield the more substituted alcohol. Fermentation of glucose by yeast produces ethanol under anaerobic conditions at around 35 °C. Also, halogenoalkanes can be hydrolysed by warm aqueous NaOH to produce alcohols.

OCR 涉及的主要合成路线包括烯烃水合、糖类的发酵以及卤代烷的亲核取代。烯烃水合使用水蒸气和酸催化剂 (H₃PO₄)，在高温高压下进行，遵循马氏规则生成取代较多的醇。葡萄糖在酵母作用下、35 °C 左右的厌氧条件下发酵生成乙醇。此外，卤代烷可通过温热 NaOH 水溶液水解制得醇。

CH₂=CH₂ + H₂O → CH₃CH₂OH (hydration, acid catalyst)
CH₂=CH₂ + H₂O → CH₃CH₂OH（水合，酸催化）
C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂ (fermentation)
C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂（发酵）
CH₃CH₂Br + NaOH → CH₃CH₂OH + NaBr (nucleophilic substitution)
CH₃CH₂Br + NaOH → CH₃CH₂OH + NaBr（亲核取代）

5. Reaction with Sodium | 与钠的反应

Alcohols react with sodium metal to form alkoxide ions and hydrogen gas. This reaction is less vigorous than the reaction of sodium with water, producing steady effervescence. It serves as a test for the –OH group. The general equation is 2 ROH + 2 Na → 2 RO⁻Na⁺ + H₂.

醇与金属钠反应生成醇钠和氢气。该反应不如钠与水的反应剧烈，会平稳地冒泡。可用于检验 –OH 基团。一般方程式为：2 ROH + 2 Na → 2 RO⁻Na⁺ + H₂。

For ethanol: 2 CH₃CH₂OH + 2 Na → 2 CH₃CH₂O⁻Na⁺ + H₂. The ionic species formed is sodium ethoxide.

以乙醇为例：2 CH₃CH₂OH + 2 Na → 2 CH₃CH₂O⁻Na⁺ + H₂。生成的离子化合物为乙醇钠。

6. Oxidation Reactions: Primary, Secondary and Tertiary | 氧化反应：一级、二级和三级醇

Oxidation of alcohols is a key distinguishing feature. Under reflux with acidified potassium dichromate(VI) (H⁺/Cr₂O₇²⁻), primary alcohols are first oxidised to aldehydes and then to carboxylic acids. The orange dichromate turns green as Cr³⁺ is formed. To isolate the aldehyde, distillation is used to remove it from the oxidising mixture before further oxidation occurs. Secondary alcohols are oxidised to ketones, which resist further oxidation. Tertiary alcohols are not oxidised under these conditions because they lack a hydrogen atom on the carbon bearing the –OH.

醇的氧化是关键的区分特征。一级醇在酸性重铬酸钾 (H⁺/Cr₂O₇²⁻) 回流条件下，先被氧化成醛，进而被氧化成羧酸。橙色的重铬酸盐变成绿色的 Cr³⁺。若要分离醛，需使用蒸馏法，在进一步氧化之前将其从混合体系中移出。二级醇被氧化成酮，酮难以继续氧化。三级醇在此条件下不被氧化，因为连有 –OH 的碳上没有氢原子。

Representative equations using [O] for the oxidising agent:

用 [O] 表示氧化剂的代表性方程式：

Primary: RCH₂OH + [O] → RCHO + H₂O, then RCHO + [O] → RCOOH
一级：RCH₂OH + [O] → RCHO + H₂O，然后 RCHO + [O] → RCOOH
Secondary: RCH(OH)R’ + [O] → RCOR’ + H₂O
二级：RCH(OH)R’ + [O] → RCOR’ + H₂O
Tertiary: no reaction
三级：不反应

7. Esterification (Reaction with Carboxylic Acids) | 酯化反应（与羧酸的反应）

Alcohols react with carboxylic acids in the presence of a concentrated sulfuric acid catalyst under reflux to form esters and water. This is a reversible condensation reaction, where the –OH from the carboxylic acid and the –H from the alcohol’s –OH are eliminated as water. Esters have characteristic sweet, fruity smells and are used as solvents and plasticisers.

醇与羧酸在浓硫酸催化下回流反应，生成酯和水。这是一个可逆的缩合反应，羧酸的 –OH 与醇 –OH 中的 –H 结合成水脱除。酯具有特征性的香甜果味，可用作溶剂和增塑剂。

General equation: RCOOH + R’OH ⇌ RCOOR’ + H₂O. Example: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (ethyl ethanoate).

通式：RCOOH + R’OH ⇌ RCOOR’ + H₂O。示例：CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O（乙酸乙酯）。

8. Reaction with Hydrogen Halides | 与氢卤酸的反应

Alcohols undergo nucleophilic substitution with hydrogen halides (HCl, HBr, HI) to form halogenoalkanes. The reaction rate depends on the hydrogen halide and the class of alcohol. For tertiary alcohols, the reaction occurs rapidly at room temperature by shaking with concentrated HCl; for secondary and primary alcohols, a catalyst such as anhydrous ZnCl₂ and heating are required. Alternatively, alcohols can be converted using PCl₅, PCl₃, or SOCl₂ in separate laboratory methods.

醇与氢卤酸 (HCl, HBr, HI) 发生亲核取代生成卤代烷。反应速率取决于氢卤酸的种类和醇的级别。三级醇与浓盐酸在室温下振摇即可迅速反应；二级和一级醇则需要无水 ZnCl₂ 催化剂并加热。此外，实验室还可通过 PCl₅、PCl₃ 或 SOCl₂ 将醇转化为卤代烷。

General reaction: ROH + HX → RX + H₂O. Using phosphorus halides: 3 ROH + PCl₃ → 3 RCl + H₃PO₃; ROH + PCl₅ → RCl + POCl₃ + HCl; ROH + SOCl₂ → RCl + SO₂ + HCl.

总反应：ROH + HX → RX + H₂O。使用卤化磷的方法：3 ROH + PCl₃ → 3 RCl + H₃PO₃；ROH + PCl₅ → RCl + POCl₃ + HCl；ROH + SOCl₂ → RCl + SO₂ + HCl。

9. Dehydration (Elimination) to Alkenes | 脱水（消除）生成烯烃

Heating an alcohol with a concentrated acid catalyst, such as H₂SO₄ or H₃PO₄, results in elimination of water to produce an alkene. This is an E1 or E2 mechanism depending on conditions. The major product follows Zaitsev’s rule: the more substituted alkene is favoured. For unsymmetrical alcohols, a mixture of isomeric alkenes can form. Dehydration can also be carried out by passing alcohol vapour over heated aluminium oxide (Al₂O₃) at around 350 °C.

将醇与浓酸催化剂（如 H₂SO₄ 或 H₃PO₄）共热，会发生消除反应脱去一分子水生成烯烃。反应机理依据条件为 E1 或 E2。主产物遵循扎伊采夫规则：更取代的烯烃占优势。对于不对称醇，可能生成异构烯烃混合物。脱水也可通过将醇蒸气通过约 350 °C 的热氧化铝 (Al₂O₃) 来实现。

Example: CH₃CH₂OH → CH₂=CH₂ + H₂O (with H⁺/heat). For butan-2-ol, the major product is but-2-ene rather than but-1-ene.

示例：CH₃CH₂OH → CH₂=CH₂ + H₂O (H⁺/加热)。对于丁-2-醇，主要产物为丁-2-烯而非丁-1-烯。

10. Distinguishing Tests and Lucas Reagent | 鉴别试验与卢卡斯试剂

The Lucas test distinguishes between primary, secondary, and tertiary alcohols based on their reactivity with concentrated HCl in the presence of anhydrous ZnCl₂ (Lucas reagent). Tertiary alcohols produce cloudiness immediately at room temperature due to the formation of an insoluble chloroalkane. Secondary alcohols produce cloudiness within 5–10 minutes, while primary alcohols remain clear unless heated. Another test is oxidation with acidified dichromate: primary and secondary alcohols turn the solution green, but tertiary alcohols do not react.

卢卡斯试验利用醇与浓盐酸/无水 ZnCl₂（卢卡斯试剂）的反应性差异区分一、二、三级醇。三级醇在室温下即刻变浑浊（生成不溶性氯代烷）；二级醇在 5–10 分钟内变浑浊；一级醇除非加热否则保持澄清。另一项试验是用酸性重铬酸盐氧化：一级和二级醇使溶液变绿，三级醇不反应。

Additionally, the iodoform test can identify alcohols with a methyl group adjacent to the –OH (i.e., ethanol or secondary alcohols with a methyl substituent). A positive result gives a pale yellow precipitate of CHI₃.

此外，碘仿试验可鉴别 –OH 相邻处有甲基的醇（如乙醇或带有甲基取代的二级醇）。阳性结果为生成淡黄色的碘仿 (CHI₃) 沉淀。

11. Spectroscopic Identification: IR and Mass Spectrometry | 光谱鉴定：红外与质谱

Infrared spectroscopy shows a broad, strong peak around 3200–3550 cm⁻¹ for the O–H stretch in alcohols. Hydrogen bonding causes broadening. The C–O stretch appears near 1000–1300 cm⁻¹. In mass spectra, alcohols often give a molecular ion peak (M⁺) and fragments from loss of water (M – 18) or cleavage at the α-carbon. For example, primary alcohols show a prominent peak at m/z 31 (CH₂OH⁺).

红外光谱中，醇的 O–H 伸缩振动在 3200–3550 cm⁻¹ 处呈现宽而强的吸收峰，氢键导致峰形变宽。C–O 伸缩振动出现在 1000–1300 cm⁻¹ 附近。在质谱中，醇通常给出分子离子峰 (M⁺)，以及失去一分子水 (M – 18) 或在 α-碳断裂产生的碎片。例如，一级醇常出现 m/z = 31 的显著峰 (CH₂OH⁺)。

For ethanol, the mass spectrum shows peaks at m/z 46 (M⁺), 45 (M – 1), 31 (CH₂OH⁺), and 29 (C₂H₅⁺). The IR spectrum has a broad O–H band and no C=O band unless oxidation has occurred.

乙醇的质谱显示 m/z = 46 (M⁺)、45 (M – 1)、31 (CH₂OH⁺) 和 29 (C₂H₅⁺) 的峰。其红外光谱有宽 O–H 吸收带，不含 C=O 峰（除非发生氧化）。

12. Summary of Key Reactions and Overview Table | 重点反应总结与总览表

Below is a concise summary table of the principal reactions of alcohols required for the OCR examination.

下表简要概括 OCR 考试中要求的醇的主要反应。

Reaction \| 反应	Reagents/Conditions \| 试剂/条件	Product \| 产物
Oxidation of 1° alcohol	K₂Cr₂O₇/H⁺, distil (for aldehyde) or reflux (for acid)	Aldehyde then carboxylic acid
一级醇氧化	K₂Cr₂O₇/H⁺，蒸馏得醛，回流得酸	醛，然后羧酸
Oxidation of 2° alcohol	K₂Cr₂O₇/H⁺, reflux	Ketone
二级醇氧化	K₂Cr₂O₇/H⁺，回流	酮
Esterification	Carboxylic acid, conc. H₂SO₄, reflux	Ester + H₂O
酯化	羧酸，浓 H₂SO₄，回流	酯 + 水
Reaction with HX	HCl/ZnCl₂ or PX₃/PCl₅/SOCl₂	Haloalkane
与 HX 反应	HCl/ZnCl₂ 或 PX₃/PCl₅/SOCl₂	卤代烷
Dehydration	Conc. H₂SO₄ or Al₂O₃, heat	Alkene
脱水	浓 H₂SO₄ 或 Al₂O₃，加热	烯烃
With sodium	Sodium metal, room temp.	Alkoxide + H₂
与钠反应	金属钠，室温	醇钠 + 氢气

A solid grasp of these transformations, together with their mechanisms where required, will enable you to tackle synthesis and problem-solving questions with confidence. Always connect properties to bonding and structure for a deeper chemical understanding.

扎实掌握这些转化及其机理（在要求的情况下），将使你能够自信地应对合成和问题解决题目。始终将性质与键合和结构相联系，以获得更深层次的化学理解。

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A-Level OCR Chemistry: Alcohols Revision Essentials | A-Level OCR 化学：醇 考点精讲