📚 Alcohols, Carboxylic Acids and Related Compounds | 醇、羧酸及相关化合物
Alcohols and carboxylic acids are central functional groups in A-Level organic chemistry. This topic links structure, naming, physical properties, preparation, oxidation, esterification and condensation polymers, and it provides a foundation for understanding many related compounds such as aldehydes, ketones, esters, acyl chlorides and acid anhydrides.
醇和羧酸是 A-Level 有机化学的核心官能团。本主题将结构、命名、物理性质、制备、氧化、酯化和缩聚反应联系起来，为理解醛、酮、酯、酰氯和酸酐等相关化合物奠定基础。
Examiners repeatedly test the same skills: drawing and interpreting structures, predicting reagents and conditions, explaining trends in boiling point and acidity, and writing balanced equations for oxidation and condensation. Working through this revision guide section by section will therefore cover most of the organic chemistry questions that appear on Paper 1, Paper 2 and the practical papers.
考官反复考查的技能其实很固定：绘制与解读结构、预测试剂和条件、解释沸点与酸性趋势，以及书写氧化和缩合的配平方程式。因此逐节完成本篇复习指南，就能覆盖 Paper 1、Paper 2 及实验卷中绝大多数有机化学题目。
1. Functional Groups and Homologous Series | 官能团与同系物
Alcohols are organic compounds containing at least one hydroxyl group, –OH, bonded to a saturated carbon atom. The homologous series of saturated monohydric alcohols has the general formula CₙH₂ₙ₊₁OH or CₙH₂ₙ₊₂O, and members differ by a CH₂ unit with a relative molecular mass difference of 14.
醇是含有至少一个羟基(–OH)且羟基连接在饱和碳原子上的有机化合物。饱和一元醇同系物的通式为 CₙH₂ₙ₊₁OH 或 CₙH₂ₙ₊₂O，相邻同系物相差一个 CH₂ 单元，相对分子质量相差 14。
Carboxylic acids contain the carboxyl group, –COOH, which combines a carbonyl C=O and a hydroxyl –OH on the same carbon. Related compounds in this topic include aldehydes (–CHO), ketones (C=O between two carbon atoms), esters (–COO–), acyl chlorides (–COCl) and acid anhydrides.
羧酸含羧基(–COOH)，它由同一碳上的羰基 C=O 和羟基 –OH 组成。本主题涉及的相关化合物包括醛(–CHO)、酮(两个碳原子之间的 C=O)、酯(–COO–)、酰氯(–COCl)和酸酐。
Understanding the functional group is essential because it controls the chemical behaviour of each family, and because the same functional group can be identified and named systematically wherever it appears in a molecule.
理解官能团十分重要，因为它决定了每一类化合物的化学行为，也因为无论官能团出现在分子的哪个位置，都可以被系统识别和命名。
| Family 类别 | Functional group 官能团 | General formula 通式 | Example 举例 |
| Alcohol 醇 | –OH | CₙH₂ₙ₊₁OH | CH₃CH₂OH |
| Aldehyde 醛 | –CHO | CₙH₂ₙO | CH₃CHO |
| Ketone 酮 | C=O | CₙH₂ₙO | CH₃COCH₃ |
| Carboxylic acid 羧酸 | –COOH | CₙH₂ₙ₊₁COOH | CH₃COOH |
| Ester 酯 | –COO– | CₙH₂ₙO₂ | CH₃COOCH₂CH₃ |
| Acyl chloride 酰氯 | –COCl | CₙH₂ₙ₊₁COCl | CH₃COCl |
2. Naming Alcohols and Carboxylic Acids | 醇和羧酸的命名
IUPAC names of alcohols are formed by identifying the longest carbon chain carrying the –OH group. The ending -e of the parent alkane is replaced by -ol, and a number is used if the position is ambiguous. Examples include methanol, ethanol, propan-1-ol, propan-2-ol and 2-methylpropan-2-ol.
醇的 IUPAC 命名是先找出带有 –OH 的最长碳链，把母体烷烃词尾 -e 改为 -ol，如果位置不明确则加上编号。例如甲醇、乙醇、丙-1-醇、丙-2-醇和 2-甲基丙-2-醇。
When two or more hydroxyl groups are present the suffixes -diol and -triol are used with the lowest possible locants, as in ethane-1,2-diol and propane-1,2,3-triol. If a higher-priority group such as –COOH is present, the hydroxyl group is named as the prefix hydroxy-, for example 3-hydroxybutanoic acid.
当分子中有两个或更多羟基时，使用后缀 -diol 和 -triol，并给出尽可能小的编号，如乙-1,2-二醇和丙-1,2,3-三醇。若分子中存在优先级更高的 –COOH，则羟基改用前缀 hydroxy- 表示，例如 3-羟基丁酸。
Carboxylic acids are named by selecting the longest chain containing the –COOH group and replacing -e with -oic acid; the carbon of the carboxyl group is always numbered 1, so no locant is needed. Dicarboxylic acids use -dioic acid, for example ethanedioic acid and hexanedioic acid.
羧酸的命名是选择含 –COOH 的最长碳链，把 -e 改为 -oic acid；羧基碳始终编号为 1，因此无需标注位置。二元羧酸使用 -dioic acid，例如乙二酸和己二酸。
Esters are named as alkyl alkanoates: the alkyl part comes from the parent alcohol and the alkanoate part from the parent acid. Acyl chlorides replace -oic acid by -oyl chloride, and acid anhydrides are named as alkanoic anhydrides.
酯的命名是"某酸某酯"(alkyl alkanoate)：烷基部分来自醇，烷酸酯部分来自酸。酰氯把 -oic acid 改为 -oyl chloride，酸酐则命名为"某酸酐"(alkanoic anhydride)。
CH₃COOCH₂CH₃ = ethyl ethanoate 乙酸乙酯 CH₃COCl = ethanoyl chloride 乙酰氯 (CH₃CO)₂O = ethanoic anhydride 乙酸酐
3. Structure, Bonding and Intermolecular Forces | 结构、成键与分子间作用力
The oxygen atom in an alcohol is sp³ hybridised with two lone pairs, giving a C–O–H bond angle of about 104.5°, while the carbon bearing the –OH group is approximately tetrahedral with bond angles close to 109.5°. Oxygen is much more electronegative than carbon and hydrogen, so both the C–O and O–H bonds are polar.
醇中氧原子为 sp³ 杂化，带两对孤对电子，C–O–H 键角约为 104.5°；连有 –OH 的碳原子近似四面体，键角接近 109.5°。氧的电负性远高于碳和氢，因此 C–O 键和 O–H 键都是极性键。
The polarity produces permanent dipole-dipole attractions and, more importantly, hydrogen bonding between the lone pairs on oxygen and the partially positive hydrogen of another molecule, written as O–H···O. Hydrogen bonding explains the anomalously high boiling points of alcohols and their solubility in water.
这种极性带来永久偶极-偶极作用，更重要的是氧上孤对电子与另一分子带部分正电的氢之间形成氢键，记作 O–H···O。氢键解释了醇异常高的沸点及其在水中的溶解性。
In the carboxyl group the carbon is bonded to two oxygen atoms, and the group is planar with bond angles of about 120°. When a carboxylic acid loses a proton, the resulting carboxylate ion is stabilised by delocalisation: the negative charge is shared equally over both oxygen atoms, so the two C–O bonds become identical in length (about 0.127 nm) and are intermediate between a single and a double bond.
在羧基中碳与两个氧相连，整个基团为平面结构，键角约 120°。羧酸失去质子后形成的羧酸根离子因离域而稳定：负电荷平均分布在两个氧上，两条 C–O 键长度相同(约 0.127 nm)，介于单键与双键之间。
R–COOH + H₂O ⇌ R–COO⁻ + H₃O⁺
This delocalisation, together with the polar O–H bond, is why carboxylic acids are far more acidic than alcohols, in which no such resonance stabilisation of the alkoxide ion is possible.
这种离域作用，加上极性 O–H 键，正是羧酸酸性远强于醇的原因——醇形成的烷氧负离子无法获得类似的共振稳定。
4. Physical Properties: Boiling Points and Solubility | 物理性质：沸点与溶解性
Boiling points of alcohols increase steadily with chain length because the number of electrons rises and London forces between molecules become stronger; branching lowers the boiling point because the molecules pack less efficiently. Carboxylic acids boil at higher temperatures than alcohols of similar relative molecular mass because they form hydrogen-bonded dimers, effectively doubling the mass of the species that must be separated.
醇的沸点随碳链增长而稳定升高，因为电子数增加、分子间色散力增强；支链会降低沸点，因为分子堆积效率变差。相同相对分子质量下羧酸的沸点高于醇，因为羧酸形成氢键二聚体，实际上使需要克服的粒子质量加倍。
| Compound 化合物 | Mr | Boiling point 沸点 |
| Methanol 甲醇 | 32 | 65 °C |
| Ethanol 乙醇 | 46 | 78 °C |
| Propan-1-ol 丙-1-醇 | 60 | 97 °C |
| Methanoic acid 甲酸 | 46 | 101 °C |
| Ethanoic acid 乙酸 | 60 | 118 °C |
Solubility in water depends on the balance between the hydrophilic –OH or –COOH group, which forms hydrogen bonds with water, and the hydrophobic hydrocarbon chain. Methanol, ethanol and propan-1-ol are completely miscible with water; solubility falls sharply from butan-1-ol onwards. Dicarboxylic acids such as ethanedioic acid are more soluble than monocarboxylic acids of the same chain length because they have two hydrogen-bonding groups.
水溶性取决于亲水的 –OH 或 –COOH(能与水形成氢键)与疏水的烃链之间的平衡。甲醇、乙醇和丙-1-醇与水完全互溶；从丁-1-醇开始溶解度急剧下降。同碳链长度下，乙二酸等二元羧酸比一元羧酸更易溶，因为它有两个可形成氢键的基团。
5. Preparation of Alcohols | 醇的制备
Ethanol is made industrially by fermentation or by catalytic hydration of ethene. In fermentation, glucose from sugar cane or maize is converted by yeast enzymes under anaerobic conditions at about 25–35 °C, giving a dilute aqueous solution of ethanol that must be distilled; the process is renewable but slow and batch-based.
乙醇在工业上由发酵法或乙烯催化水合法制取。发酵法中，来自甘蔗或玉米的葡萄糖在约 25–35 °C、无氧条件下经酵母酶作用转化，得到稀乙醇水溶液，须再蒸馏；该过程可再生，但速度慢且为间歇操作。
C₆H₁₂O₆ → 2CH₃CH₂OH + 2CO₂
Catalytic hydration uses ethene from cracking, steam and a phosphoric(V) acid catalyst on a silica support at about 300 °C and 60 atm. It is continuous, fast and gives a very pure product, but the ethene comes from a fossil source. Comparing the two routes is a classic exam question about sustainability, atom economy and purity.
催化水合法以裂解得到的乙烯为原料，通入水蒸气，在约 300 °C、60 atm 下用负载于硅胶上的磷酸催化剂反应。该方法连续、快速、产物很纯，但乙烯来自化石原料。比较这两条路线是考查可持续性、原子经济性和纯度的经典题目。
In the laboratory, alcohols are also prepared by warming a haloalkane with aqueous sodium hydroxide under reflux, which is a nucleophilic substitution, and by reducing an aldehyde or ketone. Lithium aluminium hydride, LiAlH₄, in dry ether is a powerful reducing agent that delivers hydride ions, H⁻, to the polar carbonyl carbon; sodium tetrahydridoborate(III), NaBH₄, in water or ethanol is milder and safer and reduces aldehydes and ketones but not carboxylic acids.
实验室中，醇还可通过卤代烃与氢氧化钠水溶液回流加热(亲核取代)制得，或通过还原醛、酮制得。在干醚中的氢化铝锂 LiAlH₄ 是强还原剂，能把氢负离子 H⁻ 送到极性的羰基碳上；水或乙醇中的硼氢化钠 NaBH₄ 较温和安全，可还原醛和酮，但不能还原羧酸。
Carboxylic acids themselves can be reduced to primary alcohols by LiAlH₄ in dry ether, which is useful
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