📚 Organic Chemistry II: Carbonyls, Carboxylic Acids and Chirality | 有机化学II：羰基化合物、羧酸与手性
This revision guide covers the core Edexcel A-Level Chemistry Topic 17 content on aldehydes, ketones, carboxylic acids, esters, acyl chlorides and optical isomerism. You will learn how the polar C=O bond controls nucleophilic addition, why carboxylic acids are weak acids, how esterification and acyl chloride reactions proceed, and how chirality gives rise to optical isomers.
本复习指南涵盖 Edexcel A-Level 化学第 17 单元的核心内容，包括醛、酮、羧酸、酯、酰氯和旋光异构。你将学习极性 C=O 键如何主导亲核加成、羧酸为什么是弱酸、酯化和酰氯反应如何进行，以及手性如何产生旋光异构体。
1. The Carbonyl Group and Polarity | 羰基与极性
The carbonyl group C=O is polar because oxygen is more electronegative than carbon. The electron density is drawn towards oxygen, making the carbon partially positive (δ⁺) and the oxygen partially negative (δ⁻).
羰基 C=O 具有极性，因为氧的电负性大于碳。电子云被拉向氧，使碳带有部分正电荷（δ⁺），氧带有部分负电荷（δ⁻）。
This polarity makes aldehydes and ketones susceptible to attack by nucleophiles at the carbonyl carbon. Aldehydes are generally more reactive than ketones because ketones have two electron-donating alkyl groups that reduce the partial positive charge on the carbonyl carbon and cause steric hindrance.
这种极性使醛和酮容易在羰基碳上受到亲核试剂进攻。醛通常比酮更活泼，因为酮有两个给电子烷基，降低了羰基碳上的部分正电荷，并产生空间位阻。
2. Naming Aldehydes and Ketones | 醛和酮的命名
In IUPAC naming, aldehydes use the suffix -al and ketones use -one. The longest carbon chain containing the carbonyl group is identified, and the carbonyl carbon is numbered to give the lowest possible locant.
在 IUPAC 命名中，醛使用后缀 -al，酮使用后缀 -one。先找出包含羰基的最长碳链，并给羰基碳最低位次编号。
For example, CH₃CHO is ethanal, CH₃COCH₃ is propanone, and CH₃CH₂CHO is propanal. If the aldehyde group is attached to a ring, the suffix -carbaldehyde is used.
例如，CH₃CHO 是乙醛，CH₃COCH₃ 是丙酮，CH₃CH₂CHO 是丙醛。如果醛基连接在环上，则使用后缀 -carbaldehyde。
3. Physical Properties and Solubility | 物理性质与溶解性
Short-chain aldehydes and ketones are soluble in water because the polar carbonyl group can form hydrogen bonds with water molecules. However, they cannot form hydrogen bonds with themselves because there is no hydrogen atom bonded to the highly electronegative oxygen.
短链醛和酮可溶于水，因为极性羰基能与水分子形成氢键。但它们自身之间不能形成氢键，因为没有氢原子直接连接在高电负性的氧上。
Boiling points of aldehydes and ketones are lower than the corresponding alcohols but higher than alkanes of similar molar mass. This is because alcohols form strong intermolecular hydrogen bonds, while carbonyl compounds only have permanent dipole-dipole forces and van der Waals forces.
醛和酮的沸点低于相应的醇，但高于相对分子质量相近的烷烃。这是因为醇能形成强的分子间氢键，而羰基化合物只有永久偶极-偶极力和范德华力。
4. Nucleophilic Addition Mechanism | 亲核加成机理
Nucleophilic addition is the characteristic reaction of aldehydes and ketones. A nucleophile attacks the δ⁺ carbonyl carbon, and the π bond of C=O breaks to form a tetrahedral alkoxide intermediate.
亲核加成是醛和酮的特征反应。亲核试剂进攻 δ⁺ 的羰基碳，C=O 的 π 键断裂，形成四面体醇盐中间体。
The general mechanism with hydrogen cyanide is shown below. The cyanide ion CN⁻ acts as the nucleophile, and the product is a hydroxynitrile.
与氰化氢反应的一般机理如下所示。氰根离子 CN⁻ 作为亲核试剂，产物是羟基腈。
RCHO + HCN ⇌ RCH(OH)CN
This reaction is important because it increases the length of the carbon chain by one carbon atom. The product is a mixture of enantiomers when the aldehyde is unsymmetrical.
该反应很重要，因为它将碳链延长一个碳原子。当醛不对称时，产物是两种对映体的混合物。
5. Reduction of Carbonyl Compounds | 羰基化合物的还原
Aldehydes can be reduced to primary alcohols, and ketones can be reduced to secondary alcohols. The most common reducing agent is aqueous sodium borohydride, NaBH₄, which provides a source of hydride ions H⁻.
醛可被还原为伯醇，酮可被还原为仲醇。最常用的还原剂是硼氢化钠水溶液 NaBH₄，它提供氢负离子 H⁻。
The reduction mechanism involves nucleophilic addition of the hydride ion to the carbonyl carbon, followed by protonation of the alkoxide intermediate by water or dilute acid.
还原机理包括氢负离子对羰基碳的亲核加成，然后醇盐中间体被水或稀酸质子化。
RCHO + 2[H] → RCH₂OH
Lithium aluminium hydride LiAlH₄ can also reduce carbonyl compounds, but it must be used in dry ether because it reacts violently with water.
氢化铝锂 LiAlH₄ 也能还原羰基化合物，但必须在无水乙醚中使用，因为它与水剧烈反应。
6. Testing for Carbonyl Compounds | 羰基化合物的检验
2,4-dinitrophenylhydrazine (2,4-DNPH or Brady’s reagent) reacts with aldehydes and ketones to give an orange or yellow precipitate. This confirms the presence of a carbonyl group but does not distinguish between an aldehyde and a ketone.
2,4-二硝基苯肼（2,4-DNPH 或 Brady 试剂）与醛和酮反应生成橙色或黄色沉淀。这可以确认羰基的存在，但不能区分醛和酮。
Tollens’ reagent (ammoniacal silver nitrate) oxidises aldehydes but not ketones. A positive test is the formation of a silver mirror on the inside of the test tube.
托伦试剂（氨性硝酸银）能氧化醛但不能氧化酮。正反应结果是在试管内壁形成银镜。
Fehling’s or Benedict’s solution also oxidises aldehydes, giving a brick-red precipitate of copper(I) oxide. Ketones give no reaction because they cannot be easily oxidised.
斐林试剂或本尼迪克特试剂也能氧化醛，生成砖红色的氧化亚铜沉淀。酮不发生反应，因为它们不易被氧化。
7. Carboxylic Acids: Structure and Acidity | 羧酸：结构与酸性
Carboxylic acids contain the functional group -COOH. They are weak acids, meaning they only partially dissociate in water to form carboxylate ions and hydrogen ions.
羧酸含有官能团 -COOH。它们是弱酸，在水中只部分电离，生成羧酸根离子和氢离子。
RCOOH ⇌ RCOO⁻ + H⁺
The acidity arises because the negative charge on the carboxylate ion is delocalised across two oxygen atoms. This makes the anion more stable and shifts the equilibrium to the right.
酸性来源于羧酸根离子上的负电荷在两个氧原子上离域。这使阴离子更稳定，使平衡向右移动。
Electron-withdrawing groups near the -COOH group increase acid strength by stabilising the carboxylate ion further. Electron-donating groups decrease acid strength.
-COOH 附近的吸电子基通过进一步稳定羧酸根离子而增强酸性。给电子基则降低酸性。
8. Reactions of Carboxylic Acids | 羧酸的反应
Carboxylic acids react with metals, bases and carbonates to form salts. For example, ethanoic acid reacts with sodium carbonate to produce sodium ethanoate, water and carbon dioxide gas.
羧酸与金属、碱和碳酸盐反应生成盐。例如，乙酸与碳酸钠反应生成乙酸钠、水和二氧化碳气体。
Carboxylic acids react with alcohols in the presence of a strong acid catalyst such as concentrated sulfuric acid to form esters. This is called esterification and is a reversible condensation reaction.
羧酸在强酸催化剂（如浓硫酸）存在下与醇反应生成酯。这称为酯化反应，是一种可逆的缩合反应。
RCOOH + R’OH ⇌ RCOOR’ + H₂O
The alcohol provides the alkyl group that replaces the -OH of the carboxylic acid, while water is eliminated. Concentrated sulfuric acid also acts as a dehydrating agent to shift the equilibrium towards the ester.
醇提供烷基取代羧酸中的 -OH，同时脱去水。浓硫酸也作为脱水剂，使平衡向生成酯的方向移动。
9. Acyl Chlorides and Acid Anhydrides | 酰氯和酸酐
Acyl chlorides have the functional group -COCl and are much more reactive than carboxylic acids. They react with water, alcohols, ammonia and primary amines at room temperature to give carboxylic acids, esters, amides and N-substituted amides respectively.
酰氯的官能团是 -COCl，其反应活性远高于羧酸。它们在室温
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