📚 Carbonyl Compounds: Aldehydes and Ketones | 羰基化合物：醛与酮
p is at the end of a carbon chain, while in ketones it is attached to two carbon-containing groups. This article reviews the structure, naming, physical properties, preparation and key reactions of aldehydes and ketones for Cambridge A-Level Chemistry, including nucleophilic addition, oxidation, reduction and the standard qualitative tests.
羰基化合物含有与氧原子双键相连的碳原子 C=O。醛的羰基位于碳链末端，而酮的羰基连接两个含碳基团。本文为剑桥 A-Level 化学复习梳理醛与酮的结构、命名、物理性质、制备与主要反应，包括亲核加成、氧化、还原以及标准定性测试。
1. The Carbonyl Group | 羰基
ause oxygen is more electronegative than carbon. The carbon atom carries a partial positive charge, δ+, making it attackable by nucleophiles; the oxygen atom carries a partial negative charge, δ−, and can be protonated. This polarity is the key reason why carbonyl compounds undergo nucleophilic addition rather than electrophilic addition.
羰基 C=O 具有极性，因为氧的电负性大于碳。碳原子带部分正电荷 δ+，易受亲核试剂进攻；氧原子带部分负电荷 δ−，可被质子化。这种极性是羰基化合物发生亲核加成而不是亲电加成的关键原因。
e bond undergoes nucleophilic addition. The π electrons are pulled towards oxygen, leaving the π* orbital accessible to electron-rich species.
与烯烃中非极性、发生亲电加成的 C=C 双键不同，C=O 双键发生亲核加成。π 电子被拉向氧，使电子丰富的物种能进攻 π* 轨道。
The bond angle around a carbonyl carbon is approximately 120°, consistent with sp² hybridisation and a trigonal planar arrangement. The carbonyl carbon and the two attached atoms lie in the same plane.
羰基碳周围的键角约为 120°，符合 sp² 杂化和平面三角形结构。羰基碳与所连的两个原子处于同一平面。
C=O: polar, sp² hybridised, bond angle ≈ 120°
2. Naming Aldehydes and Ketones | 醛和酮的命名
For aldehydes, select the longest carbon chain containing the -CHO group and replace the final -e of the parent alkane with -al. The carbon atom of the -CHO group is always numbered 1. For example, methanal, ethanal, propanal and benzaldehyde are common aldehydes. If two -CHO groups are present, the suffix -dial is used, as in ethanedial.
对于醛，选择含有 -CHO 基团的最长碳链，将母体烷烃末尾的 -e 替换为 -al。-CHO 基团的碳原子始终编号为 1。例如，甲醛 methanal、乙醛 ethanal、丙醛 propanal 和苯甲醛 benzaldehyde 是常见醛。若含有两个 -CHO 基团，则使用后缀 -dial，如乙二醛 ethanedial。
replace the final -e with -one. The carbonyl carbon must be given the lowest possible number. For example, propanone, butanone, pentan-2-one and phenylethanone are common ketones. If two carbonyl groups are present, the suffix -dione is used.
对于酮，选择含有 C=O 基团的最长碳链，将末尾的 -e 替换为 -one。羰基碳必须获得尽可能小的编号。例如，丙酮 propanone、丁酮 butanone、戊-2-酮 pentan-2-one 和苯乙酮 phenylethanone 是常见酮。若含有两个羰基，则使用后缀 -dione。
Common names such as formaldehyde, acetaldehyde and acetone are still widely used, but IUPAC names are preferred in exams.
甲醛、乙醛和丙酮等俗名仍广泛使用，但考试中更推荐 IUPAC 名称。
Substituents are listed alphabetically with position numbers, for example 3-methylbutanal.
取代基按字母顺序排列并标明位置编号，例如 3-甲基丁醛。
In aromatic aldehydes and ketones, the benzene ring is often treated as a substituent, as in benzaldehyde and phenylethanone.
在芳香醛和芳香酮中，苯环通常视为取代基，如苯甲醛和苯乙酮。
3. Physical Properties | 物理性质
ns between the polar C=O groups. However, their boiling points are lower than those of alcohols of similar mass because they do not have O-H groups and cannot form hydrogen bonds with each other.
与相对分子质量相近的烷烃相比，醛和酮的沸点更高，因为极性 C=O 基团之间存在永久偶极-偶极相互作用。然而，它们的沸点低于质量相近的醇，因为它们没有 O-H 基团，分子间不能形成氢键。
Small aldehydes and ketones such as methanal, ethanal and propanone are soluble in water because they can form hydrogen bonds with water molecules through the lone pairs on the carbonyl oxygen. Solubility decreases as the hydrocarbon chain becomes longer, because the non-polar part becomes more significant.
甲醛、乙醛和丙酮等小分子醛酮可溶于水，因为它们能通过羰基氧上的孤对电子与水分子形成氢键。随着烃链增长，非极性部分变得更显著，溶解度下降。
nd gives a strong absorption around 1700 cm⁻¹. This is a useful way to identify the presence of a carbonyl group in an unknown organic compound.
在红外光谱中，C=O 键在约 1700 cm⁻¹ 处产生强吸收。这是识别未知有机化合物中是否存在羰基的有效方法。
4. Preparation of Aldehydes and Ketones | 醛和酮的制备
Primary alcohols can be oxidised to aldehydes using acidified potassium dichromate, K₂Cr₂O₇/H₂SO₄, with heating. The aldehyde must be distilled off as it forms to prevent further oxidation to a carboxylic acid.
伯醇可用酸化重铬酸钾 K₂Cr₂O₇/H₂SO₄ 加热氧化为醛。生成的醛必须及时蒸馏出来，以防止进一步氧化成羧酸。
Secondary alcohols are oxidised to ketones under reflux with acidified potassium dichromate. Ketones are resistant to further oxidation under these conditions because there is no hydrogen atom attached to the carbonyl carbon.
仲醇在回流条件下用酸化重铬酸钾氧化为酮。在这些条件下，酮不易进一步氧化，因为羰基碳上没有连接的氢原子。
Tertiary alcohols are generally resistant to oxidation because the carbon bearing the -OH group has no hydrogen atom available for removal. Other preparative routes include the hydration of alkynes and the catalytic dehydrogenation of alcohols over copper at high temperature.
叔醇通常不易氧化，因为连有 -OH 的碳上没有可供脱去的氢原子。其他制备途径包括炔烃水合以及醇在高温下通过铜催化脱氢。
Primary alcohol → aldehyde → carboxylic acid
Secondary alcohol → ketone
5. Nucleophilic Addition: Mechanism | 亲核加成：机理
forming a tetrahedral intermediate with a negatively charged oxygen. The oxygen is then protonated to give the final addition product, usually an alcohol.
在亲核加成中，亲核试剂进攻带部分正电荷的羰基碳。C=O 键的 π 电子转移到氧原子上，形成带负电荷氧的四面体中间体。随后氧被质子化，得到最终加成产物，通常是醇。
ttacks an electrophile. In carbonyl compounds, the electron-poor carbon is attacked by a nucleophile because the oxygen withdraws electron density.
该机理不同于烯烃的亲电加成。在烯烃中，富电子的 C=C 键进攻亲电试剂。在羰基化合物中，由于氧吸电子，缺电子的碳被亲核试剂进攻。
The reactivity of the carbonyl group depends on both electronic and steric factors. Aldehydes are generally more reactive than ketones because the carbonyl carbon in an aldehyde has less steric hindrance and a smaller electron-donating effect from attached carbon groups.
羰基的反应性取决于电子效应和空间效应。醛通常比酮更活泼，因为醛的羰基碳空间位阻较小，且相连碳基团的给电子效应较弱。
Nu⁻ + C=O → Nu-C-O⁻ → Nu-C-OH
6. Addition of Hydrogen Cyanide | 氰化氢加成
o form 2-hydroxy nitriles, also called cyanohydrins. HCN is a weak acid, so a small amount of base such as NaCN or KCN is used to generate the cyanide ion, CN⁻, which is the attacking nucleophile.
氰化氢 HCN 可加成到醛和酮的 C=O 双键上，生成 2-羟基腈，也称为氰醇。HCN 是弱酸，因此需加入少量 NaCN 或 KCN 等碱以产生进攻亲核试剂氰离子 CN⁻。
The mechanism involves nucleophilic attack by CN⁻ on the carbonyl carbon, followed by protonation of the resulting oxygen anion. The product contains both a hydroxyl group and a nitrile group on the same carbon atom.
机理包括 CN⁻ 对羰基碳的亲核进攻，随后生成的氧负离子被质子化。产物在同一碳原子上同时含有羟基和腈基。
This reaction is important because it extends the carbon chain by one carbon atom. Hydrolysis of the nitrile group under acidic conditions gives an α-hydroxy acid, which can be used in further synthesis.
该反应很重要，因为它将碳链延长一个碳原子。腈基在酸性条件下水解可得到 α-羟基酸，可用于进一步合成。
CH₃CHO + HCN → CH₃CH(OH)CN
7. Reduction of Carbonyl Compounds | 羰基化合物的还原
Sodium tetrahydridoborate, NaBH₄, is a common reducing agent for carbonyl compounds in the laboratory. It provides hydride ions, H⁻, which attack the carbonyl carbon. Aldehydes are reduced to primary alcohols, and ketones are reduced to secondary alcohols.
硼氢化钠 NaBH₄ 是实验室中常用的羰基化合物还原剂。它提供氢负离子 H⁻，进攻羰基碳。醛被还原为伯醇，酮被还原为仲醇。
Lithium tetrahydridoaluminate, LiAlH₄, is a stronger reducing agent and also reduces aldehydes and ketones to alcohols. Catalytic hydrogenation with hydrogen gas and a nickel catalyst, H₂/Ni, can also reduce carbonyl compounds, but the conditions are more vigorous.
四氢铝酸锂 LiAlH₄ 是更强的还原剂，也能将醛和酮还原为醇。使用氢气与镍催化剂的催化加氢 H₂/Ni 也可还原羰基化合物，但条件更剧烈。
NaBH₄ is often preferred because it is selective for the carbonyl group and can be used in aqueous or alcoholic solution under mild conditions. This makes it useful when other reducible groups are present in the molecule.
NaBH₄ 通常更受青睐，因为它对羰基具有选择性，可在水溶液或醇溶液中温和地进行。当分子中存在其他可还原基团时，这一点很有用。
Aldehyde → primary alcohol; Ketone → secondary alcohol
8. Oxidation of Aldehydes | 醛的氧化
Aldehydes are readily oxidised to carboxylic acids because the carbonyl carbon still has a hydrogen atom attached. Ketones are not easily oxidised under normal laboratory conditions because the carbonyl carbon is bonded to two carbon-containing groups and has no hydrogen atom for easy removal.
醛容易被氧化为羧酸，因为羰基碳上仍连有一个氢原子。酮在常规实验室条件下不易氧化，因为羰基碳与两个含碳基团相连，没有易于脱去的氢原子。
Acidified potassium dichromate, K₂Cr₂O₇/H₂SO₄, oxidises aldehydes to carboxylic acids. The orange dichromate ion, Cr₂O₇²⁻, is reduced to the green chromium(III) ion, Cr³⁺, so the colour change is from orange to green.
酸化重铬酸钾 K₂Cr₂O₇/H₂SO₄ 可将醛氧化为羧酸。橙色的重铬酸根离子 Cr₂O₇²⁻ 被还原为绿色的铬(III)离子 Cr³⁺，因此颜色由橙色变为绿色。
Fehling's solution contains copper(II) ions in alkaline solution. When warmed with an aldehyde, a brick-red precipitate of copper(I) oxide, Cu₂O, forms. Ketones do not react, so Fehling's test is a useful way to distinguish aldehydes from ketones.
斐林试剂含有碱性溶液中的铜(II)离子。与醛共热时，会生成砖红色的氧化亚铜 Cu₂O 沉淀。酮不反应，因此斐林测试是区分醛和酮的有效方法。
R-CHO + [O] → R-COOH
9. Tests to Distinguish Aldehydes and Ketones | 区分醛与酮的测试
Tollens' reagent is an ammoniacal silver nitrate solution containing [Ag(NH₃)₂]⁺ ions. When warmed with an aldehyde, silver ions are reduced to metallic silver, producing a silver mirror on the inside of the test tube. Ketones do not react with Tollens' reagent.
托伦试剂是含有 [Ag(NH₃)₂]⁺ 离子的氨性硝酸银溶液。与醛共热时，银离子被还原为金属银，在试管内壁形成银镜。酮不与托伦试剂反应。
Fehling's solution gives a brick-red precipitate of Cu₂O with aldehydes, but remains blue with ketones. Acidified potassium dichromate turns from orange to green with aldehydes, but shows no colour change with ketones under normal conditions.
斐林试剂与醛反应生成砖红色 Cu₂O 沉淀，但与酮混合时仍为蓝色。酸化重铬酸钾与醛反应由
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