📚 Aldehydes and Ketones: Key Exam Points | 醛和酮考点精讲
group. This article covers their structure, nomenclature, preparation, characteristic reactions, and key distinction tests that are essential for A-Level Chemistry exam success.
醛和酮是有机化学中基本的羰基化合物，均含有活泼的 >C=O 基团。本文全面梳理其结构、命名、制备方法、特征反应以及关键的鉴别实验，这些内容都是 A-Level 化学考试中必须掌握的考点。
1. Functional Group and Structure | 官能团与结构
Carbonyl group: Aldehydes have the functional group – he chain. Ketones have >C=O with two alkyl or aryl groups attached to the carbonyl carbon.
羰基：醛的官能团为 –CHO，羰基碳位于碳链末端；酮的羰基碳两边各连接一个烃基，官能团写作 >C=O。
e higher electronegativity of oxygen, making the carbonyl carbon electron-deficient and an electrophilic centre. The oxygen carries a partial negative charge and can act as a base or hydrogen bond acceptor.
C=O 键因氧的电负性更高而具有极性，使得羰基碳缺电子，成为亲电中心。氧带部分负电荷，可作为碱或氢键受体。
Resonance forms: the true structure is a hybrid between a polarised double bond and a dipolar single bond (C⁺–O⁻).
共振式：真实结构是极化双键与偶极单键（C⁺–O⁻）之间的杂化体。
2. Nomenclature | 命名规则
IUPAC rules: Aldehydes are named by replacing the terminal -e of the alkane with -al. The carbonyl carbon is always carbon 1 and does not need numbering. Methanal, ethanal, propanal are the simplest examples.
IUPAC 规则：醛的命名是将对应烷烃词尾的 -e 改为 -al，羰基碳永远定位为 1 号，无需标出。最简单的例子是甲醛（methanal）、乙醛（ethanal）和丙醛（propanal）。
Ketones are named with the suffix -one, and the position of the carbonyl group must be given unless it is unambiguous. The chain is numbered to give the carbonyl carbon the lowest possible number. Propanone, butan-2-one, pentan-3-one.
酮以 -one 结尾，除位置明确外必须标明羰基的位次。编号时使羰基碳的数字最小。例如丙酮（propanone）、丁-2-酮（butan-2-one）、戊-3-酮（pentan-3-one）。
Common names are also used: acetone for propanone, acetaldehyde for ethanal.
俗名也常使用：acetone 指丙酮，acetaldehyde 指乙醛。
3. Physical Properties | 物理性质
The polarity of the carbonyl group gives aldehydes and ketones permanent dipole–dipole forces, leading to higher boiling points than alkanes of similar molecular mass, but lower than the corresponding alcohols, because they lack O–H hydrogen bond donors.
羰基的极性使醛酮分子间存在永久偶极–偶极作用力，因此沸点比相对分子质量相近的烷烃高，但比相应的醇低，因为它们没有 O–H 氢键供体。
Short-chain aldehydes and ketones (methanal, ethanal, propanone) are miscible with water because the carbonyl oxygen can form hydrogen bonds with water. Solubility decreases as the hydrocarbon chain lengthens.
低级醛酮（甲醛、乙醛、丙酮）可以与水混溶，因为羰基氧能与水分子形成氢键。随着碳链增长，溶解度下降。
4. Preparation of Aldehydes and Ketones | 醛和酮的制备
Aldehydes are prepared by the controlled oxidation of primary alcohols using acidified potassium dichromate(VI). The aldehyde must be distilled out of the reaction mixture as it forms to prevent further oxidation to the carboxylic acid.
醛可由伯醇的控制氧化制备，使用酸化重铬酸钾（VI）作氧化剂。必须在生成时立即将醛蒸馏出来，以免进一步氧化为羧酸。
Ketones are obtained by oxidising secondary alcohols under similar conditions. The ketone is not easily oxidised further, so distillation is not as critical.
酮可由仲醇在类似条件下氧化得到。由于酮不易被继续氧化，无需严格控制蒸馏。
Other routes include reduction of acyl chlorides (Rosenmund reduction) for aldehydes, and Friedel–Crafts acylation for aromatic ketones, but alcohol oxidation is the most common exam pathway.
其他路线包括酰氯的还原（Rosenmund 还原）制醛，以及傅–克酰基化制取芳香酮，但氧化醇是考试中最常见的制备途径。
5. Nucleophilic Addition: General Mechanism | 亲核加成一般机理
The carbonyl carbon is electrophilic; a nucleophile (Nu⁻) attacks the δ+ carbon, breaking the π bond and shifting the electron pair onto oxygen to form a tetrahedral alkoxide intermediate with a negative charge on oxygen.
羰基碳具有亲电性；亲核试剂（Nu⁻）进攻 δ+ 碳，使π键断开，电子对转移到氧上，形成一个氧带负电荷的四面体醇盐中间体。
C=O + Nu⁻ → Nu–C–O⁻
(e.g., by H₂O or H⁺) gives the final addition product. The overall result is addition of H–Nu across the C=O bond.
随后醇盐中间体质子化（被 H₂O 或 H⁺），得到最终加成产物。净结果相当于 H–Nu 加成到 C=O 双键上。
The reaction is often reversible. When the nucleophile is H⁻ (from NaBH₄ or LiAlH₄), it is effectively irreversible because the alkoxide is a strong base that does not eliminate hydride.
该反应常为可逆过程。当亲核试剂为 H⁻（来自 NaBH₄ 或 LiAlH₄）时，反应实际上不可逆，因为烷氧负离子是强碱，不会脱去氢负离子。
If the carbonyl compound is unsymmetrical or the addition creates a chiral centre, a racemic mixture is obtained because the nucleophile can attack from either face of the planar carbonyl with equal probability.
如果羰基化合物不对称，或加成产生手性中心，产物为外消旋混合物，因为亲核试剂可以从平面羰基的两面以相同概率进攻。
6. Addition of Hydrogen Cyanide | 与氰化氢的加成
Hydrogen cyanide adds across the C=O bond to form cyanohydrins (hydroxynitriles). The reaction is catalysed by base (CN⁻ is generated). It is important for chain extension and synthetic utility.
氰化氢与 C=O 加成生成羟腈（氰醇）。反应由碱催化（产生 CN⁻），在碳链增长和有机合成中非常重要。
RCHO + HCN → RCH(OH)CN
RCOR' + HCN → RC(OH)(R')CN
The mechanism: cyanide ion acts as the nucleophile, attacking carbonyl carbon; the alkoxide then picks up H⁺ from HCN or water.
机理：氰根离子作为亲核试剂进攻羰基碳，生成的烷氧负离子再从 HCN 或水中夺取质子。
HCN is toxic and volatile; it is often generated in situ from NaCN (or KCN) and dilute sulfuric acid.
HCN 剧毒且易挥发，通常使用 NaCN（或 KCN）与稀硫酸现场发生。
7. Reaction with 2,4-Dinitrophenylhydrazine (Brady's Reagent) | 与 2,4-二硝基苯肼反应
2,4-DNPH reacts with both aldehydes and ketones to form a yellow-orange precipitate of the corresponding 2,4-dinitrophenylhydrazone. This is a condensation (addition–elimination) reaction, and it confirms the presence of a carbonyl group.
2,4-二硝基苯肼（2,4-DNPH）与醛和酮均反应，生成对应的 2,4-二硝基苯腙黄色或橙色沉淀。这是一个缩合反应（加成–消除），用于确证羰基的存在。
>C=O + H₂N–NH–C₆H₃(NO₂)₂ → >C=N–NH–C₆H₃(NO₂)₂ + H₂O
The derivative can be purified and its melting point measured. Comparison with literature data helps identify the original carbonyl compound.
生成的衍生物可提纯后测定熔点，与文献数据对比可帮助鉴定原始羰基化合物。
This test does not distinguish between aldehydes and ketones; both give a positive result.
该实验无法区分醛和酮，两者均为阳性结果。
8. Oxidation: Distinguishing Aldehydes from Ketones | 氧化反应：鉴别醛和酮
Aldehydes are easily oxidised to carboxylic acids. Ketones resist oxidation under mild conditions because C–C bond breaking is required. This difference forms the basis of chemical tests.
醛容易被氧化为羧酸，而酮在温和条件下不易氧化，因为这需要断裂 C–C 键。这一区别构成了化学鉴别实验的基础。
Tollens' reagent (ammoniacal silver nitrate): aldehydes reduce Ag⁺ to metallic silver, forming a silver mirror on the test tube. Ketones give no reaction.
托伦试剂（银氨溶液）：醛将 Ag⁺ 还原为金属银，在试管壁形成银镜。酮无此反应。
RCHO + 2[Ag(NH₃)₂]⁺ + 2OH⁻ → RCOO⁻ + 2Ag↓ + 4NH₃ + 2H₂O
Fehling's solution (alkaline Cu²⁺ complexed with tartrate): aldehydes (but not aromatic aldehydes) reduce the blue solution to a brick-red precipitate of Cu₂O. Ketones give no colour change.
斐林试剂（铜离子与酒石酸根的碱性络合物）：脂肪醛可将蓝色溶液还原为砖红色的 Cu₂O 沉淀（芳香醛无此反应）。酮无颜色变化。
Benedict's reagent is similar, using citrate complex, and gives a positive test with aliphatic aldehydes.
本尼迪克试剂类似，使用柠檬酸盐络合物，脂肪醛呈阳性。
Thus, Tollens’ test is the most general test for aldehydes, while Fehling’s/Benedict’s distinguish aliphatic aldehydes from ketones and aromatic aldehydes.
因此，托伦试剂是最通用的检醛试剂，而斐林/本尼迪克试剂可区分脂肪醛与酮及芳香醛。
9. Iodoform Test (Triiodomethane Reaction) | 碘仿试验（三碘甲烷反应）
The iodoform test is positive for methyl ketones (CH₃CO–R) and compounds that can be oxidised to methyl ketones or ethanal, such as ethanol and secondary alcohols with a CH₃CH(OH)– group.
碘仿试验对甲基酮（CH₃CO–R）以及能被氧化成甲基酮或乙醛的化合物呈阳性，例如乙醇和含有 CH₃CH(OH)– 结构的仲醇。
Reaction conditions: aqueous iodine (I₂) and sodium hydroxide. The methyl group is tri-iodinated, followed by cleavage to give the yellow precipitate iodoform (CHI₃) with a characteristic antiseptic smell.
反应条件：碘的氢氧化钠溶液。甲基被三碘取代后发生断裂，生成黄色碘仿（CHI₃）沉淀，有特殊消毒水气味。
CH₃COCH₃ + 3I₂ + 4NaOH → CHI₃↓ + CH₃COONa + 3NaI + 3H₂O
Aldehydes with no methyl carbonyl group (e.g., propanal) and ketones without a CH₃CO– group give a negative result. This test helps narrow down structural possibilities.
没有甲基酮结构的醛（如丙醛）以及不含 CH₃CO– 的酮呈阴性反应。该试验有助于缩小结构判断范围。
10. Reduction of Carbonyl Compounds | 羰基化合物的还原
Both aldehydes and ketones can be reduced to alcohols. Sodium borohydride (NaBH₄) in water or ethanol is the typical selective reducing agent. Aldehydes yield primary alcohols; ketones give secondary alcohols.
醛和酮均可被还原为醇。硼氢化钠（NaBH₄）在水或乙醇中是常用的选择性还原剂。醛还原得伯醇，酮得仲醇。
RCHO + 2[H] → RCH₂OH
RCOR' + 2[H] → RCH(OH)R'
The mechanism is nucleophilic addition, where hydride ion (H⁻) from NaBH₄ attacks the carbonyl carbon, followed by protonation of the alkoxide intermediate. This reaction is effectively irreversible.
机理为亲核加成：来自 NaBH₄ 的氢负离子（H⁻）进攻羰基碳，然后醇盐中间体质子化。该反应实际上不可逆。
Lithium aluminium hydride (LiAlH₄) is a stronger reducing agent that also reduces carboxylic acids and esters, but NaBH₄ is preferred for aldehydes and ketones as it is safer and more chemoselective.
氢化铝锂（LiAlH₄）还原性更强，亦可还原羧酸和酯，但 NaBH₄ 更常用于醛酮，因为它更安全且具有化学选择性。
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