Preparation of Aldehydes and Ketones | 醛和酮的制备方法汇总

📚 Preparation of Aldehydes and Ketones | 醛和酮的制备方法汇总

Understanding the preparation of aldehydes and ketones is fundamental to mastering carbonyl chemistry at the A-Level. This comprehensive guide covers every method you need to know for your CIE examinations, with clear distinctions between laboratory and industrial routes.

理解醛和酮的制备方法是掌握A-Level羰基化合物化学的基础。本综合指南涵盖CIE考试所需的每一种制备方法，并清晰区分实验室路线与工业路线。

1. Oxidation of Alcohols | 醇的氧化

The most common laboratory method for preparing aldehydes and ketones involves the oxidation of primary and secondary alcohols. Tertiary alcohols cannot be oxidised under normal conditions because they lack a hydrogen atom on the carbon bearing the -OH group.

实验室制备醛和酮最常用的方法涉及伯醇和仲醇的氧化。叔醇在正常条件下不能被氧化，因为连接-OH基团的碳原子上没有氢原子。

Primary alcohols → aldehydes (gentle oxidation) → carboxylic acids (vigorous oxidation)

伯醇 → 醛（温和氧化）→ 羧酸（剧烈氧化）
Secondary alcohols → ketones (standard oxidation)

仲醇 → 酮（标准氧化）

For preparing aldehydes from primary alcohols, the aldehyde must be distilled off immediately as it forms. This prevents further oxidation to the carboxylic acid. In contrast, ketones are resistant to further oxidation, so reflux can be used.

从伯醇制备醛时，醛一旦生成必须立即蒸馏出来。这样可以防止其进一步氧化为羧酸。相比之下，酮对进一步氧化具有抵抗性，因此可以使用回流操作。

Primary alcohol + [O] → Aldehyde + H₂O

Secondary alcohol + [O] → Ketone + H₂O

Typical oxidising agents include acidified potassium dichromate(VI) (K₂Cr₂O₇/H₂SO₄) and acidified potassium manganate(VII) (KMnO₄/H₂SO₄). The colour change for the dichromate test is orange → green, indicating a successful oxidation.

典型的氧化剂包括酸化的重铬酸钾（K₂Cr₂O₇/H₂SO₄）和酸化的高锰酸钾（KMnO₄/H₂SO₄）。重铬酸盐测试的颜色变化为橙色→绿色，表明氧化成功。

2. Ozonolysis of Alkenes | 烯烃的臭氧分解

Ozonolysis is an elegant method for preparing aldehydes and ketones by cleaving the carbon-carbon double bond of an alkene using ozone (O₃), followed by a reductive work-up with zinc and water (Zn/H₂O).

臭氧分解是一种巧妙的制备方法，利用臭氧（O₃）切断烯烃的碳碳双键，然后通过锌和水（Zn/H₂O）进行还原性后处理。

If the C=C carbon bears one alkyl group and one hydrogen → aldehyde

如果C=C碳上连有一个烷基和一个氢 → 生成醛
If the C=C carbon bears two alkyl groups (no H) → ketone

如果C=C碳上连有两个烷基（无氢）→ 生成酮
If the C=C carbon bears two hydrogens → methanal

如果C=C碳上连有两个氢 → 生成甲醛

For example, propene undergoes ozonolysis to give a mixture of methanal and ethanal. This method is particularly valuable because it reveals the structure of the original alkene from the carbonyl products.

例如，丙烯经臭氧分解生成甲醛和乙醛的混合物。该方法尤其有价值，因为可以从羰基产物推断原烯烃的结构。

R₂C=CR₂ + O₃ + Zn/H₂O → 2 R₂C=O

Note that this is a reductive work-up. If an oxidative work-up (H₂O₂) is used instead, carboxylic acids are formed, not aldehydes or ketones.

注意这是还原性后处理。如果改用氧化性后处理（H₂O₂），则生成的是羧酸，而非醛或酮。

3. Hydration of Alkynes | 炔烃的水合反应

Alkynes can be converted to carbonyl compounds through acid-catalysed hydration, known as the Kucherov reaction. The reaction involves the addition of water across the triple bond, with enol intermediates tautomerising to the more stable carbonyl form.

炔烃可以通过酸催化水合反应转化为羰基化合物，这被称为库切罗夫反应。该反应涉及水在三键上的加成，烯醇中间体互变异构为更稳定的羰基形式。

Typically performed using mercury(II) sulfate (HgSO₄) in dilute sulfuric acid, the reaction proceeds as follows:

通常使用硫酸汞（HgSO₄）和稀硫酸进行，反应过程如下：

Ethyne → ethanal (acetaldehyde): HC≡CH + H₂O → CH₃CHO

乙炔 → 乙醛：HC≡CH + H₂O → CH₃CHO
Propyne → propanone (acetone): CH₃C≡CH + H₂O → CH₃COCH₃

丙炔 → 丙酮：CH₃C≡CH + H₂O → CH₃COCH₃

For terminal alkynes, the hydration follows Markovnikov’s rule, with the OH group attaching to the more substituted carbon. This always produces a ketone (except for ethyne itself, which gives ethanal).

对于末端炔烃，水合反应遵循马氏规则，OH基团连接到取代程度更高的碳上。这总是生成酮（但乙炔本身除外，它生成乙醛）。

4. Acid-Catalysed Hydration of Alkenes | 烯烃的酸催化水合

While this route is more commonly discussed in the context of alcohol preparation, its reverse direction is worth noting. Industrially, the hydration of ethene was historically important for ethanol production, but it is not a direct route to carbonyls.

虽然该路线更多是在醇的制备语境中讨论，但其逆方向值得注意。在工业上，乙烯水合曾是生产乙醇的重要方法，但它不是直接制备羰基化合物的路线。

However, the oxidation of the resulting secondary alcohols from alkene hydration does provide an indirect route to ketones. For example, propene hydrates to propan-2-ol, which can be oxidised to propanone.

然而，烯烃水合生成的仲醇经氧化确实提供了制备酮的间接路线。例如，丙烯水合生成丙-2-醇，可进一步氧化为丙酮。

The CIE syllabus emphasises the direct preparation of carbonyls from alcohols and alkynes, so this method is supplementary understanding rather than a core required route.

CIE考纲强调从醇和炔烃直接制备羰基化合物，因此该方法作为补充理解而非核心要求路线。

5. Friedel-Crafts Acylation | 傅里德尔-克拉夫茨酰化反应

The Friedel-Crafts acylation reaction is the most important method for preparing aromatic ketones. An acyl chloride reacts with an aromatic ring (typically benzene) in the presence of an anhydrous aluminium chloride (AlCl₃) catalyst.

傅里德尔-克拉夫茨酰化反应是制备芳香酮最重要的方法。在无水氯化铝（AlCl₃）催化下，酰氯与芳香环（通常是苯）发生反应。

C₆H₆ + RCOCl → C₆H₅COR + HCl

The mechanism involves generation of the acylium ion (RCO⁺), which acts as an electrophile in electrophilic aromatic substitution. Unlike Friedel-Crafts alkylation, acylation does not suffer from over-reaction because the electron-withdrawing carbonyl group deactivates the ring after the first substitution.

该反应的机理涉及酰基正离子（RCO⁺）的生成，它在亲电芳香取代中充当亲电试剂。与傅里德尔-克拉夫茨烷基化不同，酰化不会发生过反应，因为羰基的吸电子效应在第一次取代后使环失活。

The same reaction can be performed with acid anhydrides, which are often preferred industrially because they are less hazardous than acyl chlorides. For example, ethanoyl chloride reacts with benzene to give phenylethanone (acetophenone).

该反应也可以使用酸酐进行，工业上通常更偏好酸酐，因为它们比酰氯更安全。例如，苯与乙酰氯反应生成苯乙酮。

6. Grignard Reaction Followed by Hydrolysis | 格氏反应后水解

Grignard reagents (RMgX) are powerful carbon nucleophiles that react with various carbonyl electrophiles to produce alcohols, which can then be oxidised to carbonyls. However, a more direct approach involves using nitriles or Weinreb amides.

格氏试剂（RMgX）是强碳亲核试剂，能与各种羰基亲电体反应生成醇，然后可氧化为羰基化合物。然而，更直接的方法涉及使用腈或温勒布酰胺。

When a Grignard reagent reacts with a nitrile (RCN), the initial adduct undergoes hydrolysis to produce a ketone. This is a key chain-extension method.

当格氏试剂与腈（RCN）反应时，初始加合物经水解生成酮。这是一种关键的碳链增长方法。

RCN + R’MgX → R-CO-R’ (after H₃O⁺ work-up)

For aldehyde synthesis, a less reactive electrophile such as ethyl methanoate or a Weinreb amide (N-methoxy-N-methylamide) is required. The Weinreb amide route is particularly chemoselective, as it forms a stable chelated intermediate that prevents over-addition.

对于醛的合成，需要使用反应性较弱的亲电体，如甲酸乙酯或温勒布酰胺（N-甲氧基-N-甲基酰胺）。温勒布酰胺路线具有特别好的化学选择性，因为它形成的稳定螯合中间体能防止过度加成。

This method is more characteristic of advanced organic synthesis (A2 level extension) and illustrates how nucleophilic addition can construct carbonyl frameworks.

此方法更具高级有机合成特征（A2级别拓展），展示了亲核加成如何构建羰基骨架。

7. Partial Reduction of Carboxylic Acid Derivatives | 羧酸衍生物的部分还原

Carboxylic acids can be reduced to primary alcohols using lithium aluminium hydride (LiAlH₄), but partial reduction to aldehydes is difficult because the aldehyde is more reactive than the acid. Specialist methods circumvent this problem.

羧酸用氢化铝锂（LiAlH₄）还原得到伯醇，但部分还原为醛比较困难，因为醛比酸更具反应性。专业方法可以规避这个问题。

Rosenmund reduction is the classic method. An acyl chloride is hydrogenated over a palladium catalyst which has been poisoned with barium sulfate (BaSO₄). The poisoned catalyst prevents over-reduction to the alcohol.

罗森蒙德还原是经典方法。酰氯在经硫酸钡（BaSO₄）毒化的钯催化剂上进行氢化。毒化催化剂防止过度还原为醇。

RCOCl + H₂ → RCHO + HCl (Pd/BaSO₄ catalyst)

Alternatively, lithium tri(tert-butoxy)aluminium hydride, LiAl(O-t-Bu)₃H, is a milder reducing agent that converts acyl chlorides to aldehydes without attacking the aldehyde product.

另外，三(叔丁氧基)氢化铝锂，LiAl(O-t-Bu)₃H，是一种更温和的还原剂，能将酰氯转化为醛而不会进一步还原醛产物。

For ketone synthesis, organocadmium reagents (R₂Cd) react with acyl chlorides, although this method is seldom tested at A-Level due to toxicity concerns.

对于酮的合成，有机镉试剂（R₂Cd）能与酰氯反应，但由于毒性问题，此方法在A-Level中很少考查。

8. Dehydrogenation of Alcohols | 醇的脱氢反应

Industrial preparation of aldehydes and ketones commonly employs catalytic dehydrogenation of alcohols, passing alcohol vapours over a copper or silver catalyst at high temperature. This avoids the use of stoichiometric oxidising agents.

工业上制备醛和酮常用醇的催化脱氢法，将醇蒸气在高温下通过铜或银催化剂。这避免了使用化学计量的氧化剂。

RCH₂OH → RCHO + H₂ (Cu catalyst, ~300°C)

R₂CHOH → R₂C=O + H₂ (Cu catalyst, ~300°C)

This method is suitable for methanol → methanal and for converting secondary alcohols to ketones. The hydrogen gas produced can be recycled as fuel. However, the CIE syllabus focuses more on the dichromate oxidation method in the laboratory context.

该法适用于甲醇→甲醛以及仲醇转化为酮。产生的氢气可回收利用。然而，CIE考纲更侧重于实验室情境下的重铬酸盐氧化法。

A related industrial route is the Ostwald process variant where methane is partially oxidised to methanal, but this is volatile and considered beyond the core syllabus.

相关的工业路线是奥斯特瓦尔德工艺变体，甲烷部分氧化生成甲醛，但该路线不够稳定，超出核心考纲范围。

9. Oxidative Cleavage of Alkenes (Potassium Manganate) | 烯烃的氧化裂解（高锰酸钾）

Under vigorous conditions, alkenes can be cleaved using hot, concentrated acidified KMnO₄. This produces ketones from tetra-substituted or tri-substituted alkene carbons bearing no hydrogen.

在剧烈条件下，烯烃可用热的浓酸性KMnO₄裂解。四取代或三取代且无氢的烯烃碳生成酮。

This method is significant for structural determination. By identifying the ketone fragments, the position of the original double bond can be deduced. However, if the alkene carbon bears a hydrogen, a carboxylic acid is formed instead.

该方法对结构测定具有重要意义。通过鉴定酮碎片，可以推断原双键的位置。然而，如果烯烃碳上连有氢，则生成羧酸而非酮。

C=C with H on both carbons → mixture of carboxylic acids

双键两端碳均有氢 → 羧酸混合物
C=C with H on one carbon only → carboxylic acid + ketone

仅一个双键碳上有氢 → 羧酸 + 酮
C=C with no H on either carbon → ketone + ketone

双键两端碳均无氢 → 酮 + 酮

This distinction is frequently tested in CIE Paper 4 multiple-choice questions involving structural elucidation.

这一区别在CIE Paper 4的选择题中经常考查，涉及结构解析。

10. Hydrolysis of Geminal Dihalides | 偕二卤代烃的水解

Vicinal dihalides and geminal dihalides behave differently under hydrolysis. A geminal dihalide of the type RCHCl₂ can be hydrolysed with aqueous alkali to produce an aldehyde RCHO.

邻二卤代烃和偕二卤代烃在水解时表现不同。RCHCl₂类型的偕二卤代烃可用碱水溶液水解生成醛RCHO。

RCHCl₂ + 2NaOH → RCHO + 2NaCl + H₂O

For geminal dihalides of type R₂CCl₂, hydrolysis yields a ketone R₂C=O. The mechanism involves substitution of both chlorines by hydroxide, followed by elimination of water from the unstable gem-diol intermediate.

对于R₂CCl₂类型的偕二卤代烃，水解生成酮R₂C=O。机理涉及两个氯被氢氧根取代，随后不稳定的偕二醇中间体脱去一分子水。

Although this method is less commonly examined directly, it reinforces the principle that halogen oxidation state influences the oxidation level of the carbon product. It is occasionally used in synthetic routes within examination questions.

虽然该方法直接考查较少，但它强化了卤素的氧化态影响碳产物氧化水平的原理。在考试题的合成路线中偶尔会用到。

11. Laboratory Comparison Summary | 实验室方法对比总结

Method	Reagents	Product	Key Notes
Alcohol oxidation	K₂Cr₂O₇/H₂SO₄	Aldehyde / Ketone	Distil for aldehydes; reflux for ketones
Ozonolysis	O₃ then Zn/H₂O	Aldehyde / Ketone	Reductive work-up; reveals alkene structure
Alkyne hydration	HgSO₄, H₂SO₄	Mainly ketones	Markovnikov addition; ethyne → ethanal
Friedel-Crafts acylation	RCOCl, AlCl₃	Aromatic ketones	No over-acylation; carbocation not formed
Rosenmund reduction	H₂, Pd/BaSO₄	Aldehydes	Acyl chloride starting material

When choosing a synthetic route, consider the availability of starting materials, the desired regiochemistry, and whether over-reaction is possible. Primary alcohols give aldehydes only under careful distillation; secondary alcohols cleanly produce ketones; alkynes provide a direct route to carbonyls without oxidation state ambiguity.

选择合成路线时，需考虑起始原料的可获得性、所需区域化学以及是否可能发生过度反应。伯醇仅在仔细蒸馏下生成醛；仲醇干净地生成酮；炔烃提供了无氧化态歧义性的直接羰基合成路线。

12. Exam Focus and Common Pitfalls | 考试重点与常见误区

Common pitfall 1: Students often forget that aldehyde oxidation requires immediate distillation. In the laboratory, if a primary alcohol is refluxed with excess oxidising agent, the final product is the carboxylic acid, not the aldehyde.

常见误区1：学生经常忘记醛的氧化需要立即蒸馏。在实验室中，如果伯醇与过量氧化剂回流，最终产物是羧酸，而不是醛。

Common pitfall 2: In ozonolysis questions, students misidentify whether the carbon is substituted or not. Always count the bonds: a carbon with one alkyl group and one H is aldehyde-forming; a carbon with two alkyl groups is ketone-forming.

常见误区2：在臭氧分解题目中，学生误判碳是否有取代基。务必数化学键：一个烷基加一个H的碳生成醛；两个烷基的碳生成酮。

Common pitfall 3: For alkyne hydration, some students expect ethyne to give methanal. It gives ethanal because the initially formed vinyl alcohol tautomerises to the carbonyl with the −CH₃ group.

常见误区3：对于炔烃水合，有些学生期望乙炔生成甲醛。实际上生成乙醛，因为初始形成的烯醇互变异构为含−CH₃的羰基化合物。

Key equations to memorise: alcohol + [O] → carbonyl; RCN + R’MgX + H₃O⁺ → ketone; C₆H₆ + RCOCl (AlCl₃) → aromatic ketone. These three represent the core transformations in exam questions.

需记忆的关键方程式：醇 + [O] → 羰基化合物；RCN + R’MgX + H₃O⁺ → 酮；C₆H₆ + RCOCl (AlCl₃) → 芳香酮。这三个代表了考题中的核心转化。

Examiner’s tip: In synthesis problems, always state the condition (heat under reflux, distil, etc.) in addition to the reagent. CIE mark schemes frequently allocate one mark for the reagent and one for the condition.

考官提示：在合成题中，除试剂外，务必说明条件（回流加热、蒸馏等）。CIE评分方案通常为试剂和条件各分配一分。

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Preparation of Aldehydes and Ketones | 醛和酮的制备方法汇总