Preparation of Aldehydes and Ketones | 醛和酮的制备

📚 Preparation of Aldehydes and Ketones | 醛和酮的制备

Aldehydes and ketones are central functional groups in organic chemistry, containing the carbonyl group C=O. Their preparation is a key topic in A-level Cambridge Chemistry, focusing on controlled oxidation of alcohols, hydration of alkynes, oxidative cleavage of alkenes, and selective reduction of carboxylic acid derivatives. Understanding the need for specific conditions – such as distillation versus reflux – is essential to avoid over-oxidation or unwanted side products.

醛和酮是含有羰基 C=O 的重要有机官能团。它们的制备是剑桥 A-level 化学的核心考点，涉及醇的选择性氧化、炔烃的水合、烯烃的氧化裂解以及羧酸衍生物的选择性还原。掌握反应所需的条件——例如蒸馏与回流的选择——对于防止过度氧化或副反应至关重要。

1. Oxidation of Primary Alcohols to Aldehydes | 伯醇氧化制备醛

Primary alcohols can be oxidised to aldehydes using a warm, acidified solution of potassium dichromate(VI), K₂Cr₂O₇, in dilute H₂SO₄. The immediate challenge is that aldehydes are themselves easily oxidised further to carboxylic acids. To obtain a good yield of the aldehyde, it must be removed from the reaction mixture as soon as it is formed. This is achieved by distillation: the aldehyde has a significantly lower boiling point than the corresponding alcohol or carboxylic acid because it cannot form intermolecular hydrogen bonds. The apparatus is set up for distillation rather than reflux, and the aldehyde is collected in a cooled receiver. The colour change from orange (Cr₂O₇²⁻) to green (Cr³⁺) confirms the oxidation has taken place.

伯醇可以用温热的重铬酸钾(VI)酸性溶液（K₂Cr₂O₇/H₂SO₄）氧化成醛。但醛本身极易被进一步氧化成羧酸，因此要想获得较高产率的醛，必须在醛生成后立即将其从反应混合物中转移出来。这通过蒸馏来实现：醛的沸点显著低于相应的醇和羧酸，因为醛分子间无法形成氢键。实验装置采用蒸馏而非回流，产物用冷却的接收瓶收集。反应过程中溶液从橙色（Cr₂O₇²⁻）变为绿色（Cr³⁺），证明氧化反应已经发生。

The general equation for this controlled oxidation is:
RCH₂OH + [O] → RCHO + H₂O
If the aldehyde remains in contact with the oxidising agent, it undergoes further oxidation to the carboxylic acid:
RCHO + [O] → RCOOH

该控制氧化的通式如下：
RCH₂OH + [O] → RCHO + H₂O
如果生成的醛继续与氧化剂接触，会进一步氧化为羧酸：
RCHO + [O] → RCOOH

2. Oxidation of Secondary Alcohols to Ketones | 仲醇氧化制备酮

Secondary alcohols are oxidised to ketones using the same acidified potassium dichromate(VI) reagent. Unlike aldehydes, ketones are resistant to further oxidation under these conditions because breaking a carbon–carbon bond would be required. Therefore, the reaction can safely be carried out under reflux, allowing the oxidation to go to completion without fear of over-oxidation. The colour change from orange to green is again observed. The product ketone can be separated by distillation after the reaction.

仲醇同样可使用酸化重铬酸钾氧化成酮。与醛不同，酮在这些条件下难以继续氧化，因为要继续氧化就需要断裂碳-碳键。因此，反应可以安全地在回流条件下进行，使氧化彻底完成而不必担心过度氧化。实验中同样可观察到溶液由橙色变为绿色。产物酮可在反应结束后通过蒸馏分离。

The general equation for the oxidation of a secondary alcohol is:
RCH(OH)R’ + [O] → RCOR’ + H₂O

仲醇氧化的通式为：
RCH(OH)R’ + [O] → RCOR’ + H₂O

3. Distillation vs Reflux in Alcohol Oxidation | 醇氧化中的蒸馏与回流

The choice between distillation and reflux when oxidising alcohols is dictated by the boiling points and susceptibility to further oxidation. Primary alcohols, aldehydes and carboxylic acids all form hydrogen bonds with water and among themselves, but the aldehyde lacks an –OH group; it cannot act as a hydrogen-bond donor, only as an acceptor. This makes the aldehyde’s boiling point substantially lower, so it can be distilled off as it forms. In contrast, to drive the oxidation of a secondary alcohol to completion, reflux ensures a sustained high temperature without loss of volatile material, and since the ketone is not easily oxidised further, no special separation is needed during the reaction.

醇氧化实验中蒸馏与回流的选择取决于各组分的沸点以及产物被进一步氧化的倾向。伯醇、醛和羧酸都能与水及彼此形成氢键，但醛没有羟基，不能作为氢键供体，只能作为受体，因此醛的沸点明显较低，可以在生成时就被蒸馏出来。反之，为使仲醇的氧化进行彻底，回流可以维持较高的反应温度而不损失挥发组分，而且由于酮不易继续氧化，反应过程中无需特殊的产物分离。

4. Hydration of Alkynes – A Versatile Route | 炔烃的水合 —— 通用合成路线

Alkynes can be hydrated to form carbonyl compounds using mercury(II) sulfate as a catalyst in the presence of aqueous sulfuric acid, HgSO₄/H₂SO₄. The reaction proceeds via Markovnikov addition of water: the enol intermediate immediately tautomerises to the more stable carbonyl compound. Ethyne is the only alkyne that yields an aldehyde; its hydration gives ethanal (acetaldehyde). All other alkynes produce ketones. For example, propyne gives propanone, and but-2-yne gives butanone.

炔烃可以在硫酸汞-硫酸水溶液催化下水合生成羰基化合物，反应通过马氏规则加水：首先生成的烯醇中间体迅速互变异构为更稳定的羰基形式。乙炔是唯一能产生醛的炔烃，其水合生成乙醛。其他所有炔烃水合均生成酮，例如丙炔生成丙酮，丁-2-炔生成丁酮。

Ethyne hydration:
CH≡CH + H₂O → [CH₂=CHOH] → CH₃CHO
Propyne hydration:
CH₃C≡CH + H₂O → CH₃COCH₃

乙炔水合：
CH≡CH + H₂O → [CH₂=CHOH] → CH₃CHO
丙炔水合：
CH₃C≡CH + H₂O → CH₃COCH₃

5. Ozonolysis of Alkenes – Oxidative Cleavage | 烯烃的臭氧化 —— 氧化裂解

Ozonolysis is a two-stage process that cleaves the C=C double bond and forms carbonyl compounds. The alkene is first treated with ozone (O₃) at low temperature to form an ozonide, which is then reduced in the presence of a mild reducing agent such as zinc and acetic acid, or dimethyl sulfide. The nature of the substituents around the double bond determines whether aldehydes, ketones, or a mixture of both are obtained. Symmetrical alkenes give a single carbonyl product, whilst unsymmetrical alkenes give a mixture. This reaction is not only a preparative method but also a valuable tool for determining the position of a double bond in unknown alkenes.

臭氧化是一个两步过程，可断裂碳碳双键并生成羰基化合物。烯烃先在低温下用臭氧（O₃）处理生成臭氧化物，然后在温和还原剂存在下（如锌和乙酸，或二甲硫醚）还原。双键周围取代基的性质决定了产物是醛、酮还是二者的混合物。对称烯烃得到单一羰基产物，而不对称烯烃得到混合物。该反应不仅是醛酮的制备方法，还是确定未知烯烃中双键位置的重要工具。

For example, ozonolysis of 2-methylbut-2-ene, (CH₃)₂C=CHCH₃, gives propanone and ethanal:
(CH₃)₂C=CHCH₃ + O₃ → (CH₃)₂C=O + CH₃CHO

例如，2-甲基丁-2-烯（(CH₃)₂C=CHCH₃）的臭氧化生成丙酮和乙醛：
(CH₃)₂C=CHCH₃ + O₃ → (CH₃)₂C=O + CH₃CHO

6. Friedel-Crafts Acylation for Aromatic Ketones | 傅-克酰基化制备芳香酮

Friedel-Crafts acylation is an electrophilic substitution reaction used to introduce an acyl group into an aromatic ring, yielding an aromatic ketone. The reaction typically involves an acyl chloride (RCOCl) and an aromatic substrate (such as benzene) in the presence of anhydrous aluminium chloride as a Lewis acid catalyst. The AlCl₃ activates the acyl chloride, generating a highly electrophilic acylium ion, which then attacks the aromatic ring. The product is an aryl ketone, ArCOR, and HCl gas is evolved.

傅-克酰基化反应是一种亲电取代反应，可在芳香环上引入酰基，生成芳香酮。反应通常使用酰氯（RCOCl）和芳香族底物（如苯），在无水三氯化铝路易斯酸催化下进行。AlCl₃ 活化酰氯产生强亲电的酰基正离子，进而进攻芳香环，产物为芳基酮 ArCOR，同时生成 HCl 气体。

The general equation for the acylation of benzene is:
C₆H₆ + RCOCl → C₆H₅COR + HCl
It is worth noting that formyl chloride (HCOCl) is unstable, so this method cannot be used to prepare aromatic aldehydes directly.

苯的酰基化通式为：
C₆H₆ + RCOCl → C₆H₅COR + HCl
值得注意的是，甲酰氯（HCOCl）不稳定，因此无法通过该方法直接制备芳香醛。

7. Rosenmund Reduction – Acyl Chlorides to Aldehydes | 罗森蒙德还原 —— 由酰氯制醛

The Rosenmund reduction allows the partial reduction of an acyl chloride to an aldehyde without further reducing the aldehyde to a primary alcohol. This is achieved by hydrogenating the acyl chloride in the presence of a poisoned palladium catalyst, Pd/BaSO₄, which is deactivated by the addition of a sulphur-containing compound such as quinoline. The catalytic hydrogenation stops at the aldehyde stage, producing HCl as a by-product. The aldehyde can be removed by distillation to minimise over-reduction.

罗森蒙德还原可将酰氯部分还原为醛，而不会进一步将醛还原为伯醇。该方法通过在含硫喹啉毒化的钯-硫酸钡催化剂（Pd/BaSO₄）存在下加氢来实现。催化加氢反应在醛阶段停止，副产物为 HCl。生成的醛可通过蒸馏移出，以尽量减少继续还原。

The general transformation is:
RCOCl + H₂ → RCHO + HCl

通用的反应为：
RCOCl + H₂ → RCHO + HCl

8. Stephen Reaction – Nitriles to Aldehydes | 斯蒂芬反应 —— 由腈制醛

The Stephen reaction provides another route to aldehydes by treating a nitrile with anhydrous tin(II) chloride (SnCl₂) in the presence of dry hydrogen chloride gas, followed by hydrolysis. The nitrile is reduced to an imine hydrochloride (RCH=NH·HCl), which is then hydrolysed with water to release the aldehyde. The method is particularly useful for preparing aromatic aldehydes, such as benzaldehyde from benzonitrile, and avoids over-reduction to the amine.

斯蒂芬反应提供了由腈制醛的另一条途径。腈在干燥氯化氢气体存在下用无水氯化亚锡（SnCl₂）处理，随后水解。腈首先被还原为亚胺盐酸盐（RCH=NH·HCl），再经水解释放醛。该方法特别适用于制备芳香醛，如由苯甲腈制备苯甲醛，并且可避免过度还原成胺。

The sequence can be summarised as:
RCN + 2[H] → RCH=NH·HCl
RCH=NH·HCl + H₂O → RCHO + NH₄Cl

反应过程可概括为：
RCN + 2[H] → RCH=NH·HCl
RCH=NH·HCl + H₂O → RCHO + NH₄Cl

9. Comparison of Key Synthetic Routes | 主要合成路线对比

When selecting a method to prepare an aldehyde or a ketone, the starting material, selectivity, and ease of work-up must be considered. Oxidation of a primary alcohol is the most straightforward laboratory route to an aldehyde, but it requires careful distillation to avoid over-oxidation. Oxidation of a secondary alcohol is even simpler and proceeds under reflux to give ketones cleanly. Hydration of alkynes is an important industrial pathway, while ozonolysis offers a way to cleave alkenes into carbonyl fragments. For non-oxidative routes to aldehydes, Rosenmund and Stephen reductions are valuable, especially when sensitive functional groups are present that would not tolerate oxidative conditions. For aromatic ketones, Friedel-Crafts acylation is the method of choice.

在选择制备醛或酮的方法时，需要考虑起始原料、选择性以及后处理的难易程度。伯醇氧化是实验室制备醛最直接的途径，但必须小心蒸馏，以防过度氧化。仲醇氧化更为简单，在回流条件下即可洁净地生成酮。炔烃水合是一条重要的工业路线，而臭氧化可将烯烃裂解为羰基碎片。对于非氧化的醛制备途径，罗森蒙德和斯蒂芬还原很有价值，尤其适用于含有不能耐受氧化条件的敏感官能团的底物。制备芳香酮时，傅-克酰基化是首选方法。

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Published by TutorHao | Chemistry Revision Series | aleveler.com

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