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 both contain the carbonyl group C=O, but they differ in the number of alkyl or hydrogen substituents attached to the carbonyl carbon. In an aldehyde the carbonyl carbon is bonded to at least one hydrogen atom and one organic group, while in a ketone it is bonded to two organic groups. The most important laboratory preparation of these two families is the controlled oxidation of alcohols, although other routes such as catalytic dehydrogenation and ozonolysis are also useful.

醛和酮都含有羰基 C=O，但区别在于羰基碳上连接的烷基或氢原子数量不同。醛的羰基碳至少连接一个氢原子和一个有机基团，而酮的羰基碳连接两个有机基团。这两类化合物最重要的实验室制备方法是醇的受控氧化，催化脱氢和臭氧分解等路线也有应用价值。

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1. The Carbonyl Group and the Alcohol Oxidation Ladder | 羰基与醇的氧化阶梯

Primary, secondary and tertiary alcohols are classified by the number of carbon atoms bonded to the carbon bearing the -OH group. This classification determines the product of oxidation. A primary alcohol can be oxidised first to an aldehyde and then to a carboxylic acid; a secondary alcohol is oxidised to a ketone; a tertiary alcohol is resistant to oxidation under mild conditions because the carbon bearing the -OH has no hydrogen atom attached to it.

伯醇、仲醇和叔醇是按连接 -OH 的碳原子上所连碳原子数来分类的。这一分类决定了氧化产物。伯醇可先被氧化为醛，然后进一步氧化为羧酸；仲醇被氧化为酮；叔醇在温和条件下不易被氧化，因为连接 -OH 的碳原子上没有氢原子。

The carbonyl group is polar because oxygen is more electronegative than carbon. This polarity explains many of the physical and chemical properties of aldehydes and ketones, but the preparation routes depend mainly on the oxidation state of the starting alcohol carbon.

由于氧的电负性比碳大，羰基具有极性。这一极性可以解释醛和酮的许多物理和化学性质，但制备路线主要取决于起始醇碳原子的氧化态。

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2. Preparing Aldehydes: Partial Oxidation of Primary Alcohols | 醛的制备：伯醇的部分氧化

An aldehyde is made from a primary alcohol by warming it with acidified potassium dichromate(VI), K₂Cr₂O₇ dissolved in dilute sulfuric acid. The reaction mixture is heated gently and the aldehyde is distilled off immediately as it forms. The oxidising agent is represented by [O].

伯醇与酸化重铬酸钾(VI)，即 K₂Cr₂O₇ 溶于稀硫酸，温热反应可生成醛。反应混合物应缓慢加热，醛一旦生成就立即被蒸馏出来。氧化剂通常用 [O] 表示。

The general equation for the oxidation of a primary alcohol to an aldehyde is:

伯醇氧化生成醛的一般方程式为：

RCH₂OH + [O] → RCHO + H₂O

For ethanol, the equation is:

以乙醇为例，方程式为：

CH₃CH₂OH + [O] → CH₃CHO + H₂O

The colour change observed during this reaction is from orange to green, because the orange dichromate(VI) ion, Cr₂O₇²⁻, is reduced to the green chromium(III) ion, Cr³⁺. This colour change is a simple test for a primary or secondary alcohol that can be oxidised.

反应过程中可观察到溶液由橙色变为绿色，因为橙色的重铬酸根离子 Cr₂O₇²⁻ 被还原为绿色的 Cr³⁺ 离子。这一颜色变化可作为伯醇或仲醇能被氧化的简单判断依据。

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3. Practical Control: Distillation from the Reaction Mixture | 实践控制：从反应混合物中蒸馏

To stop the aldehyde from being oxidised further to a carboxylic acid, the aldehyde must be removed from the oxidising mixture as quickly as possible. Aldehydes have lower boiling points than the corresponding alcohols and carboxylic acids because they cannot form hydrogen bonds between their own molecules. For example, ethanal boils at 21 °C, whereas ethanol boils at 78 °C.

为了防止醛进一步氧化为羧酸，必须尽快把醛从氧化混合物中移走。醛的沸点比相应的醇和羧酸低，因为醛分子之间不能形成氢键。例如，乙醛的沸点为 21 °C，而乙醇的沸点为 78 °C。

The practical set-up therefore uses a simple distillation apparatus. The reaction flask contains the primary alcohol and acidified potassium dichromate(VI). Gentle heating vapourises the aldehyde, which passes into the condenser and is collected in a receiving flask. The thermometer bulb should be placed at the side arm of the distillation head to measure the vapour temperature.

因此实验装置采用简易蒸馏装置。反应瓶中加入伯醇和酸化重铬酸钾(VI)。缓慢加热使醛汽化，蒸气进入冷凝管后被收集到接收瓶中。温度计水银球应放在蒸馏头支管口处，以测量蒸气温度。

Because the aldehyde is volatile, it is removed before it can be oxidised further. This is the key difference between preparing an aldehyde and preparing a carboxylic acid from a primary alcohol.

由于醛具有挥发性，它能够在被进一步氧化之前就被移出体系。这正是从伯醇制备醛与制备羧酸之间的关键区别。

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4. Over-oxidation: Why Reflux Gives a Carboxylic Acid | 过度氧化：为何回流会生成羧酸

If a primary alcohol is heated under reflux with an excess of acidified potassium dichromate(VI), the aldehyde is not removed. The aldehyde remains in contact with the oxidising agent and is oxidised further to a carboxylic acid. The general equation for this second step is:

如果伯醇与过量酸化重铬酸钾(VI) 在回流条件下加热，醛不会被移走。醛继续与氧化剂接触，被进一步氧化为羧酸。第二步反应的一般方程式为：

RCHO + [O] → RCOOH

For ethanol, the full sequence is:

以乙醇为例，完整反应为：

CH₃CH₂OH + 2[O] → CH₃COOH + H₂O

The full ionic equation for the oxidation of ethanol to ethanoic acid using dichromate(VI) is:

用重铬酸钾(VI) 将乙醇氧化为乙酸的完整离子方程式为：

3CH₃CH₂OH + 2Cr₂O₇²⁻ + 16H⁺ → 3CH₃COOH + 4Cr³⁺ + 11H₂O

Note that this equation shows the overall oxidation to ethanoic acid, not the aldehyde. To prepare an aldehyde, you must use distillation and avoid excess oxidising agent. To prepare a carboxylic acid, reflux and excess oxidising agent are used.

注意，该方程式表示最终氧化成乙酸，而非醛。要制备醛，必须使用蒸馏并避免氧化剂过量。要制备羧酸，则应使用回流和过量氧化剂。

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5. Preparing Ketones: Oxidation of Secondary Alcohols | 酮的制备：仲醇的氧化

A secondary alcohol is oxidised to a ketone by heating it with acidified potassium dichromate(VI). Unlike an aldehyde, the ketone does not need to be distilled off immediately because ketones are not easily oxidised further under these conditions. The reaction can be carried out under reflux to complete the conversion.

仲醇与酸化重铬酸钾(VI) 加热可被氧化为酮。与醛不同，酮不需要立即蒸馏出来，因为在这些条件下酮不易被进一步氧化。反应可在回流条件下进行，使转化更加完全。

The general equation for the oxidation of a secondary alcohol to a ketone is:

仲醇氧化生成酮的一般方程式为：

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