醛和酮的制备

酮和醛都是带有羰基的简单化学分子。羰基在碳和氧之间具有双键。由于羰基中的碳原子缺乏 OH 或 Cl 等反应性基团，因此这些化学分子很简单。

醛和酮的制备

下面讨论一些制备醛和酮的一般方法：

来自酒精：

• By oxidation – Aldehydes can be prepared by the oxidation of primary alcohols with normal oxidizing agents such as acidified potassium dichromate, potassium permanganate, chromium oxide, and sulphuric acid.

RCH₂OH (Primary Alcohol) + [O] → RCHO (Aldehyde) + H₂O

CH₃CH₂OH (Ethanol) + [O] → CH₃CHO (Acetaldehyde) + H₂O

Aldehydes undergo readily oxidation to carboxylic acids. therefore, to prevent further oxidation of aldehydes, these are distilled off as soon as they are formed. Oxidation of aldehydes is carried out under controlled conditions.

Ketones can be prepared by the oxidation of secondary alcohols with similar oxidizing agents.

C(CH₃)(CH₃)HOH (Isopropyl alcohol) + [O] → C(CH₃)(CH₃)=O (Acetone) + H₂O

C(CH₃)(H₃CH₂C)HOH (sec-Butyl alcohol) + [O] → C(CH₃)(CH₃H₂C)=O (Ethyl methyl ketone) + H₂O

• By the catalytic dehydrogenation of alcohols – Aldehydes and Ketones can be prepared by the dehydrogenation of alcohols. It is carried out by passing the vapour of alcohol over reduced copper at 573 K.

Primary alcohols give aldehydes while secondary alcohols give ketones.

CH₃CH₂OH (Ethyl alcohol) → CH₃CHO (Acetaldehyde) + H₂

CH₃—C(OH)H—CH₃ (Propan-2-ol) → CH₃—C(CH₃)=O (Propanone)

来自羧酸

• By Catalytic Decomposition – Aldehydes and ketones can be prepared by catalytic decomposition of carboxylic acids. This can be done by heating a mixture of methanoic acid or other acids to 573 K in the presence of manganous oxide which acts as a catalyst.

CH₃COOH (Ethanoic acid) + HOOCH (Methanoic acid) → HCHO (Methanal) + CO₂ + H₂O

Ketones can be prepared by passing the vapor of fatty acids over MnO at 573 K.

CH₃COOH (Ethanoic acid) + HOOCC₂H₅ (Propanoic acid) → CH₃COC₂H₅ (Butan-2-one) + CO₂ + H₂O  

• By Distillation of Calcium Salts – Aldehydes and ketones can also be prepared by distilling the calcium salts of the acids.

Ca(HCOO)(HCOO) (Calcium formate) → HCHO (Methanal) + CaCO₃

This method is, however not very suitable for the preparation of aldehydes because the yield is very low. This is because when a mixture of two calcium salts is heated three products are formed. For example, dry distillation of a mixture of calcium formate gives a mixture of formaldehyde. Similarly, this method cannot be used for the preparation of unsymmetrical ketone because it gives a mixture of three ketones.

For example, dry distillation of a mixture of calcium acetate will give a mixture of acetone.

   Ca(CH₃COO)(CH₃COO) (Calcium acetate) → CH₃COCH₃ (Propanone)+ CaCO₃

Cyclic ketones are formed when calcium salts of dicarboxylic acids are heated.

来自碳氢化合物

• By hydration of alkynes – Aldehydes and ketones can be prepared by the hydration of alkynes in the presence of dil. H2SO4 and HgSO4 as catalysts. Water adds to alkynes to form unstable enol intermediates which rearrange to form aldehydes or ketones.

HC≡CH (Acetylene) + H₂O → CH₂=CH(OH) (Vinyl alcohol) ⇌ CH₃CHO (Acetaldehyde)

Hydration of alkynes other than acetylene gives ketones.

• By Hydroboration-oxidation reaction: The alkynes can be converted into aldehydes and ketones by hydroboration- oxidation reaction. Borane adds to an alkyne forming vinylic borane, which on oxidation with H2O2 gives aldehydes and ketones. The symmetrical non-terminal alkynes give a single ketone while unsymmetrical non-terminal alkynes give a mixture of both possible ketones in which the methyl ketones predominate.

CH₃C≡C—CH₃ (But-2-yne) → CH₃—C(BH₂)=CH—CH₃ (Vinyl borane) → CH₃—C(OH)=CH—CH₃ (Enol) ⇌ CH₃—C=O—CH₂CH₃ (Butan-2-one)

Terminal alkynes give aldehydes. However, to avoid complications due to double bond addition of diborane, bulky sterically hindered boranes such as bis (1,2-dimethyl propyl) borane commonly known as disiamylborane is used in place of diborane.

CH₃CH₂CH₂C≡CH (Pent-1-yne) → CH₃CH₂CH₂CH₂CHO (Pentanal)  

• By ozonolysis of alkenes: Alkenes react with ozone to form ozonide which on subsequent cleavage with zinc dust and water gives aldehydes and ketones. It is clear that if the carbon forming the double bond carries an H-atom attached to it, aldehydes are formed otherwise ketones are formed.

 

• By Wacker’s process: Alkenes can be converted to aldehydes and ketones by treating with an acidified aqueous solution of palladium chloride containing a catalytic amount of cupric chloride in the presence of air or oxygen. This method is known as Wacker’s process.

 CH₃CH=CH₂ (Propene) + PdCl₂ + H₂O → CH₃COCH₃ (Acetone) + Pd + 2HCl

来自宝石二卤化物

The gem dihalides containing two halogen atoms on the same carbon atom on hydrolysis give a carbonyl group.

CH₃CHCl₂ (1,1-Dichloroethane) → [CH₃CH(OH)₂] (Unstable) → CH₃CHO (Ethanal)

醛的制备

来自酸性氯化物

在负载于硫酸钡上的钯催化剂存在下，酰氯通过催化氢化转化为醛。加入少量硫或喹啉会使催化混合物中毒。该反应称为Rosemund 还原。

来自烷基氰化物和酯的还原

在无水乙醚中用氯化亚锡和盐酸还原烷基氰化物，然后水解得到醛。这个反应被称为斯蒂芬还原。

来自芳烃

芳香醛由芳香烃通过以下方法制备：

• 通过甲苯的氧化：强氧化剂，如酸化或碱性KMnO ₄ ，酸化K ₂ Cr ₂ O ₇ ，浓。 HNO ₃等通过中间醛的氧化将甲苯及其衍生物氧化成苯甲酸。

然而，可以用合适的试剂在醛阶段停止氧化，这些试剂将甲基基团转化为难以进一步氧化的中间体。为此目的使用以下方法：

• 在乙酸酐中使用氧化铬：当使用三氧化铬和乙酸酐氧化烷基侧链芳环时，制备芳香醛。形成的醛立即被乙酸酐乙酰化，形成二乙酸偕，这不会被进一步氧化。因此，乙酸酐的函数是防止醛进一步氧化成酸。分离形成的偕二乙酸酯并用酸水溶液水解，得到相应的芳香醛。

类似地，对硝基甲苯产生对硝基苯甲醛。

可以注意到，通过用乙酸酐将醛捕获为偕二乙酸盐，可以防止苯甲醛进一步氧化为苯甲酸。宝石二乙酸盐不能被进一步氧化。

• 氯化铬的用途：烷基苯也可以用氯化铬的 CCl4 溶液氧化成苯甲醛。生成棕色铬络合物，遇水分解生成相应的苯甲醛。这种反应称为Etard反应。

在侧链含有比-CH 3 基高的基团的情况下，链的末端碳原子被CrO ₂ Cl ₂氧化成-CHO基。

• 通过 Gattermann Koch 反应：在无水 AlCl ₃或氯化亚铜存在下，通过用一氧化碳和氯化氢处理将苯或其衍生物转化为苯甲醛或取代苯甲醛。

这个反应被称为Gattermann Koch 反应。

从氢氰酸

醛可以从氢氰酸用格氏试剂处理制备，随后水解得到醛。

HC≡ N + CH ₃ MgBr → CH3—CH=NMgBr → CH ₃ CHO + NH ₃ + Mg(OH)Br

氢氰酸加成产物乙醛

Reimer-Tiemann 反应

酚醛是通过在大约 343 K 的氢氧化钠水溶液中用氯仿处理苯酚而获得的。该反应称为Reimer-Tiemann 反应。

酮的制备

来自酰氯

酮可以通过用二烷基镉处理由酰氯制备。为此目的所需的二烷基镉是通过格氏试剂和无水氯化镉之间的反应制备的。

2RMgX（格氏试剂）+ CdCl ₂ → R ₂ Cd（二烷基镉）+ 2Mg(X)Cl

2CH ₃ CH ₂ MgBr (格氏试剂) + CdCl ₂ (氯化镉) → (CH ₃ CH ₂ ) ₂ Cd (二乙基镉) + 2MgBrCl

通过格氏试剂从腈类中提取

脂肪族和芳香族酮都可以通过用合适的格氏试剂处理烷基或芳基腈然后酸水解来制备。

1-苯基丙酮也可以通过乙基溴化镁与苄腈作用，然后酸水解来制备。

通过 Friedel Crafts 反应由苯或取代苯

芳族酮可以通过弗里德尔·克拉夫茨酰化或苯甲酰化在路易斯酸如无水氯化铝的存在下用酰氯处理芳烃来制备。

Friedel Crafts 反应是亲电取代反应的典型例子。二苯甲酮也可以通过 Friedel Crafts 的碳酰氯与过量苯的反应来制备。

通过 Fries 重排从苯酯

在 CS ₂作为溶剂的存在下，酚酸酯或苯酯与无水 AlCl ₃一起加热会发生重排，其中酰基从酚氧原子迁移到苯环的邻位和对位，得到一种混合物邻和对酚酮。

这种反应称为薯条重排。

示例问题

问题1：用乙醇制备乙醛时，一生成就被蒸馏掉。解释。

回答：

Aldehyde is easily oxidizable to acetic acid. Therefore, to prevent its oxidation, it is distilled out as soon as it is formed.

问题 2：脂肪醛不显示位置异构体。为什么？

回答：

In the case of aliphatic aldehydes, the —CHO group is always present at the end. Therefore, they do not show position isomerism.

问题3：什么是福尔马林？

回答：

Formalin is a 40% aqueous solution of formaldehyde.

问题 4：醛和酮与什么反应？

回答：

Aldehydes and Ketones react with primary amines to form a class of compounds called imines. An unshared pair of electrons on the nitrogen of the amine is attracted to the partial-positive carbon of the carbonyl group.

问题5：醛和酮生成什么产物？

回答：

Most aldehydes and ketones react with 2°- amines to give products known as enamines. These are acid-catalyzed reversible reactions in which water is lost. enamines are easily converted back to their carbonyl precursors by acid-catalyzed hydrolysis.

问题 6：醛和酮是酸性还是碱性？

回答：

Aldehydes and Ketones are hydrogen bond acceptors; this makes them have considerable solubilities in water. Ketones such as acetone are good solvents because they dissolve both aqueous and organic compounds. Acetone is a polar, aprotic solvent. Reactions with acids:- The carbonyl oxygen is weakly basic.

问题7：醛是醇吗？

回答：

Alcohol with its —OH group bonded to a carbon atom that is bonded to no or one other carbon atom will form an aldehyde. Alcohol with its —OH group attached to two other carbon atoms will form a ketone.