Preparation of Aldehydes and Ketones

We know that organic compounds (Ketones and aldehydes) production is possible in industrial scale and laboratory scale. However, do you know that ketone, as well as aldehyde production, occurs naturally in many living organisms? Ketone generation in the form of ribulose-1,5-bisphosphate is one of the steps of photosynthesis and help in the formation of the necessary organic compounds during photosynthesis.

Ketones are present as sugars and are called ketoses. It is also present in most vertebrates including humans as ketone bodies. Now, that you know how organisms produce ketone, try finding how living organisms generate aldehydes. In this topic, we will learn how the preparation of Aldehydes and Ketones is possible by various chemical reactions.

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Aldehydes and Ketones

Aldehydes and Ketones are simple organic compounds containing a carbonyl group. Carbonyl group contains carbon-oxygen double bond. These organic compounds are simple because the carbon atom presents in the carbonyl group lack reactive groups such as OH or Cl.

Aldehydes

An aldehyde is one of the classes of carbonyl group-containing alkyl group on one end and hydrogen on the other end. The R and Ar denote alkyl or aryl member respectively. In the condensed form, the aldehyde is written as –CHO.

Ketones

Ketone is a member of the carbonyl group-containing alkyl or aryl group on both the end of the carbonyl group. The compound formula is RC(=O)R’. In this case, R and R’ are the different carbon containing substituents.

Method of Preparation of Aldehydes and Ketones

Aldehydes and Ketones can be prepared by a number of methods. Let’s discuss the method one by one.

Formation by Oxidation of Alcohols

Oxidation of primary and secondary alcohols leads to the formation of aldehydes and ketones. The oxidation is possible with the help of common oxidizing agents are KMnO₄, K₂Cr₂O₇, and CrO₃. Strong oxidizing agents helps in the oxidation of the primary alcohol to aldehyde then to a carboxylic acid.

Primary alcohols having low molecular weight can undergo oxidation and form aldehydes. The reaction mixture after aldehyde formation can avoid further oxidation if the reaction temperature is modulated so that the boiling point of the aldehyde is lower than the alcohol which helps in the distillation of aldehyde from the reaction mixture soon after its formation. Hence, it is important to maintain the reaction temperature slightly more than 349K. Refer to the reaction below

Aldehyde and Ketone preparation is possible by oxidation of primary and secondary alcohol by agents such as PCC (pyridinium chlorochromate), Collins reagents (Chromium trioxide-pyridine complex), and Cu at 573 K.

Collins Reagents (Chromium trioxide-pyridine complex)

Collin’s reagent or chromium trioxide-pyridine complex is a good oxidizing reagent for conversion of primary alcohol to aldehydes. Additionally, an advantage of Collins reagent is that it helps to cease further oxidation of aldehydes to carboxylic acids. However, the reaction with Collins reagent is possible in a non-aqueous medium such as CH₂Cl₂.

PCC (pyridinium chlorochromate)

The mixture of pyridine along with CrO₃ and HCl in dichloromethane leads to the formation of Pyridine chlorochromate or PCC (C₅H₅NH⁺CrO₃ Cl^–).

Ketones can be prepared by using similar oxidizing agents from secondary alcohols.

Formation by Dehydrogenation of Alcohols

This preparation method applies in case of conversion of volatile alcohols to aldehydes. It is generally used in industrial application. Vapours of alcohol are passed through heavy metal catalysts such as Cu or Ag in this technique. Primary alcohol produces aldehyde whereas secondary alcohol produces ketones, respectively.

For example, alcohols undergo dehydrogenation when vapours of primary alcohol or secondary alcohol pass through copper gauze at a temperature of 573 K. Refer to the example below to see how n-propyl alcohol leads to the formation of propionaldehyde in the dehydrogenation process.

It is possible to use different metal catalysts such as copper or silver under heating conditions during dehydrogenation of alcohol. However, this technique is apt for conversion of valuable alcohols to aldehydes. Furthermore, it is useful in industrial applications.

Moreover, this is one of the better methods for preparation of aldehydes and ketones because further oxidation is not possible of aldehydes. Hence, there is no risk of conversion of aldehydes to carboxylic acids.

Ketone and Aldehyde Preparation from Hydrocarbons

This method is further divided into two separate methods. They are

• By ozonolysis of alkenes
• By hydration of alkynes

Ozonolysis of Alkenes

Formation of aldehyde and ketone is possible by ozonolysis of alkenes. Ozonolysis is a reaction method in which addition of ozone molecules or O₃ to an alkene compound leads to the formation of ozonide. Reduction of the ozonide compound with the help of zinc dust and water produces the smaller molecules, which in this case will be the respective aldehydes and ketones.

The reaction produces aldehydes, ketones and in some cases both the compounds on the basis of the substitution arrangement of the alkene compounds.

Refer to the example below to observe how an alkene on ozonolysis leads to the formation of the ozonide compound. This compound further undergoes subsequent reductive cleavage with Zn dust and water or H₂/Pd to produce carbonyl compounds.

Zinc Dust helps in removing the H₂O₂ thereby ceasing the chances of further oxidation of carbonyl compounds to acids. Refer to the examples below to see how choosing a suitable alkene for this method helps to achieve the proper aldehyde or ketone.

Hydration of Alkynes

Alkynes follow Markovnikov’s rule in the presence of a proper catalyst to produce ketones. All alkynes react with water in the presence of HgSO₄ and H₂SO₄  to form ketones. However, the reaction of ethyne with water in the presence of the catalyst (HgSO₄ and H₂SO₄) leads to the formation of acetaldehyde. This is an only exception where alkyne on hydration produces acetaldehyde. Rest all the alkyne on hydration produces ketones.

Solved Examples for You

Identify and name the reagents that help in carrying out the following reactions

1. Cyclohexanol to cyclohexanone-
2. Hexan-1-ol to hexanal
3. But-2-ene to ethanol
4. Allyl alcohol to propenal

Solution

1. Anhydrous CrO₃
2. C₅H₅NH⁺CrO₃Cl (PCC)
3. O3 /H2O-Zn dust
4. PCC