Did you know the sweet-smelling component in vanilla ice cream is an aldehyde? Its vanillin, chemically known as \(3-\)Methoxy\(-4-\)hydroxybenzaldehyde. The liquid in nail polish remover is propanone, commonly known as acetone. Both vanillin and acetone are carbonyl compounds and differ in their respective functional groups. But how are these carbonyl compounds prepared? Let’s learn everything about their preparation in detail. 

Preparation of Aldehydes

The aldehydes can be prepared with the help of the following methods:

1. By Oxidation Of Primary Alcohols

a). By using PCC:

Mild oxidising agents such as Pyridinium chlorochromate (PCC) are used to oxidise primary alcohols to form aldehydes. PCC does not oxidise aldehydes to carboxylic acids.

b). By using Dess-Martin Periodinane (DMP):

PCC is being replaced by Dess-Martin periodinane (DMP). This is because DMP has several practical advantages over PCC, such as producing higher yields and requiring less rigorous reaction conditions. Like PCC, it oxidises primary alcohols to aldehydes without further oxidation of aldehydes to carboxylic acid.

2. By Dehydrogenation Of Primary Alcohols

The process in which the removal of hydrogen takes place is known as dehydrogenation. It is a catalytic \({\text{E2}}\) elimination reaction in which copper is used as a catalyst.
When the vapours of primary alcohol are passed through a heated copper metal surface, it forms aldehyde.

The reaction stops once the aldehyde is formed. As the reaction involves the removal of hydrogen, it is also called an oxidation reaction.

3. From Hydrocarbons

a) By Ozonolysis of alkenes

Ozonolysis is a two-step process in which the conversion of alkene into an ozonide takes place, followed by its reductive cleavage to yield carbonyl compounds.

When ozone is passed through a solution of alkene in some inert solvent such as \({\text{C}}{{\text{H}}_2}{\text{C}}{{\text{l}}_2},{\text{CHC}}{{\text{l}}_3},{\text{CC}}{{\text{l}}_4}\) at a low temperature \(\left( {{\text{196 – 200}}\;{\text{K}}} \right),\) alkenes are oxidised to their respective ozonides. The ozonides so formed are unstable and explosive. Hence, they are not isolated but are reduced, in situ ( produced during the reaction), with zinc dust and water or \({{\text{H}}_{\text{2}}}{\text{/pd}}\) to give aldehydes and ketones or sometimes a mixture of both (reductive cleavage) depending upon the structure of alkene.

Ozonolysis is a versatile method to identify unknown alkenes. For example,

b) By hydration of ethyne

Ethyne or acetylene is the simplest hydrocarbon with a triple bond between the two carbon atoms. In the hydration of ethyne, a water molecule is added to the triple bond to form a very unstable intermediate. Ethyne is converted to vinyl alcohol (ethenol) which tautomerises to form ethanal. All other alkynes except ethyne undergo hydration to give ketones.

4. From Nitriles (Stephen Reaction)

a) Stephen Reaction, also known as Stephen Aldehyde Synthesis or Stephen Reduction, is an organic reaction used to prepare aldehydes \(\left( {{\text{R – CHO}}} \right)\) from nitriles \(\left( {{\text{R – CN}}} \right){\text{.}}\) The reaction takes place in the presence of tin(II) chloride \(\left( {{\text{SnC}}{{\text{l}}_{\text{2}}}} \right){\text{,}}\) hydrochloric acid \(\left( {{\text{HCl}}} \right)\) to form corresponding imine hydrochlorides, which on acid hydrolysis, give corresponding aldehydes.

b) DIBAL-H is also used to reduce nitriles to imines partially. The imines are then hydrolysed to aldehydes upon the addition of water.

5. From Esters

DIBAL-H is also used to reduce esters to their corresponding aldehydes. The temperature of the reaction mixture is kept very low \(\left( {{\rm{ – 7}}{{\rm{0}}^{\rm{o}}}{\rm{C}}} \right){\rm{.}}\)

6. From Acyl Chlorides (Rosenmunds Reaction)

Acid chlorides are easily reduced to the corresponding aldehydes by passing hydrogen gas through boiling xylene solution in the presence of Palladium \(\left( {{\rm{pd}}} \right)\) catalyst over \({{\mathop{\rm BaSO}\nolimits} _4}.\)

The Pd catalyst is poisoned with \({{\mathop{\rm BaSO}\nolimits} _4}\) because the untreated catalyst is too reactive and will further reduce aldehydes to alcohols. To avoid over reduction and to terminate the reaction at the aldehyde stage, \({{\rm{pd}}}\) is poisoned with \({{\mathop{\rm BaSO}\nolimits} _4}.\) 

The reactions are given below:

Acid chlorides are converted into aldehydes.

Example: Acetyl chloride is converted into acetaldehyde.

Benzoyl chloride is converted into benzaldehyde.

Formaldehyde cannot be prepared through Rosenmund’s reaction since formyl chloride, \({\rm{HCOCl}}\), is unstable at room temperature. This reaction is used only for the preparation of aldehydes but not for ketones.

Preparation of Aromatic Aldehydes

1. By oxidation of Methylbenzene (Toluene)

a) Using Chromyl chloride (Etard Reaction)

Etard Reaction is the partial oxidation of methylbenzene or toluene, which takes place with the help of chromyl chloride in carbon disulphide and carbon tetrachloride solution. Etard reaction is a two-step reaction in which a chromium complex is formed in the first step, followed by its hydrolysis to form the desired aldehyde.

b) Using Chromic oxide 

Chromic oxide is a mild oxidising agent which is used to oxidise toluene to benzaldehyde in the presence of acetic anhydride \({\left( {{\rm{C}}{{\rm{H}}_3}{\rm{CO}}} \right)_2}{\rm{O}}{\rm{.}}\)

When toluene reacts with \({\rm{Cr}}{{\rm{O}}_3}\) in the presence of \({\left( {{\rm{C}}{{\rm{H}}_3}{\rm{CO}}} \right)_2}{\rm{O}}{\rm{,}}\) the methyl group is oxidised and forms an intermediate known as benzylidene diacetate. When this intermediate reacts with aqueous \({\rm{NaOH}}\), it changes into benzaldehyde with the removal of a water molecule.

Benzaldehyde so formed does not undergo further oxidation into benzoic acid because chromic oxide is a mild oxidising agent.

c) Side chain chlorination followed by hydrolysis

Monochlorination of toluene is a two-step reaction. 

Step 1: Toluene reacts with chlorine in the presence of sunlight at \({\rm{383}}\,{\rm{K}}\) to give Benzal Chloride. The substitution of halogen takes place at the side chain rather than at the hydrogen of the aromatic ring.

Step 2: The Benzal chloride formed in step 1 undergoes hydrolysis to form benzaldehyde.

2. By Gatterman Koch Reaction

In Gattermann – Koch reaction, benzene is treated with carbon monoxide in an acidic medium in the presence of anhydrous aluminium chloride to give benzaldehyde. In this reaction, anhydrous aluminium chloride works as a catalyst. It is an electrophilic substitution reaction. 

Preparation of Ketones

1. By oxidation of Secondary alcohols

The oxidation of secondary alcohol produces a ketone. In this reaction, loss of hydrogen takes place from the hydroxyl group and the hydrogen-bonded to the carbon attached to the oxygen atom. The oxygen then forms a double bond with the carbon attached to it. This results in the formation of a ketone, as \({{\rm{R}}_1} – {\rm{CO}}{{\rm{R}}_2}.\) Ketones are quite resistant to further oxidation, but this may occur with strong oxidants and produce esters or carboxylic acids.

2. By dehydrogenation of Secondary alcohols

During oxidation of secondary alcohol, both \({\rm{C – O}}\) and \({\rm{O – H}}\) bonds are broken to form \( > {\rm{C}} = {\rm{O}}\) bonds. When vapours of secondary alcohols are passed over heated copper at \(573\;{\rm{K,}}\) dehydrogenation of alcohols takes place, and ketones are produced. 

3. From Hydrocarbons

a) Ozonolysis of alkene

Like aldehydes, Ozonolysis of alkenes produces ketone. In Ozonolysis, when ozone is passed through a solution of alkene in an inert solvent of \({\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{l}}_2},{\rm{CHC}}{{\rm{l}}_3},{\rm{CC}}{{\rm{l}}_4}\) at a low temperature \((196 – 200\;{\rm{K}}),\) alkenes are oxidised to their respective ozonides, which further reacts with zinc dust and water or \({{\rm{H}}_{\rm{2}}}{\rm{/Pd}}\) to give aldehydes and ketones or sometimes a mixture of both (reductive cleavage). The product (aldehyde or ketone) formed depends upon the structure of the alkene. This is illustrated below:

b) By hydration of alkynes

In the hydration of alkynes, a water molecule is added to the triple bond to form a very unstable intermediate. Alkynes are converted to vinyl alcohol (enol) which tautomerises to form ketones. All other alkynes except ethyne undergo hydration to give ketones.

4. From acyl chlorides

Dialkylcadmium is prepared by the reaction of cadmium chloride with the Grignard reagent. Acyl chlorides react with dialkylcadmium to yield ketones.

5. From nitriles

Treatment of nitriles with Grignard reagent followed by hydrolysis yields ketones.

6. From benzene or substituted benzenes

Benzene or substituted benzene on reacting with an acid chloride in the presence of Lewis acid such as anhydrous aluminium chloride forms the corresponding ketone. This reaction is Friedel-Crafts acylation reaction and proceeds through an electrophilic aromatic substitution reaction whereby the acyl group adds to the aromatic ring with the loss of \({\rm{HX}}\).

Aluminium chloride \(\left( {{\rm{AlC}}{{\rm{l}}_{\rm{3}}}} \right)\) is often used, but \({\rm{FeC}}{{\rm{l}}_3}\) and other Lewis acids will also do the job.

Here’s a general example of the Friedel-Crafts acylation:

Summary

Aldehydes and Ketones are an important class of organic compounds, especially due to their widespread applications. From paint thinners to cakes and pastries, these compounds are widely used in our day-to-day lives. Hence, it is important to learn their preparation methods. In this article, we learned the various methods through which aldehydes and ketones can be formed. We also learned about the preparation of aromatic aldehydes.

FAQs on Preparation of Aldehydes and Ketones

Q.1. What is the Preparation of Aldehydes and Ketones?
Ans: Aldehydes are prepared by:
1. Oxidation of primary alcohols
2. Dehydrogenation of primary alcohols
3. Hydration of ethyne
4. Reduction of nitriles (Stephen Reaction) and esters
5. Rosenmund Reaction
Aromatic aldehydes are exclusively prepared by:
1. Oxidation of toluene using chromyl chloride (Etard Reaction)
2. Oxidation of toluene using chromic oxide
3. Sidechain chlorination of toluene followed by hydrolysis
4. Gatterman Koch Reaction
Ketones are prepared by:
1. Oxidation of secondary alcohols
2. Dehydrogenation of secondary alcohols
3. Ozonolysis of alkenes
4. Hydration of alkynes
5. Reduction of acyl chlorides
6. Friedel Craft’s Acylation

Q.2. How do you prepare aldehyde and ketone by Ozonolysis?
Ans: Ozonolysis is a two-step process in which the conversion of alkene into an ozonide takes place, followed by its reductive cleavage to yield carbonyl compounds.

When ozone is passed through a solution of alkene in some inert solvent such as \({\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{l}}_2},{\rm{CHC}}{{\rm{l}}_3},{\rm{CC}}{{\rm{l}}_4}\) at a low temperature \((196 – 200\;{\rm{K}}),\) alkenes are oxidised to their respective ozonides. The ozonides so formed are unstable and explosive. Hence, they are not isolated but are reduced, in situ ( produced during the reaction), with zinc dust and water or \({{\rm{H}}_{\rm{2}}}{\rm{/Pd}}\) to give aldehydes and ketones or sometimes a mixture of both (reductive cleavage) depending upon the structure of alkene.
Ozonolysis is a versatile method to identify an unknown alkene. For example,

Q.3. What is meant by aldehyde?
Ans: Aldehydes are organic compounds with an aldehydic \(\left( {{\rm{ – CHO}}} \right)\) functional group. The aldehyde functional groups are made up of a carbon atom linked to an oxygen atom via a double bond and a hydrogen atom via a single bond. Aldehydes have the general structural formula \({\rm{R – CHO,}}\) where \({\rm{R}}\) denotes the alkyl or aryl group.

An aldehyde group always lies at the end of a carbon chain. The general molecular formula of the homologous aldehyde series is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{{\rm{2n + 1}}}}{\rm{CHO,}}\) where \({\rm{n = 1,2,3}}……\)

Q.4. What are natural aldehydes?
Ans: Aldehydes that occur naturally are known as natural aldehydes. Sugars, the bark of cinnamon, almonds all consists of aldehydes. These can also be synthesised in laboratories

Q.5. Is aldehyde acidic or basic?
Ans: Alpha hydrogen atoms in aldehydes are acidic in nature. Due to this, bases can easily remove acidic hydrogen to form intermediate carbanion.

Study Uses of Aldehydes and Ketones

We hope this article on the preparation of aldehydes and ketones is helpful to you. If you have any questions related to this page or in general about aldehydes preparation, reach us through the comment section below and we will get back to you as soon as possible.