Preparation of Aldehydes: Methods & Uses

Aldehydes are a type of carbonyl compound. Carbonyl Compounds are defined as compounds that have a carbonyl group (that is C=O group). The carbonyl group are an essential functional group in organic chemistry. These compounds form an integral part of organic chemistry, and the primary members of this group are ketones, carboxylic acids, and aldehydes. Aldehydes are one of the two types of carbonyl compounds. They contain a carbonyl group known as simple organic compounds. Aldehydes can be prepared by a number of ways and this article will discuss different methods of preparing aldehydes as well as some common aldehydes with their IUPAC names.

Key Terms: Aldehydes, Formaldehyde, Benzaldehyde, Oxidation, Dehydrogenation, Ozonolysis, Hydration, Toluene

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What are Aldehydes?

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Aldehyde is one of the most common groups in organic chemistry. In these chemical compounds, a carbonyl group is single-bonded to a hydrogen atom. The Carbonyl group is a carbon that is double-bonded to an oxygen atom, it can also bond to a hydrogen atom or carbon-hydrogen chain which is known as the R group. 

The aldehydes are found in natural and synthetic hormones. They give almonds their sweet aroma and cinnamon their distinct odour. Aldehydes found in almonds are known as benzaldehyde and the aldehyde found in the bark of cinnamon is cinnamaldehyde.

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Methods for Preparation of Aldehydes

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Aldehydes are chemical compounds where hydrogen and carbon are attached to carbonyl groups. These organic compounds are surrounded by hydrogen and the Alkyl group (R). The Carbonyl group is an aldehyde containing an alkyl group on one end and hydrogen on the other end. A condensed form of aldehyde is written as -CHO. 

Aldehydes as organic compounds have diverse properties. Smaller size aldehydes are more soluble in water. Formaldehyde and Acetaldehyde are completely soluble in water. Pungent odours are linked with more volatile aldehydes. 

1. From Oxidation of Alcohol

Aldehydes can be formed by the oxidation of primary and secondary alcohols respectively. KMnO₄, CrO₃, and K₂Cr₂O₇ are used as oxidizing agents.

There is a problem when aldehydes are made by oxidising primary alcohols.

The aldehyde produced can get oxidised further to a carboxylic acid due to the acidified potassium dichromate (VI) solution. To solve this problem, that is, to stop at the aldehyde we need to:

• Use excess alcohol – this way there wouldn’t be enough oxidizing agent present to carry out the second stage and oxidize the aldehyde to form a carboxylic acid.
• Distil the aldehyde as soon as it gets formed. This way it doesn't stay in the mixture to be oxidized further.

1.1 Collin’s Reagent

Collin’s Reagent is a 1:2 mixture of chromium trioxide and pyridine in dichloromethane. It is a good oxidizing reagent for the preparation of aldehydes by the oxidation of primary alcohols as it helps to prevent further oxidation to a carboxylic acid. This action is only possible in a non-aqueous medium like CH₂Cl₂.

Collin’s Reagent Reaction

1.2 Dehydrogenation of Alcohol

Dehydrogenation of alcohols is used to convert volatile alcohols into aldehydes. It has a wide variety of uses in industrial applications. Primary alcohols in this technique are passed over heavy metal catalysts like Cu to obtain an aldehyde. 

Example: when a primary alcohol undergoes dehydrogenation; its vapours pass over the copper gauze at 573 K temperature. The n-Propyl alcohol undergoes dehydrogenation after passing over copper at 573 K temperature to form a proportion aldehyde. 

During the dehydrogenation, the metal catalysts use silver under certain heating conditions. 

This is one of the best methods for preparation of aldehydes from alcohol as in this method aldehydes don’t get oxidized further, therefore there is no risk of formation of carboxylic acid. 

2. From Hydrocarbons

Aldehydes can also be obtained from hydrocarbons through:

2.1 Ozonolysis of Alkenes

Aldehydes are obtained by reacting zinc dust and water which are products of Ozonolysis. 

2.2 Hydration of Alkenes

When water is added to ethyne in presence of H₂SO₄ and HgSO₄, it gives acetaldehyde. For any other alkyne, ketones would be produced.

2.3 From Acyl Chloride (acid chloride)

This reaction with acyl chloride is called Rosenmund reduction. In this reaction acid chloride is hydrogenated over catalyst, palladium on barium sulphate. Aldehyde formation happens after the partial poisoning of this reaction which is done by the addition of compounds such as sulphur or quinolone. This is an important step for formation of aldehydes.

*Sulphur or quinolone behaves as poison for this reaction and causes partial poisoning to stop further reduction of aldehydes into alcohols. 

2.4. From Nitriles and Esters

We can also prepare Aldehydes with nitriles. Reduction of nitriles with compound Stannous Chloride (SnCl₂) in presence of HCl leads to the formation of the nitrile compound’s corresponding imine form. This imine compound then undergoes complete hydrolysis to form corresponding aldehydes. The reaction is known as Stephen’s Reduction.

Nitriles can also undergo reduction by compound DIBAL-H which is di-isobutyl aluminium hydride to form imines. These imines further undergo hydrolysis forming aldehyde compounds. 

Esters also undergo this reduction with DIBAL-H to form aldehydes.

3. Oxidation of Methylbenzene or Toluene

Toluene or Methylbenzene and its derivatives get oxidised by use of strong oxidising agents. If we use suitable reagents that convert methyl group to intermediate which is difficult to oxidise further, this way we can stop the oxidation at the aldehyde stage. We can use the following methods:

3.1 Using Chromyl Chloride Cro₂Cl₂

A chromium complex is formed from a methyl group when chromyl chloride oxidises. Hydrolysis on this gives benzaldehyde. This reaction is also known as benzaldehyde. In Etard’s reaction, Toluene undergoes an oxidation process with reagent chromyl chloride (CrO₂Cl₂) which is present as a solution in CCl₄ or CS₂ which forms chromium complex.

3.2 Using Chromic Oxide CrO₂

When treated with chromic oxide in acetic anhydride, toluene and its substitutes are converted to benzylidene diacetate. The benzylidene diacetate can be hydrolysed to benzaldehyde with aqueous acid.

3.3 Gatterman - Koch Reaction

When Benzene or its derivatives are treated with carbon monoxide and hydrogen chloride in presence of a Lewis acid such as anhydrous aluminium chloride/cuprous chloride. This gives benzaldehyde or substituted benzaldehyde.

3.4 Side-Chain Halogenation

Side chain chlorination of Benzene will give benzal chloride which when hydrolysed gives benzaldehyde. This is one of the commercial methods for benzaldehyde manufacturing.

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Common Aldehydes with IUPAC Names and Structures

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Some of the common aldehydes and their structure and IUPAC Names are as given below:

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Uses of Aldehydes

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Some of the common uses of aldehydes are:

1. Aldehydes play a major role in the physiological processes of the human body:

• Aldehyde is present in the retina which is given by Vitamin A and is very necessary for eyesight.
• It is also found in many hormones natural and synthetic. 

2. Aldehyde condensation polymers are used in many industrial processes such as Bakelite during plastic production.
3. Formica is used in the formation of a laminate tabletop material. 
4. Aldehydes are important for the synthesis of solvents, perfumes, pharmaceuticals, and dyes.

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Things to Remember

• Aldehydes are one of the most important classes of organic compounds containing a carbonyl group. 
• Aldehydes are prepared by different methods like dehydrogenation or controlled oxidation of primary alcohols and controlled or reduction of acyl halides selectively. 
• The Aromatic aldehydes are also prepared through oxidation of (i) methylbenzene with chromyl chloride or CrO₃ in the presence of acetic anhydride, (ii) formulation of arenes with carbon monoxide and hydrochloric acid in the presence of anhydrous aluminium chloride, and (iii) cuprous chloride or by hydrolysis of benzal chloride.
• Aldehydes are used for physiological processes in the body. 
• Vitamin A contains aldehyde present in the retina which is necessary for eyesight. They are also used in natural and synthetic polymers. 
• Aldehydes also have industrial use.

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