Preparation of Aldehydes

Do you know aldehyde play a major role in human vision? Also, it is important in many physiological processes. Vitamin A contains aldehyde found in the retina which is necessary for eyesight. It is also found in many natural and synthetic hormones. Aldehyde condensation polymers find its use in many industrial processes such as Bakelite during plastic production, Formica for the formation of laminate tabletop material. By now, we also know that aldehydes are important in the synthesis of solvents, perfumes, pharmaceutical, and dyes. Therefore, in this topic, it is very crucial to understand the preparation of aldehydes.

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Preparation of Aldehydes from Acid Chloride or Acyl Chloride

Acyl chloride/acid chloride undergoes hydrogenation in the presence of a catalyst such as barium sulfate (BaSO4) or Palladium (Pd) to form aldehydes. Aldehyde formation with this process is possible after the partial poisoning of the reaction by the addition of compounds such as sulfur or quinolone. This is an important step for the formation of aldehydes. This is also known as Rosenmund’s Reaction.

Points to Remember in Rosenmund’s Reaction

In this reaction, Sulphur or quinolone behaves as poison for catalysts and causes partial poisoning to stop further reduction of aldehydes into alcohols. However, it is not possible to prepare formaldehyde from this reaction because the acyl chloride form, formyl chloride, is not stable at room temperature. It is not possible to prepare Ketones by this reaction.

Preparation of Aldehydes from Nitriles and Esters

Preparation of Aldehydes is possible with the help of nitriles. Reduction of nitriles with the compound Stannous Chloride (SnCl₂) in the presence of HCl leads to the formation of the nitrile compound’s corresponding imine form. The imine compound undergoes hydrolysis to yield the corresponding aldehydes. The reaction is known as Stephen’s Reduction.

Moreover, nitriles can undergo reduction by the compound DIBAL-H or di-isobutyl aluminium hydride for the formation of imines. The imines further undergo hydrolysis thereby forming aldehyde compounds.

Similarly, esters can also undergo reduction with DIBAL-H to form aldehydes.

Videos on Aldehydes, Ketones & Carboxylic Acids

Preparation of Aromatic Aldehydes from Hydrocarbons

Formation of Aromatic Aldehyde, benzaldehydes and the derivatives of benzaldehyde, is possible with the help of aromatic hydrocarbons primarily by methods mentioned below.

• Oxidation of methylbenzene
• Side chain chlorination
• Gatterman – Koch reaction

Preparation of Aromatic Aldehydes by Oxidation of Methylbenzene

Toluene and the derivatives of toluene undergo oxidation with the help of a strong oxidizing agent to form benzoic acids. However, it is possible to stop the reaction at the aldehyde stage with the help of proper reagents. The reagents can convert the methyl group to an intermediate that cannot undergo further oxidation easily. Oxidation of methylbenzene or toluene falls under two categories on the basis of reagents used in the reaction

• Use of chromyl chloride
• Use of chromic oxide
• Side chain halogenation

Oxidation of Methylbenzene or Toluene Using Chromyl Chloride

Oxidising agent chromyl chloride can oxidize and convert methyl group to a chromium complex. The chromium complex undergoes hydrolysis to produce benzaldehyde. We refer to this reaction as Etard Reaction. In this reaction, methylbenzene/toluene undergo oxidation process with the reagent of chromyl chloride (CrO₂Cl₂) present in solution form in CCl₄ or in CS₂ thereby forming chromium complex.

Oxidation of Methylbenzene or Toluene Using Chromic Oxide

It is possible to oxidize toluene or substituted toluene to aldehydes on treatment with reagents such as Chromium oxide, chromium trioxide, with acetic anhydride. This reaction leads to the formation of benzylidene diacetate. The intermediate or in this case benzylidene diacetate can undergo further hydrolysis to corresponding benzaldehyde with aqueous acid.

Side Chain Halogenation

Preparation of aldehydes is possible by side chain halogenation, more specifically side chain chlorination, followed by hydrolysis. Side chain chlorination of toluene yields benzal chloride which undergoes hydrolysis leads to the formation of benzaldehyde.  The preparation technique is also the commercial way of benzaldehyde manufacture.

By Gatterman – Koch Reaction

When benzene and its derivatives undergo treatment with carbon monoxide and HCl in the presence of a Lewis acid such as cuprous chloride/ anhydrous aluminium chloride leads to the formation of benzaldehyde or substitution of benzaldehyde compounds. This reaction method refers to as Gatterman-Koch Reaction

Solved Examples for You

Question: Complete the reactions

Solution: