Gattermann reaction

Gattermann Reaction

The Gattermann Reaction, also known as the Gattermann Formylation, is a chemical reaction that involves the formylation of aromatic compounds. This reaction is of significant importance in the field of organic chemistry, particularly in the synthesis of aldehydes. The Gattermann Reaction is named after the German chemist Ludwig Gattermann, who developed this method in the late 19th century.

Overview

The Gattermann Reaction allows for the direct introduction of an aldehyde group (-CHO) into an aromatic ring through the use of a carbon monoxide (CO) source and hydrochloric acid (HCl) in the presence of a Lewis acid catalyst such as aluminum chloride (AlCl3) or zinc chloride (ZnCl2). The reaction is similar to the Friedel-Crafts Acylation but uses different reagents and conditions.

Mechanism

The mechanism of the Gattermann Reaction involves several key steps:

1. Activation of the aromatic compound: The Lewis acid catalyst coordinates to the aromatic compound, increasing its reactivity towards electrophilic substitution.
2. Generation of the formyl cation: Carbon monoxide (CO), in the presence of hydrochloric acid (HCl), forms a formyl cation (H-C+=O), which is the actual electrophile in the reaction.
3. Electrophilic attack: The formyl cation attacks the activated aromatic ring, leading to the formation of an aldehyde group directly attached to the ring.

Variants

There are two main variants of the Gattermann Reaction:

• Gattermann-Koch Reaction: This variant involves the use of carbon monoxide (CO) and hydrochloric acid (HCl) under high pressure, with aluminum chloride (AlCl3) as the catalyst. It is particularly useful for the formylation of benzene and its derivatives.
• Gattermann-Salicylaldehyde Synthesis: This method is used for the synthesis of salicylaldehyde from phenols. It employs zinc chloride (ZnCl2) as the catalyst and uses hydrogen cyanide (HCN) instead of carbon monoxide as the carbon source.

Applications

The Gattermann Reaction is widely used in organic synthesis for the preparation of aromatic aldehydes, which are valuable intermediates in the production of dyes, fragrances, pharmaceuticals, and agrochemicals. Its ability to introduce aldehyde groups directly onto aromatic rings makes it a powerful tool in the synthesis of complex organic molecules.

Limitations

Despite its utility, the Gattermann Reaction has some limitations. It generally requires harsh conditions, such as the use of strong Lewis acid catalysts and high pressure (in the case of the Gattermann-Koch Reaction). Additionally, it may not be suitable for substrates that are sensitive to acid or prone to polymerization under the reaction conditions.

See Also

• Friedel-Crafts Acylation
• Vilsmeier-Haack Reaction
• Reimer-Tiemann Reaction