Tishchenko reaction

Tishchenko reaction is a chemical reaction that involves the dimerization of an aldehyde in the presence of an alkoxide. This reaction is named after the Russian chemist Vyacheslav Evgen'evich Tishchenko, who discovered it in the early 20th century. The Tishchenko reaction is an important process in organic chemistry, particularly in the synthesis of esters from aldehydes. It is a special case of the more general Cannizzaro reaction, differing in that it uses a catalyst to facilitate the reaction.

Mechanism

The Tishchenko reaction proceeds through a mechanism that involves the formation of a half-ester intermediate. Initially, the alkoxide catalyst deprotonates the aldehyde to form an alkoxide intermediate. This intermediate then attacks another molecule of aldehyde to form a half-ester. Subsequently, a second molecule of aldehyde reduces this half-ester to the final ester product, while itself being oxidized to a carboxylic acid. However, in the presence of the catalyst, the carboxylic acid can further react to form more ester product.

Catalysts

The choice of catalyst is crucial for the Tishchenko reaction. Commonly used catalysts include aluminum alkoxides, such as aluminum isopropoxide. These catalysts are effective because they are strong Lewis acids, which can activate the carbonyl group of the aldehyde for nucleophilic attack. Other metal alkoxides, such as those of titanium and zirconium, have also been used.

Applications

The Tishchenko reaction is widely used in the synthesis of various esters, which are important in the manufacture of plastics, fragrances, and pharmaceuticals. For example, the reaction can be used to synthesize butyl acetate, a solvent in the paint and coatings industry, from acetaldehyde. It is also used in the synthesis of higher molecular weight esters, which are valuable as plasticizers and in the production of synthetic lubricants.

Limitations

While the Tishchenko reaction is a powerful tool for the synthesis of esters, it has some limitations. The reaction is generally limited to aldehydes that do not have strong electron-withdrawing groups, as these can inhibit the reaction. Additionally, the reaction conditions must be carefully controlled to prevent side reactions, such as the Aldol condensation, which can occur if the reaction mixture becomes too concentrated.

See Also

• Cannizzaro reaction
• Aldol condensation
• Esterification
• Organic synthesis