Kolbe's Reaction: Mechanism, Applications and Reactions for Phenol

Kolbe’s reaction, sometimes also known as the Kolbe Schmitt reaction after Hermann Kolbe and Rudolf Schmitt, is a type of an addition reaction. When phenol is reacted with sodium hydroxide, the phenoxide ion is produced. When it comes to electrophilic aromatic substitution reactions, the phenoxide ion produced is more reactive than phenol. As a result, it conducts an electrophilic substitution reaction with the weak electrophile carbon dioxide. The principal result is ortho-hydroxybenzoic acid (salicylic acid). Kolbe's reaction is the common name for this kind of reaction. In this article, we will have a look at Kolbe’s reaction, its mechanism, and application.

Key Terms: Kolbe’s reaction, Electrophilic aromatic substitution process, Carboxylation, Phenoxide, Ion, Ortho-hydroxybenzoic acid, Salicylic acid, Phenol, Sodium hydroxide

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What is Kolbe’s Reaction?

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Kolbe's reaction involves a process of heating a combination of sodium phenoxide and carbon dioxide under pressure at 180°C to 200°C in order to produce salicylic acid. The phenoxide ion is formed when phenol combines with sodium hydroxide.

In electrophilic aromatic substitution processes, this produced phenoxide ion is more reactive than phenol. The phenoxide ion combines with carbon dioxide (a weak electrolyte) and forms salicylate by an electrophilic substitution process. Salicylate acid is formed when the salicylate interacts with acid. It's a carboxylation reaction in which sodium phenoxide is heated to 125°C in the presence of carbon dioxide at 100 atm pressure. This reaction's product is then treated with sulfuric acid to produce salicylic acid (an aromatic hydroxy acid).

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Mechanism of Kolbe’s Reaction

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Kolbe’s reaction is a chemical reaction that involves carboxylation. When sodium phenoxide is allowed to absorb carbon dioxide and the resulting product is heated to 125 degrees Celsius at a pressure of over a hundred atmospheres, Kolbe’s reaction happens. The formation of an unstable intermediate is now complete. The mechanism of the reaction is as follows:

• The reaction is initiated when the nucleophilic addition of carbon dioxide along with the phenoxide reaction takes place in order to produce salicylate.
• This salicylate reacts with the acid to form the salicylic acid.
• The resultant product is heated at 125 degrees and an unstable compound is formed as a result of this reaction.
• This unstable intermediate undergoes a proton shift, resulting in sodium salicylate production. 
• Sulfuric acid is now used to treat this combination. 
• Salicylic acid is produced by acidifying the mixture. 
• As a result of Kolbe’s reaction, the necessary aromatic hydroxy acid – salicylic acid is created. In the mechanism, nucleophilic addition of sodium phenoxide to carbon dioxide gas results in the formation of salicylate.

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Kolbe’s Reaction of Phenol

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The major product of the reaction is ortho hydroxybenzoic acid (salicylic acid), which is generated by heating sodium phenoxide at 125⁰C in the presence of carbon dioxide gas under a pressure of about 100 atm and then acidifying the intermediate product.

Aspirin (acetylsalicylic acid) is created when salicylic acid is acetylated in the presence of acetic anhydride.

Steps:

1. The electrophilic carbon present in the ortho position reacts with the nucleophilic phenolate as in the case of an aldol reaction.
2. A non-aromatic intermediate compound, cyclohexadienone carboxylate, is formed.
3. The compound transforms into a stable aromatic enol by undergoing tautomerism.
4. The intramolecular hydrogen bond stabilizes this compound further.
5. After it undergoes acidification, the carboxylic acid is formed.

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Applications of Kolbe’s Reaction

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Some of the applications of Kolbe’s Reaction are as follows:

• 4-Hydroxybenzoic acid can be obtained when potassium hydroxide is employed in the Kolbe reaction. This is a crucial step in the production of parabens (parahydroxybenzoate or ester of para-hydroxy benzoic acid, used as a biocide in cosmetic products).
• The Kolbe reaction can also be used to produce 3-hydroxy-2-naphthoic acid, which is an important precursor in the industry for azo dyes and pigments.
• By coupling salicylic acid with acetic anhydride, aspirin can be produced. Aspirin is a popular pain reliever.

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Things to Remember

• In Kolbe’s reaction, sodium phenoxide (the sodium salt of phenol) is heated with carbon dioxide under pressure (100 atm, 125 °C) and subsequently treated with sulfuric acid. An aromatic hydroxy acid, commonly known as salicylic acid, is formed as the end product.
• Aspirin, from Kolbe’s reaction itself, is made by reacting salicylic acid with acetic anhydride, which is derived from Kolbe's reaction. Aspirin is a common pain reliever.
• Kolbe's reaction is a sort of phenolic reaction or an addition reaction. Because phenol is involved in the reaction, it is often called a phenolic reaction.
• True aromatic acids do not undergo Kolbe's electrolytic reaction because the carboxyl group is linked to the benzene ring in aromatic carboxylic acids. This makes carbon dioxide loss more challenging.
• In the chemical industry, Kolbe's reaction is utilized to synthesize chemicals. It is widely used in chemical synthesis, the medical industry, and chemical and dye manufacture.

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