Kolbe Reaction Mechanism

Kolbe's reaction is a type of addition reaction named after Sir Hermann Kolbe and Rudolf Schmitt. It is also known as Kolbe Schmitt's reaction. When phenol reacts with sodium hydroxide, phenoxide ion is generated. 

This generated phenoxide ion is more reactive as compared to phenol towards electrophilic aromatic substitution reactions.

The phenoxide ion reacts with carbon dioxide (a weak electrolyte) and undergoes an electrophilic substitution reaction. Ortho-hydroxybenzoic acid (salicylic acid) is formed in the reaction as the primary product. 

This reaction is called Kolbe's reaction.

An Example of Kolbe's Reaction is Given below:

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Details About Kolbe - Schmitt Reaction

In the earlier classes, students must have learnt about the transformation of one type of chemical substance or compounds to some other form of compounds or substances. This process is commonly known as the process of chemical reaction that occurs due to the inherent chemical properties of that substance or by some external arrangements. The electrons present in the atoms of the molecules play the role of carrying out such reactions. The chemical bonds break or get exchanged with the transfer of electrons. 

In the chapters of organic chemistry students also must have got acquainted with the carboxylic acid group of hydrocarbons.  These chains of carbon atoms have one Oxygen atom and an OH formed out of oxygen and hydrogen atoms,  molecules attached to one carbon atom. Carboxylic acid is commonly denoted by the formula of R-COOH. R can be alkyl, alkenyl, aryl, or any other group of hydrocarbon compounds.

Hermann Kolbe and Rudolf Schmitt were two chemists of Germany who have a major contribution in the field of Organic Chemistry. They together found a way to produce Salicylic acid from the sodium salt of phenol. It is an aromatic hydroxy acid formed by heating sodium phenoxide at about 125 degrees Celsius under a pressure of 100 atm and finally treating it with sulfuric acid. It became a major discovery because later this Salicylic acid was used to manufacture aspirin. And the scientists named this carboxylation chemical reaction as 'Kolbe-Schmitt reaction' or which is sometimes also referred to as the 'Kolbe process'. 

To understand the mechanism of Kolbe Reaction students need to have knowledge on nucleophilic addition of a phenoxide to carbon dioxide. Traditionally sodium phenoxide (NaOC₆H₅) is used for phenoxide. Finally, the salicylate anion transforms into Salicylic acid in the presence of an acid.

Kolbe’s Reaction – Reaction of phenol with sodium hydroxide creates phenoxide ion.

The Mechanism of Kolbe Reaction: The reaction starts with the nucleophilic addition of the phenoxide ion and carbon dioxide, thereby forming salicylate.

The salicylate further reacts with acid and forms salicylic acid. It is a carboxylation reaction in which sodium phenoxide is heated to a temperature of 125 degrees celsius and under a pressure of 100 ATMs in the presence of carbon dioxide. The resultant of this reaction is further treated with sulfuric acid to form salicylic acid (an aromatic hydroxy acid).

Kolbe Reaction Mechanism

Kolbe's reaction is a carboxylation chemical reaction. The mechanism of Kolbe’s reaction starts by passing carbon dioxide through sodium phenoxide solution. The resultant product of this reaction is heated to a temperature of 125 degrees celsius under 100 atmospheric pressure. An intermediate unstable compound is formed after this reaction.

The unstable intermediate compound undergoes a proton shift and leads to the formation of sodium salicylate. 

This mixture is further treated with sulfuric acid and the acidification of the mixture forms salicylic acid. The representation of Kolbe's reaction mechanics is mentioned below:

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Thus, salicylic acid (an aromatic hydroxy acid) is formed by Kolbe's reaction. In this reaction, you can observe that the nucleophilic addition of sodium phenoxide with carbon dioxide gas takes place to form salicylate.

Kolbe’s Reaction of Phenol

When sodium phenoxide is heated at 125 degrees celsius in the presence of carbon dioxide gas and a pressure of about 100 ATMs and further the intermediate product is acidified, ortho hydroxybenzoic acid (salicylic acid) is obtained as the main product.

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When salicylic acid undergoes acetylation in the presence of acetic anhydride, aspirin (acetylsalicylic acid) is produced.

The Mechanism for Carboxylation of Phenols

• Step 1: The nucleophilic phenolate reacts with electrophilic carbon (of carbon dioxide) in the ortho position. This reaction is similar to the Aldol reaction. 

• Step 2: The intermediate compound cyclohexadiene carboxylate (a non-aromatic compound) formed in this reaction undergoes tautomerism to become a stable aromatic enol. This compound is further stabilized by an intramolecular hydrogen bond. Carboxylic acid is formed after the compound undergoes acidification.

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Question: Explain the Following with an Example

1. Kolbe's reaction.

2. Reimer-Tiemann reaction.

3. Williamson ether synthesis.

4. Unsymmetrical ether.

Answer:

1. Kolbe's Reaction

In Kolbe's reaction, phenol is treated with sodium hydroxide to form sodium phenoxide. The sodium phenoxide reacts with carbon dioxide, and it undergoes acidification and an electrophilic substitution reaction to form ortho-hydroxybenzoic acid.

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2. Reimer-Tiemann Reaction

When phenol reacts with chloroform (CHCl₃) in the presence of sodium hydroxide, an aldehyde group (-CHO) group is generated at the ortho of the benzene ring.

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This reaction is called the Reimer-Tiemann reaction.

The intermediate product formed in this reaction is hydrolyzed in the presence of an alkali to produce salicylaldehyde.

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3. Williamson Ether Synthesis

Williamson ether synthesis is a laboratory method that is used to prepare symmetric and asymmetric ethers by the reaction of alkyl halides with sodium alkoxides.

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This reaction begins with the attack of SN₂ of the alkoxide ion on the alkyl halide. This reaction is best suitable for primary alkyl halides and yields better results.

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If a secondary or tertiary alkyl halide is used, the reaction undergoes elimination rather than substitution.

4. Unsymmetrical Ether

If two groups of ether, located on two sides of an oxygen atom, differ, the ether is called unsymmetrical ether. The two groups must have an unequal number of carbon atoms to be unsymmetrical. An example is ethyl methyl ether (CH₃-O-CH₂CH₃).

Applications of Kolbe Reaction

• If potassium hydroxide is used in place of sodium hydroxide in Kolbe's reaction, it produces 4-Hydroxybenzoic. This is an important compound for parabens (parahydroxybenzoate or ester of para-hydroxy benzoic acid, which is used as a biocide in cosmetic products).

• Kolbe's reaction is used for the production of 3-hydroxy-2-naphthoic acid. This acid is a common precursor to azo dyes and pigments.

• The salicylic acid obtained from Kolbe's reaction is used to make aspirin by making it react with acetic anhydride. Aspirin is a widely used painkiller.

Do You Know?

By using potassium hydroxide, 4-hydroxybenzoic acid is accessible. A crucial precursor for paraben class utilized for instance in the case of personal care products. 

The method works out fine in industrial synthesizing of 3 hydroxy 2 naphthoic acids. Hence, the chemistry of carboxylation increases and decreases with respect to temperature.