Kolbe Reaction Mechanism

What is Kolbe Reaction?

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 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:

[Image to be added Soon]

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

The 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:

[Image to be added Soon]

Thus, salicylic acid (an aromatic hydroxy acid) is formed by the 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.

[Image to be added Soon]

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 cyclohexadienone 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.

[Image to be added Soon]

Explain the Following with an Example

(i) Kolbe's reaction.

(ii) Reimer-Tiemann reaction.

(iii) Williamson ether synthesis.

(iv) 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.

[Image to be added Soon]

  1. 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.

[Image to be added Soon]

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.

[Image to be added Soon]

  1. 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.

[Image to be added Soon]

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.

[Image to be added Soon]

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

  1. 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 case of personal care products. 

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

FAQ (Frequently Asked Questions)

Q1. What is Kolbe's Electrolytic Method?

Ans: Kolbe's electrolytic method consists of electrochemical oxidative decarboxylation of carboxylic acid, which leads to the formation of radicals. These radicals dimerize.

This reaction is applied to the synthesis of symmetrical dimers. In some reactions, it is also used with two carboxylic acids to produce unsymmetrical dimers.

Q2. What is Kolbe's Synthesis?

Ans: Kolbe's reaction is a method by which chemical reaction occurs between CO2 and an alkaline salt of a corresponding phenol to produce aromatic o-hydroxycarboxylic acids.

Kolbe's reaction was given by a German chemist Sir A. W. H. Kolbe, in 1860 and it was improved by R. Schmitt in 1865.

Q3. Is it Possible to Prepare Ethane by Kolbe's Electrolytic Method?

Ans: Yes. It is possible to prepare ethane and other higher alkanes by Kolbe's electrolytic method.

By using Kolbe's electrolytic method, alkanes are formed by the combination of two alkyls free radical, and hence the product contains at least two carbon atoms. Since methane has only one carbon atom, it cannot be prepared by Kolbe's electrolytic method.

Q4. Can Kolbe’s Reaction be Used for Converting Sodium Acetate to Ethane?

Ans: Yes. By using Kolbe's electrolytic method, firstly, the aqueous solution of sodium acetate is electrolyzed. The acetate ions then get decomposed, thereby forming methyl radicals.

These methyl radicals combine to form ethane.