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Kolbe's Electrolytic Method - JEE Important Topic

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Last updated date: 21st May 2024
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Introduction to Kolbe's Electrolytic Method

Kolbe electrolysis, sometimes known as the Kolbe reaction, is a type of organic reaction named after Hermann Kolbe. It's best for making symmetrical dimers, but it can also be used to make unsymmetrical dimers when combined with a mixture of two carboxylic acids.


Here, we will discuss the method of Kolbe electrolysis, the mechanism of electrolysis, and how Kolbe’s reaction is used in the preparation of alkane and acetic acid.


Kolbe's Electrolytic Method

The Kolbe reaction is a decarboxylative dimerization of two carboxylic acids in its formal form (or carboxylate ions). The general reaction is


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General Reaction of Kolbe's Electrolysis


Mechanism of the Above Reaction:

Two molecules of carboxylate ions lose two electrons and two molecules of carbon dioxide to form two alkyl free radicals, which then join together to form an alkane.


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Mechanism of Kolbe's Reaction

Electrolysis

Let us now define electrolysis. The word "lysis" means "to separate or break," so electrolysis would be defined as "electric breakdown." Electrolysis is a method of causing a non-spontaneous chemical reaction to occur using electrical energy.


Electrolysis is a chemical and industrial process that uses the direct electric current (DC) to drive a chemical reaction that would otherwise be non-spontaneous. The use of an electrolytic cell in the commercial separation of elements from naturally occurring sources such as ores is important. The decomposition potential is the voltage needed for electrolysis. 


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The Process of Electrolysis


Faraday’s Law of Electrolysis

  • The amount of product created by the passage of an electric current (moles) is stoichiometrically equal to the quantity of electrons provided (moles).

  • The stoichiometry of the reaction, the size of the current flowing, and the time during which the current flows are used to compute the amount of product generated during an electrolysis operation.


Kolbe's Reaction

Let us now explain Kolbe's reaction. It is sometimes known as the Kolbe Schmitt reaction, after Hermann Kolbe and Rudolf Schmitt, and is an addition reaction. The phenoxide ion is formed when phenol reacts with sodium hydroxide. The phenoxide ion generated in electrophilic aromatic substitution processes is more reactive than phenol. As a result, the weak electrophile carbon dioxide undergoes an electrophilic substitution process. Ortho-hydroxybenzoic acid is the main product (salicylic acid). Kolbe's reaction is another name for this process.


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Kolbe’s Reaction

Kolbe's Reaction of Preparation of Alkane

Electrolysis is the process of electrolyzing a concentrated sodium salt of a carboxylic acid. In one of the products, the alkane is produced at the anode. This reaction of the formation of alkane from a derivative of the carboxylic acid is also known as Kolbe’s reaction.


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Kolbe’s Reaction to Get Alkane

Kolbe Electrolysis Mechanism

Kolbe electrolysis reaction has the following mechanism:


The sodium salt of carboxylic acid undergoes electrolysis to form alkane as the major product and carbon dioxide water and caustic soda as byproducts. 

  • An oxidation reaction occurs at the anode, which results in the loss of carbon dioxide and the formation of two methyl free radicals which then react among themselves forming ethane.

  • Reduction occurs at the anode which results in the formation of two hydroxide ions as well as two hydrogen free radicals which react among themselves, forming hydrogen molecules.


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Mechanism of Kolbe’s electrolysis.

Synthesis of Acetic Acid by Kolbe

For the first time in 1845, German chemist Hermann Kolbe produced or synthesised acetic acid from inorganic components. He combined chlorine and carbon disulfide to produce carbon tetrachloride. The electrolytic reduction to acetic acid was then followed by heat decomposition to tetrachloroethylene and aqueous chlorination to trichloroacetic acid. Only hydrocarbons were employed in the inorganic chemicals he used. As a result, Kolbe was the first to make acetic acid from carbon, hydrogen, and oxygen.


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Synthesis of Acetic Acid by Kolbe’s Reaction

Conclusion

At the anode of Kolbe's electrolytic method, the electrolysis of an aqueous solution of a potassium or sodium salt of a carboxylic acid produces an alkane with an even number of carbon atoms. The decarboxylation of the sodium salt of fatty acid occurs in this reaction. A two-stage radical process is involved in the reaction mechanism, namely, electrochemical decarboxylation produces a radical intermediate, which combines to form a covalent bond. This reaction is used in the formation of alkanes and acetic acid from carboxylic acid derivatives, which undergoes an electrolytic mechanism.

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FAQs on Kolbe's Electrolytic Method - JEE Important Topic

1. What are the importance and limitations of the Kolbe reaction?

The Importance of Kolbe’s Reaction is as Follows:

For the manufacture of 3-hydroxy-2-naphthoic acid, Kolbe's process is utilised. This acid is a common precursor to azo dyes and pigments. Salicylic acid is combined with acetic anhydride, which is obtained through Kolbe's reaction. Aspirin is a common pain reliever.


The Limitations of Kolbe’s Reaction are as Follows-

  • Alkanes with an even number of carbon atoms are the only ones that can be made through Kolbe’s reaction.

  • We cannot produce methane by this process.

2. a) Which ortho product is produced in the Kolbe reaction?

b) Which electrophile is used and how does it operate as an electrophile?

Kolbe's reaction has the following Mechanism:

The negative charge develops in the ortho position as a result of the aforesaid procedure. As a result, the electrophilic substitution occurs at the ortho position, yielding ortho hydroxybenzoic acid as a product.




Mechanism of Kolbe's Reaction.


b) Carbon dioxide is the electrophile in Kolbe's process. This is due to the fact that the phenoxide ion is more reactive toward the electrophilic aromatic substitution process than the phenol ion. The molecule undergoes a substitution reaction with carbon dioxide because it is a poor electrophile.

3. What are the factors that affect the electrolysis process?

Electrolysis is Additionally Influenced by Factors such as:

  • The Electrolyte's Nature and Condition- The electrolyte should be in a liquid state, molten or in a solution with a suitable polar solvent for electrolysis.

  • The Nature and Electrode Potential of Electrolyte's Ions- The electrolysis of two elemental ions in electrolytes is straightforward, yielding the two elements on electrolysis. 

  • The Electrode's Nature- The nature of the electrolyte can result in diverse products for the same electrolyte.

  • An Excessive Amount of Voltage at Electrodes- The electrolytic processes and products are determined by the redox potential of electrolyte ions. During electrolysis, the redox potentials of some half-reactions can sometimes exceed the thermodynamic potentials. The voltage of the half-excess reaction (over-voltage) may be unfavourable to the reaction and alter the electrolysis result.