Kolbe reaction is one type of phenolic reaction in which, phenol is used as a main product and it gives salicylic acid.
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What is kolbe reaction?
In the vast realm of organic chemistry, numerous reactions have been discovered and developed, each with its unique characteristics and applications.
Among them, the Kolbe reaction stands out as a remarkable transformation that has fascinated chemists for over a century. Named after its German discoverer, Hermann Kolbe.
This reaction holds immense significance in the field, enabling the synthesis of various organic compounds with wide-ranging applications.
In this article, we delve into the intricacies of the Kolbe reaction, exploring its mechanism, applications, and the profound impact it has had on the world of chemistry.
kolbe reaction is a type of phenol reaction. in which phenol is react with sodium hydroxide, phenoxide ion is generated. this is first step reaction and in second step reaction, phenoxide ion again react with CO2 and acidification (H+). then it give ortho hydroxy benzoic acid. this product is also know as salicylic acid.
kolbe reaction is a type of addition reaction or is a type of phenolic reaction. In which phenol is taken first, in this case phenol does not have a direct reaction.
the acidic hydrogen of phenol has to be removed. acidic hydrogen can be remove with the help of naoh (sodium hydroxide). this is shown in the reaction. and reaction is,
In this reaction, acidic hydrogen of phenol will be remove by naoh. and this reaction will form sodium phenoxide. Now, sodium phenoxide react with CO2 and also does acidification.
When sodium phenoxide react with CO2. and acidification (H+) after this reaction, hydroxyl group (COOH) comes to ortho position. this is called kolbe reaction.
Why sodium phenoxide react with CO2 then COOH comes to ortho position?
Resion is, intermolecular hydrogen bonding occur between oxygen and hydrogen. that’s why hydroxyl group attached on ortho position. this product is know as ortho hydroxyl benzoic acid. it is also called salicylic acid.
The Kolbe reaction, also known as the Kolbe electrolysis or the Kolbe-Schmitt reaction, is a vital transformation in which carboxylic acids are converted into alkanes through the decarboxylation process.
It involves the electrochemical decarboxylation of sodium or potassium salts of carboxylic acids in the presence of an electrolyte. The reaction typically occurs at high temperatures and employs a direct current (DC) electrical source.
Kolbe Reaction Mechanism
We know that, kolbe’s reaction is a phenol reaction because phenol is used during reaction.
phenol is a acidic in nature. It is easy to remove hydrogen from phenol by using sodium hydroxide.
In kolbe reaction mechanism, there are 5 steps are required for kolbe’s reaction mechanism. and all step is given below.
1 steps : Phenol is taken first, and then naoh sodium hydroxide. when phenol is react with naoh (naoh gives OH-) then OH will donate electron to hydrogen. and h2o will be out. and phenoxide ion become.
2 steps: Oxygen has electron on the phenoxide ion, they will show resonance with the ring. donating electrons will bring the electrons to the carbon.
3 steps: Due to this, oxygen to form double bond. and electrons will shift to ortho position. causing, hydrogen bonding is formed between hydrogen and oxygen.
4 steps: Now, take CO2, CO2 is also In acidic in nature. and In this case the electrons of ortho position will be shifted on carbon. if electrons shift on carbon then pi electrons will break and both the electrons will shifted on oxygen. all steps is given in the mechanism, as shown in reaction.
Hydrogen are donated electrons for making Aromaticity if hydrogen donate electrons then oxygen will get – charge. it mean due to this, negative charge will come on oxygen.
After this, acidification is carryout to form final product. salicylic acid is formed as a final product.
Kolbe Reaction Mechanism – Chemistry point of view
The Kolbe reaction proceeds through a series of steps, which can be summarized as follows:
- Ionization: The carboxylic acid is ionized in the presence of a strong base, forming the corresponding carboxylate ion.
- Electrolysis: The carboxylate ion is subjected to electrolysis in an electrochemical cell. The anion migrates to the anode (positive electrode), where it undergoes decarboxylation. This leads to the generation of a carbanion intermediate.
- Radical Formation: The carbanion intermediate is oxidized, resulting in the formation of a radical. This radical subsequently reacts with another carbanion or radical to produce an alkane.
- Termination: The radical chain reaction is eventually terminated through a combination reaction, yielding the final alkane product.
Applications of the Kolbe Reaction
The Kolbe reaction has found extensive use in various fields due to its ability to synthesize alkanes, which serve as building blocks for numerous organic compounds. Some notable applications of the Kolbe reaction include:
- Pharmaceutical Industry: The Kolbe reaction enables the synthesis of key intermediates for pharmaceutical compounds. It has been employed in the production of analgesics, anti-inflammatory drugs, and other vital medications.
- Hydrocarbon Synthesis: The reaction’s ability to generate alkanes has proven valuable in the synthesis of hydrocarbon-based fuels and lubricants. The production of long-chain alkanes from carboxylic acids plays a crucial role in the petroleum industry.
- Organic Synthesis: The Kolbe reaction serves as a valuable tool for organic chemists in the synthesis of complex molecules. It enables the construction of carbon-carbon bonds, facilitating the creation of diverse organic frameworks.
- Electrochemistry: The Kolbe reaction, being an electrochemical process, has contributed to the advancement of electrochemistry as a field. It has expanded our understanding of electrode reactions and the utilization of electricity in organic transformations.
The Kolbe reaction stands as a testament to the remarkable advancements in organic chemistry. Its ability to convert carboxylic acids into alkanes through electrochemical decarboxylation has revolutionized the synthesis of organic compounds across various industries. From pharmaceuticals to petroleum and beyond, the Kolbe reaction has provided chemists with a powerful tool for the construction of diverse molecules. As research in this field progresses, further exploration of the Kolbe reaction and its derivatives promises to unlock new possibilities and shape the future of organic chemistry.
Frequently Asked question
1. What is the main compound in kolbe’s reaction?
Phenol is the main compound in kolbe’s reaction because phenol is taken first, and then naoh is taken, after that phenol is react with naoh, then sodium phenoxide is formed and after that, sodium phenoxide react with CO2/H+, after this reaction it gives artho hydroxyl benzoic acid as a main product. this product is also know as salicylic acid.
therefore, we can say that phenol is main product in kolbe’s reaction.
2. Give the example of weak-acid and stronge-base in kolbe’s reaction?
When phenol is react with naoh, sodium hydroxide then it gives sodium phenoxide and h2 will out.
phenol – weak acid
NaoH – strong base
therefore, this is the best example of weak acid and strong base in kolbe’s reaction.
3. What is the main electrophile use in kolbe’s reaction?
Corbon dioxide – CO2
4. Give the main reaction of kolbe’s reaction and reimer – tiemann reaction?
kolb’s reaction – when phenol is react with naoh (sodium hydroxide) then it give sodium phenoxide and h2o will out. and then sodium phenoxide is react with CO2 after this reaction COOH group is attached on artho position of final product. this is called artho hydroxyl benzoic acid.
Reimer – Tiemann – when phenol is react with chloroform (chcl3) in the presence of naoh (sodium hydroxide). a -CHO group is attached on artho position of benzene ring.
5. What is the main synthesis of salicylic acid?
First synthesis – when phenol is treated with naoh then it give sodium phenoxide and h2o will out.
Second synthesis – when sodium phenoxide is treated with CO2/ acidification (H+) under pressure at 180° to 200°C. after this, it give salicylic acid as a final product. it is also know as artho hydroxyl benzoic acid.